_continuous_distns.py

#
# Author:  Travis Oliphant  2002-2011 with contributions from
#          SciPy Developers 2004-2011
#
from __future__ import division, print_function, absolute_import

import warnings

import numpy as np

from scipy.misc.doccer import inherit_docstring_from
from scipy import optimize
from scipy import integrate
import scipy.special as sc
from scipy._lib._numpy_compat import broadcast_to

from . import _stats
from ._tukeylambda_stats import (tukeylambda_variance as _tlvar,
                                 tukeylambda_kurtosis as _tlkurt)
from ._distn_infrastructure import (get_distribution_names, _kurtosis,
                                    _lazyselect, _lazywhere, _ncx2_cdf,
                                    _ncx2_log_pdf, _ncx2_pdf,
                                    rv_continuous, _skew, valarray)
from ._constants import _XMIN, _EULER, _ZETA3, _XMAX, _LOGXMAX


## Kolmogorov-Smirnov one-sided and two-sided test statistics
class ksone_gen(rv_continuous):
    """General Kolmogorov-Smirnov one-sided test.

    %(default)s

    """
    def _cdf(self, x, n):
        return 1.0 - sc.smirnov(n, x)

    def _ppf(self, q, n):
        return sc.smirnovi(n, 1.0 - q)
ksone = ksone_gen(a=0.0, name='ksone')


class kstwobign_gen(rv_continuous):
    """Kolmogorov-Smirnov two-sided test for large N.

    %(default)s

    """
    def _cdf(self, x):
        return 1.0 - sc.kolmogorov(x)

    def _sf(self, x):
        return sc.kolmogorov(x)

    def _ppf(self, q):
        return sc.kolmogi(1.0 - q)
kstwobign = kstwobign_gen(a=0.0, name='kstwobign')


## Normal distribution

# loc = mu, scale = std
# Keep these implementations out of the class definition so they can be reused
# by other distributions.
_norm_pdf_C = np.sqrt(2*np.pi)
_norm_pdf_logC = np.log(_norm_pdf_C)


def _norm_pdf(x):
    return np.exp(-x**2/2.0) / _norm_pdf_C


def _norm_logpdf(x):
    return -x**2 / 2.0 - _norm_pdf_logC


def _norm_cdf(x):
    return sc.ndtr(x)


def _norm_logcdf(x):
    return sc.log_ndtr(x)


def _norm_ppf(q):
    return sc.ndtri(q)


def _norm_sf(x):
    return _norm_cdf(-x)


def _norm_logsf(x):
    return _norm_logcdf(-x)


def _norm_isf(q):
    return -_norm_ppf(q)


class norm_gen(rv_continuous):
    """A normal continuous random variable.

    The location (loc) keyword specifies the mean.
    The scale (scale) keyword specifies the standard deviation.

    %(before_notes)s

    Notes
    -----
    The probability density function for `norm` is::

        norm.pdf(x) = exp(-x**2/2)/sqrt(2*pi)

    The survival function, ``norm.sf``, is also referred to as the
    Q-function in some contexts (see, e.g.,
    `Wikipedia's <https://en.wikipedia.org/wiki/Q-function>`_ definition).

    %(after_notes)s

    %(example)s

    """
    def _rvs(self):
        return self._random_state.standard_normal(self._size)

    def _pdf(self, x):
        return _norm_pdf(x)

    def _logpdf(self, x):
        return _norm_logpdf(x)

    def _cdf(self, x):
        return _norm_cdf(x)

    def _logcdf(self, x):
        return _norm_logcdf(x)

    def _sf(self, x):
        return _norm_sf(x)

    def _logsf(self, x):
        return _norm_logsf(x)

    def _ppf(self, q):
        return _norm_ppf(q)

    def _isf(self, q):
        return _norm_isf(q)

    def _stats(self):
        return 0.0, 1.0, 0.0, 0.0

    def _entropy(self):
        return 0.5*(np.log(2*np.pi)+1)

    @inherit_docstring_from(rv_continuous)
    def fit(self, data, **kwds):
        """%(super)s
        This function (norm_gen.fit) uses explicit formulas for the maximum
        likelihood estimation of the parameters, so the `optimizer` argument
        is ignored.
        """
        floc = kwds.get('floc', None)
        fscale = kwds.get('fscale', None)

        if floc is not None and fscale is not None:
            # This check is for consistency with `rv_continuous.fit`.
            # Without this check, this function would just return the
            # parameters that were given.
            raise ValueError("All parameters fixed. There is nothing to "
                             "optimize.")

        data = np.asarray(data)

        if floc is None:
            loc = data.mean()
        else:
            loc = floc

        if fscale is None:
            scale = np.sqrt(((data - loc)**2).mean())
        else:
            scale = fscale

        return loc, scale

norm = norm_gen(name='norm')


class alpha_gen(rv_continuous):
    """An alpha continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `alpha` is::

        alpha.pdf(x, a) = 1/(x**2*Phi(a)*sqrt(2*pi)) * exp(-1/2 * (a-1/x)**2),

    where ``Phi(alpha)`` is the normal CDF, ``x > 0``, and ``a > 0``.

    `alpha` takes ``a`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _pdf(self, x, a):
        return 1.0/(x**2)/_norm_cdf(a)*_norm_pdf(a-1.0/x)

    def _logpdf(self, x, a):
        return -2*np.log(x) + _norm_logpdf(a-1.0/x) - np.log(_norm_cdf(a))

    def _cdf(self, x, a):
        return _norm_cdf(a-1.0/x) / _norm_cdf(a)

    def _ppf(self, q, a):
        return 1.0/np.asarray(a-sc.ndtri(q*_norm_cdf(a)))

    def _stats(self, a):
        return [np.inf]*2 + [np.nan]*2
alpha = alpha_gen(a=0.0, name='alpha')


class anglit_gen(rv_continuous):
    """An anglit continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `anglit` is::

        anglit.pdf(x) = sin(2*x + pi/2) = cos(2*x),

    for ``-pi/4 <= x <= pi/4``.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x):
        return np.cos(2*x)

    def _cdf(self, x):
        return np.sin(x+np.pi/4)**2.0

    def _ppf(self, q):
        return np.arcsin(np.sqrt(q))-np.pi/4

    def _stats(self):
        return 0.0, np.pi*np.pi/16-0.5, 0.0, -2*(np.pi**4 - 96)/(np.pi*np.pi-8)**2

    def _entropy(self):
        return 1-np.log(2)
anglit = anglit_gen(a=-np.pi/4, b=np.pi/4, name='anglit')


class arcsine_gen(rv_continuous):
    """An arcsine continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `arcsine` is::

        arcsine.pdf(x) = 1/(pi*sqrt(x*(1-x)))

    for ``0 < x < 1``.

    %(after_notes)s

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _pdf(self, x):
        return 1.0/np.pi/np.sqrt(x*(1-x))

    def _cdf(self, x):
        return 2.0/np.pi*np.arcsin(np.sqrt(x))

    def _ppf(self, q):
        return np.sin(np.pi/2.0*q)**2.0

    def _stats(self):
        mu = 0.5
        mu2 = 1.0/8
        g1 = 0
        g2 = -3.0/2.0
        return mu, mu2, g1, g2

    def _entropy(self):
        return -0.24156447527049044468
arcsine = arcsine_gen(a=0.0, b=1.0, name='arcsine')


class FitDataError(ValueError):
    # This exception is raised by, for example, beta_gen.fit when both floc
    # and fscale  are fixed and there are values in the data not in the open
    # interval (floc, floc+fscale).
    def __init__(self, distr, lower, upper):
        self.args = (
            "Invalid values in `data`.  Maximum likelihood "
            "estimation with {distr!r} requires that {lower!r} < x "
            "< {upper!r} for each x in `data`.".format(
                distr=distr, lower=lower, upper=upper),
        )


class FitSolverError(RuntimeError):
    # This exception is raised by, for example, beta_gen.fit when
    # optimize.fsolve returns with ier != 1.
    def __init__(self, mesg):
        emsg = "Solver for the MLE equations failed to converge: "
        emsg += mesg.replace('\n', '')
        self.args = (emsg,)


def _beta_mle_a(a, b, n, s1):
    # The zeros of this function give the MLE for `a`, with
    # `b`, `n` and `s1` given.  `s1` is the sum of the logs of
    # the data. `n` is the number of data points.
    psiab = sc.psi(a + b)
    func = s1 - n * (-psiab + sc.psi(a))
    return func


def _beta_mle_ab(theta, n, s1, s2):
    # Zeros of this function are critical points of
    # the maximum likelihood function.  Solving this system
    # for theta (which contains a and b) gives the MLE for a and b
    # given `n`, `s1` and `s2`.  `s1` is the sum of the logs of the data,
    # and `s2` is the sum of the logs of 1 - data.  `n` is the number
    # of data points.
    a, b = theta
    psiab = sc.psi(a + b)
    func = [s1 - n * (-psiab + sc.psi(a)),
            s2 - n * (-psiab + sc.psi(b))]
    return func


class beta_gen(rv_continuous):
    """A beta continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `beta` is::

                            gamma(a+b) * x**(a-1) * (1-x)**(b-1)
        beta.pdf(x, a, b) = ------------------------------------
                                     gamma(a)*gamma(b)

    for ``0 < x < 1``, ``a > 0``, ``b > 0``, where ``gamma(z)`` is the gamma
    function (`scipy.special.gamma`).

    `beta` takes ``a`` and ``b`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self, a, b):
        return self._random_state.beta(a, b, self._size)

    def _pdf(self, x, a, b):
        return np.exp(self._logpdf(x, a, b))

    def _logpdf(self, x, a, b):
        lPx = sc.xlog1py(b - 1.0, -x) + sc.xlogy(a - 1.0, x)
        lPx -= sc.betaln(a, b)
        return lPx

    def _cdf(self, x, a, b):
        return sc.btdtr(a, b, x)

    def _ppf(self, q, a, b):
        return sc.btdtri(a, b, q)

    def _stats(self, a, b):
        mn = a*1.0 / (a + b)
        var = (a*b*1.0)/(a+b+1.0)/(a+b)**2.0
        g1 = 2.0*(b-a)*np.sqrt((1.0+a+b)/(a*b)) / (2+a+b)
        g2 = 6.0*(a**3 + a**2*(1-2*b) + b**2*(1+b) - 2*a*b*(2+b))
        g2 /= a*b*(a+b+2)*(a+b+3)
        return mn, var, g1, g2

    def _fitstart(self, data):
        g1 = _skew(data)
        g2 = _kurtosis(data)

        def func(x):
            a, b = x
            sk = 2*(b-a)*np.sqrt(a + b + 1) / (a + b + 2) / np.sqrt(a*b)
            ku = a**3 - a**2*(2*b-1) + b**2*(b+1) - 2*a*b*(b+2)
            ku /= a*b*(a+b+2)*(a+b+3)
            ku *= 6
            return [sk-g1, ku-g2]
        a, b = optimize.fsolve(func, (1.0, 1.0))
        return super(beta_gen, self)._fitstart(data, args=(a, b))

    @inherit_docstring_from(rv_continuous)
    def fit(self, data, *args, **kwds):
        """%(super)s
        In the special case where both `floc` and `fscale` are given, a
        `ValueError` is raised if any value `x` in `data` does not satisfy
        `floc < x < floc + fscale`.
        """
        # Override rv_continuous.fit, so we can more efficiently handle the
        # case where floc and fscale are given.

        f0 = (kwds.get('f0', None) or kwds.get('fa', None) or
              kwds.get('fix_a', None))
        f1 = (kwds.get('f1', None) or kwds.get('fb', None) or
              kwds.get('fix_b', None))
        floc = kwds.get('floc', None)
        fscale = kwds.get('fscale', None)

        if floc is None or fscale is None:
            # do general fit
            return super(beta_gen, self).fit(data, *args, **kwds)

        if f0 is not None and f1 is not None:
            # This check is for consistency with `rv_continuous.fit`.
            raise ValueError("All parameters fixed. There is nothing to "
                             "optimize.")

        # Special case: loc and scale are constrained, so we are fitting
        # just the shape parameters.  This can be done much more efficiently
        # than the method used in `rv_continuous.fit`.  (See the subsection
        # "Two unknown parameters" in the section "Maximum likelihood" of
        # the Wikipedia article on the Beta distribution for the formulas.)

        # Normalize the data to the interval [0, 1].
        data = (np.ravel(data) - floc) / fscale
        if np.any(data <= 0) or np.any(data >= 1):
            raise FitDataError("beta", lower=floc, upper=floc + fscale)
        xbar = data.mean()

        if f0 is not None or f1 is not None:
            # One of the shape parameters is fixed.

            if f0 is not None:
                # The shape parameter a is fixed, so swap the parameters
                # and flip the data.  We always solve for `a`.  The result
                # will be swapped back before returning.
                b = f0
                data = 1 - data
                xbar = 1 - xbar
            else:
                b = f1

            # Initial guess for a.  Use the formula for the mean of the beta
            # distribution, E[x] = a / (a + b), to generate a reasonable
            # starting point based on the mean of the data and the given
            # value of b.
            a = b * xbar / (1 - xbar)

            # Compute the MLE for `a` by solving _beta_mle_a.
            theta, info, ier, mesg = optimize.fsolve(
                _beta_mle_a, a,
                args=(b, len(data), np.log(data).sum()),
                full_output=True
            )
            if ier != 1:
                raise FitSolverError(mesg=mesg)
            a = theta[0]

            if f0 is not None:
                # The shape parameter a was fixed, so swap back the
                # parameters.
                a, b = b, a

        else:
            # Neither of the shape parameters is fixed.

            # s1 and s2 are used in the extra arguments passed to _beta_mle_ab
            # by optimize.fsolve.
            s1 = np.log(data).sum()
            s2 = sc.log1p(-data).sum()

            # Use the "method of moments" to estimate the initial
            # guess for a and b.
            fac = xbar * (1 - xbar) / data.var(ddof=0) - 1
            a = xbar * fac
            b = (1 - xbar) * fac

            # Compute the MLE for a and b by solving _beta_mle_ab.
            theta, info, ier, mesg = optimize.fsolve(
                _beta_mle_ab, [a, b],
                args=(len(data), s1, s2),
                full_output=True
            )
            if ier != 1:
                raise FitSolverError(mesg=mesg)
            a, b = theta

        return a, b, floc, fscale

beta = beta_gen(a=0.0, b=1.0, name='beta')


class betaprime_gen(rv_continuous):
    """A beta prime continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `betaprime` is::

        betaprime.pdf(x, a, b) = x**(a-1) * (1+x)**(-a-b) / beta(a, b)

    for ``x > 0``, ``a > 0``, ``b > 0``, where ``beta(a, b)`` is the beta
    function (see `scipy.special.beta`).

    `betaprime` takes ``a`` and ``b`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _rvs(self, a, b):
        sz, rndm = self._size, self._random_state
        u1 = gamma.rvs(a, size=sz, random_state=rndm)
        u2 = gamma.rvs(b, size=sz, random_state=rndm)
        return u1 / u2

    def _pdf(self, x, a, b):
        return np.exp(self._logpdf(x, a, b))

    def _logpdf(self, x, a, b):
        return sc.xlogy(a - 1.0, x) - sc.xlog1py(a + b, x) - sc.betaln(a, b)

    def _cdf(self, x, a, b):
        return sc.betainc(a, b, x/(1.+x))

    def _munp(self, n, a, b):
        if n == 1.0:
            return np.where(b > 1,
                            a/(b-1.0),
                            np.inf)
        elif n == 2.0:
            return np.where(b > 2,
                            a*(a+1.0)/((b-2.0)*(b-1.0)),
                            np.inf)
        elif n == 3.0:
            return np.where(b > 3,
                            a*(a+1.0)*(a+2.0)/((b-3.0)*(b-2.0)*(b-1.0)),
                            np.inf)
        elif n == 4.0:
            return np.where(b > 4,
                            (a*(a + 1.0)*(a + 2.0)*(a + 3.0) /
                             ((b - 4.0)*(b - 3.0)*(b - 2.0)*(b - 1.0))),
                            np.inf)
        else:
            raise NotImplementedError
betaprime = betaprime_gen(a=0.0, name='betaprime')


class bradford_gen(rv_continuous):
    """A Bradford continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `bradford` is::

        bradford.pdf(x, c) = c / (k * (1+c*x)),

    for ``0 < x < 1``, ``c > 0`` and ``k = log(1+c)``.

    `bradford` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x, c):
        return c / (c*x + 1.0) / sc.log1p(c)

    def _cdf(self, x, c):
        return sc.log1p(c*x) / sc.log1p(c)

    def _ppf(self, q, c):
        return sc.expm1(q * sc.log1p(c)) / c

    def _stats(self, c, moments='mv'):
        k = np.log(1.0+c)
        mu = (c-k)/(c*k)
        mu2 = ((c+2.0)*k-2.0*c)/(2*c*k*k)
        g1 = None
        g2 = None
        if 's' in moments:
            g1 = np.sqrt(2)*(12*c*c-9*c*k*(c+2)+2*k*k*(c*(c+3)+3))
            g1 /= np.sqrt(c*(c*(k-2)+2*k))*(3*c*(k-2)+6*k)
        if 'k' in moments:
            g2 = (c**3*(k-3)*(k*(3*k-16)+24)+12*k*c*c*(k-4)*(k-3)
                  + 6*c*k*k*(3*k-14) + 12*k**3)
            g2 /= 3*c*(c*(k-2)+2*k)**2
        return mu, mu2, g1, g2

    def _entropy(self, c):
        k = np.log(1+c)
        return k/2.0 - np.log(c/k)
bradford = bradford_gen(a=0.0, b=1.0, name='bradford')


class burr_gen(rv_continuous):
    """A Burr (Type III) continuous random variable.

    %(before_notes)s

    See Also
    --------
    fisk : a special case of either `burr` or ``burr12`` with ``d = 1``
    burr12 : Burr Type XII distribution

    Notes
    -----
    The probability density function for `burr` is::

        burr.pdf(x, c, d) = c * d * x**(-c-1) * (1+x**(-c))**(-d-1)

    for ``x > 0``.

    `burr` takes ``c`` and ``d`` as shape parameters.

    This is the PDF corresponding to the third CDF given in Burr's list;
    specifically, it is equation (11) in Burr's paper [1]_.

    %(after_notes)s

    References
    ----------
    .. [1] Burr, I. W. "Cumulative frequency functions", Annals of
       Mathematical Statistics, 13(2), pp 215-232 (1942).

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _pdf(self, x, c, d):
        return c * d * (x**(-c - 1.0)) * ((1 + x**(-c))**(-d - 1.0))

    def _cdf(self, x, c, d):
        return (1 + x**(-c))**(-d)

    def _ppf(self, q, c, d):
        return (q**(-1.0/d) - 1)**(-1.0/c)

    def _munp(self, n, c, d):
        nc = 1. * n / c
        return d * sc.beta(1.0 - nc, d + nc)
burr = burr_gen(a=0.0, name='burr')


class burr12_gen(rv_continuous):
    """A Burr (Type XII) continuous random variable.

    %(before_notes)s

    See Also
    --------
    fisk : a special case of either `burr` or ``burr12`` with ``d = 1``
    burr : Burr Type III distribution

    Notes
    -----
    The probability density function for `burr` is::

        burr12.pdf(x, c, d) = c * d * x**(c-1) * (1+x**(c))**(-d-1)

    for ``x > 0``.

    `burr12` takes ``c`` and ``d`` as shape parameters.

    This is the PDF corresponding to the twelfth CDF given in Burr's list;
    specifically, it is equation (20) in Burr's paper [1]_.

    %(after_notes)s

    The Burr type 12 distribution is also sometimes referred to as
    the Singh-Maddala distribution from NIST [2]_.

    References
    ----------
    .. [1] Burr, I. W. "Cumulative frequency functions", Annals of
       Mathematical Statistics, 13(2), pp 215-232 (1942).

    .. [2] http://www.itl.nist.gov/div898/software/dataplot/refman2/auxillar/b12pdf.htm

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _pdf(self, x, c, d):
        return np.exp(self._logpdf(x, c, d))

    def _logpdf(self, x, c, d):
        return np.log(c) + np.log(d) + sc.xlogy(c - 1, x) + sc.xlog1py(-d-1, x**c)

    def _cdf(self, x, c, d):
        return -sc.expm1(self._logsf(x, c, d))

    def _logcdf(self, x, c, d):
        return sc.log1p(-(1 + x**c)**(-d))

    def _sf(self, x, c, d):
        return np.exp(self._logsf(x, c, d))

    def _logsf(self, x, c, d):
        return sc.xlog1py(-d, x**c)

    def _ppf(self, q, c, d):
        # The following is an implementation of
        #   ((1 - q)**(-1.0/d) - 1)**(1.0/c)
        # that does a better job handling small values of q.
        return sc.expm1(-1/d * sc.log1p(-q))**(1/c)

    def _munp(self, n, c, d):
        nc = 1. * n / c
        return d * sc.beta(1.0 + nc, d - nc)
burr12 = burr12_gen(a=0.0, name='burr12')


class fisk_gen(burr_gen):
    """A Fisk continuous random variable.

    The Fisk distribution is also known as the log-logistic distribution, and
    equals the Burr distribution with ``d == 1``.

    `fisk` takes ``c`` as a shape parameter.

    %(before_notes)s

    Notes
    -----
    The probability density function for `fisk` is::

        fisk.pdf(x, c) = c * x**(-c-1) * (1 + x**(-c))**(-2)

    for ``x > 0``.

    `fisk` takes ``c`` as a shape parameters.

    %(after_notes)s

    See Also
    --------
    burr

    %(example)s

    """
    def _pdf(self, x, c):
        return burr_gen._pdf(self, x, c, 1.0)

    def _cdf(self, x, c):
        return burr_gen._cdf(self, x, c, 1.0)

    def _ppf(self, x, c):
        return burr_gen._ppf(self, x, c, 1.0)

    def _munp(self, n, c):
        return burr_gen._munp(self, n, c, 1.0)

    def _entropy(self, c):
        return 2 - np.log(c)
fisk = fisk_gen(a=0.0, name='fisk')


# median = loc
class cauchy_gen(rv_continuous):
    """A Cauchy continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `cauchy` is::

        cauchy.pdf(x) = 1 / (pi * (1 + x**2))

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x):
        return 1.0/np.pi/(1.0+x*x)

    def _cdf(self, x):
        return 0.5 + 1.0/np.pi*np.arctan(x)

    def _ppf(self, q):
        return np.tan(np.pi*q-np.pi/2.0)

    def _sf(self, x):
        return 0.5 - 1.0/np.pi*np.arctan(x)

    def _isf(self, q):
        return np.tan(np.pi/2.0-np.pi*q)

    def _stats(self):
        return np.nan, np.nan, np.nan, np.nan

    def _entropy(self):
        return np.log(4*np.pi)

    def _fitstart(self, data, args=None):
        # Initialize ML guesses using quartiles instead of moments.
        p25, p50, p75 = np.percentile(data, [25, 50, 75])
        return p50, (p75 - p25)/2
cauchy = cauchy_gen(name='cauchy')


class chi_gen(rv_continuous):
    """A chi continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `chi` is::

        chi.pdf(x, df) = x**(df-1) * exp(-x**2/2) / (2**(df/2-1) * gamma(df/2))

    for ``x > 0``.

    Special cases of `chi` are:

        - ``chi(1, loc, scale)`` is equivalent to `halfnorm`
        - ``chi(2, 0, scale)`` is equivalent to `rayleigh`
        - ``chi(3, 0, scale)`` is equivalent to `maxwell`

    `chi` takes ``df`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """

    def _rvs(self, df):
        sz, rndm = self._size, self._random_state
        return np.sqrt(chi2.rvs(df, size=sz, random_state=rndm))

    def _pdf(self, x, df):
        return np.exp(self._logpdf(x, df))

    def _logpdf(self, x, df):
        l = np.log(2) - .5*np.log(2)*df - sc.gammaln(.5*df)
        return l + sc.xlogy(df - 1., x) - .5*x**2

    def _cdf(self, x, df):
        return sc.gammainc(.5*df, .5*x**2)

    def _ppf(self, q, df):
        return np.sqrt(2*sc.gammaincinv(.5*df, q))

    def _stats(self, df):
        mu = np.sqrt(2)*sc.gamma(df/2.0+0.5)/sc.gamma(df/2.0)
        mu2 = df - mu*mu
        g1 = (2*mu**3.0 + mu*(1-2*df))/np.asarray(np.power(mu2, 1.5))
        g2 = 2*df*(1.0-df)-6*mu**4 + 4*mu**2 * (2*df-1)
        g2 /= np.asarray(mu2**2.0)
        return mu, mu2, g1, g2
chi = chi_gen(a=0.0, name='chi')


## Chi-squared (gamma-distributed with loc=0 and scale=2 and shape=df/2)
class chi2_gen(rv_continuous):
    """A chi-squared continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `chi2` is::

        chi2.pdf(x, df) = 1 / (2*gamma(df/2)) * (x/2)**(df/2-1) * exp(-x/2)

    `chi2` takes ``df`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self, df):
        return self._random_state.chisquare(df, self._size)

    def _pdf(self, x, df):
        return np.exp(self._logpdf(x, df))

    def _logpdf(self, x, df):
        return sc.xlogy(df/2.-1, x) - x/2. - sc.gammaln(df/2.) - (np.log(2)*df)/2.

    def _cdf(self, x, df):
        return sc.chdtr(df, x)

    def _sf(self, x, df):
        return sc.chdtrc(df, x)

    def _isf(self, p, df):
        return sc.chdtri(df, p)

    def _ppf(self, p, df):
        return self._isf(1.0-p, df)

    def _stats(self, df):
        mu = df
        mu2 = 2*df
        g1 = 2*np.sqrt(2.0/df)
        g2 = 12.0/df
        return mu, mu2, g1, g2
chi2 = chi2_gen(a=0.0, name='chi2')


class cosine_gen(rv_continuous):
    """A cosine continuous random variable.

    %(before_notes)s

    Notes
    -----
    The cosine distribution is an approximation to the normal distribution.
    The probability density function for `cosine` is::

        cosine.pdf(x) = 1/(2*pi) * (1+cos(x))

    for ``-pi <= x <= pi``.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x):
        return 1.0/2/np.pi*(1+np.cos(x))

    def _cdf(self, x):
        return 1.0/2/np.pi*(np.pi + x + np.sin(x))

    def _stats(self):
        return 0.0, np.pi*np.pi/3.0-2.0, 0.0, -6.0*(np.pi**4-90)/(5.0*(np.pi*np.pi-6)**2)

    def _entropy(self):
        return np.log(4*np.pi)-1.0
cosine = cosine_gen(a=-np.pi, b=np.pi, name='cosine')


class dgamma_gen(rv_continuous):
    """A double gamma continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `dgamma` is::

        dgamma.pdf(x, a) = 1 / (2*gamma(a)) * abs(x)**(a-1) * exp(-abs(x))

    for ``a > 0``.

    `dgamma` takes ``a`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self, a):
        sz, rndm = self._size, self._random_state
        u = rndm.random_sample(size=sz)
        gm = gamma.rvs(a, size=sz, random_state=rndm)
        return gm * np.where(u >= 0.5, 1, -1)

    def _pdf(self, x, a):
        ax = abs(x)
        return 1.0/(2*sc.gamma(a))*ax**(a-1.0) * np.exp(-ax)

    def _logpdf(self, x, a):
        ax = abs(x)
        return sc.xlogy(a - 1.0, ax) - ax - np.log(2) - sc.gammaln(a)

    def _cdf(self, x, a):
        fac = 0.5*sc.gammainc(a, abs(x))
        return np.where(x > 0, 0.5 + fac, 0.5 - fac)

    def _sf(self, x, a):
        fac = 0.5*sc.gammainc(a, abs(x))
        return np.where(x > 0, 0.5-fac, 0.5+fac)

    def _ppf(self, q, a):
        fac = sc.gammainccinv(a, 1-abs(2*q-1))
        return np.where(q > 0.5, fac, -fac)

    def _stats(self, a):
        mu2 = a*(a+1.0)
        return 0.0, mu2, 0.0, (a+2.0)*(a+3.0)/mu2-3.0
dgamma = dgamma_gen(name='dgamma')


class dweibull_gen(rv_continuous):
    """A double Weibull continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `dweibull` is::

        dweibull.pdf(x, c) = c / 2 * abs(x)**(c-1) * exp(-abs(x)**c)

    `dweibull` takes ``d`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self, c):
        sz, rndm = self._size, self._random_state
        u = rndm.random_sample(size=sz)
        w = weibull_min.rvs(c, size=sz, random_state=rndm)
        return w * (np.where(u >= 0.5, 1, -1))

    def _pdf(self, x, c):
        ax = abs(x)
        Px = c / 2.0 * ax**(c-1.0) * np.exp(-ax**c)
        return Px

    def _logpdf(self, x, c):
        ax = abs(x)
        return np.log(c) - np.log(2.0) + sc.xlogy(c - 1.0, ax) - ax**c

    def _cdf(self, x, c):
        Cx1 = 0.5 * np.exp(-abs(x)**c)
        return np.where(x > 0, 1 - Cx1, Cx1)

    def _ppf(self, q, c):
        fac = 2. * np.where(q <= 0.5, q, 1. - q)
        fac = np.power(-np.log(fac), 1.0 / c)
        return np.where(q > 0.5, fac, -fac)

    def _munp(self, n, c):
        return (1 - (n % 2)) * sc.gamma(1.0 + 1.0 * n / c)

    # since we know that all odd moments are zeros, return them at once.
    # returning Nones from _stats makes the public stats call _munp
    # so overall we're saving one or two gamma function evaluations here.
    def _stats(self, c):
        return 0, None, 0, None
dweibull = dweibull_gen(name='dweibull')


## Exponential (gamma distributed with a=1.0, loc=loc and scale=scale)
class expon_gen(rv_continuous):
    """An exponential continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `expon` is::

        expon.pdf(x) = exp(-x)

    for ``x >= 0``.

    %(after_notes)s

    A common parameterization for `expon` is in terms of the rate parameter
    ``lambda``, such that ``pdf = lambda * exp(-lambda * x)``. This
    parameterization corresponds to using ``scale = 1 / lambda``.

    %(example)s

    """
    def _rvs(self):
        return self._random_state.standard_exponential(self._size)

    def _pdf(self, x):
        return np.exp(-x)

    def _logpdf(self, x):
        return -x

    def _cdf(self, x):
        return -sc.expm1(-x)

    def _ppf(self, q):
        return -sc.log1p(-q)

    def _sf(self, x):
        return np.exp(-x)

    def _logsf(self, x):
        return -x

    def _isf(self, q):
        return -np.log(q)

    def _stats(self):
        return 1.0, 1.0, 2.0, 6.0

    def _entropy(self):
        return 1.0
expon = expon_gen(a=0.0, name='expon')


## Exponentially Modified Normal (exponential distribution
##  convolved with a Normal).
## This is called an exponentially modified gaussian on wikipedia
class exponnorm_gen(rv_continuous):
    """An exponentially modified Normal continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `exponnorm` is::

        exponnorm.pdf(x, K) =
            1/(2*K) exp(1/(2 * K**2)) exp(-x / K) * erfc-(x - 1/K) / sqrt(2))

    where the shape parameter ``K > 0``.

    It can be thought of as the sum of a normally distributed random
    value with mean ``loc`` and sigma ``scale`` and an exponentially
    distributed random number with a pdf proportional to ``exp(-lambda * x)``
    where ``lambda = (K * scale)**(-1)``.

    %(after_notes)s

    An alternative parameterization of this distribution (for example, in
    `Wikipedia <http://en.wikipedia.org/wiki/Exponentially_modified_Gaussian_distribution>`_)
    involves three parameters, :math:`\\mu`, :math:`\\lambda` and :math:`\\sigma`.
    In the present parameterization this corresponds to having ``loc`` and
    ``scale`` equal to :math:`\\mu` and :math:`\\sigma`, respectively, and
    shape parameter :math:`K = 1/\\sigma\\lambda`.

    .. versionadded:: 0.16.0

    %(example)s

    """
    def _rvs(self, K):
        expval = self._random_state.standard_exponential(self._size) * K
        gval = self._random_state.standard_normal(self._size)
        return expval + gval

    def _pdf(self, x, K):
        invK = 1.0 / K
        exparg = 0.5 * invK**2 - invK * x
        # Avoid overflows; setting np.exp(exparg) to the max float works
        #  all right here
        expval = _lazywhere(exparg < _LOGXMAX, (exparg,), np.exp, _XMAX)
        return 0.5 * invK * expval * sc.erfc(-(x - invK) / np.sqrt(2))

    def _logpdf(self, x, K):
        invK = 1.0 / K
        exparg = 0.5 * invK**2 - invK * x
        return exparg + np.log(0.5 * invK * sc.erfc(-(x - invK) / np.sqrt(2)))

    def _cdf(self, x, K):
        invK = 1.0 / K
        expval = invK * (0.5 * invK - x)
        return _norm_cdf(x) - np.exp(expval) * _norm_cdf(x - invK)

    def _sf(self, x, K):
        invK = 1.0 / K
        expval = invK * (0.5 * invK - x)
        return _norm_cdf(-x) + np.exp(expval) * _norm_cdf(x - invK)

    def _stats(self, K):
        K2 = K * K
        opK2 = 1.0 + K2
        skw = 2 * K**3 * opK2**(-1.5)
        krt = 6.0 * K2 * K2 * opK2**(-2)
        return K, opK2, skw, krt
exponnorm = exponnorm_gen(name='exponnorm')


class exponweib_gen(rv_continuous):
    """An exponentiated Weibull continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `exponweib` is::

        exponweib.pdf(x, a, c) =
            a * c * (1-exp(-x**c))**(a-1) * exp(-x**c)*x**(c-1)

    for ``x > 0``, ``a > 0``, ``c > 0``.

    `exponweib` takes ``a`` and ``c`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x, a, c):
        return np.exp(self._logpdf(x, a, c))

    def _logpdf(self, x, a, c):
        negxc = -x**c
        exm1c = -sc.expm1(negxc)
        logp = (np.log(a) + np.log(c) + sc.xlogy(a - 1.0, exm1c) +
                negxc + sc.xlogy(c - 1.0, x))
        return logp

    def _cdf(self, x, a, c):
        exm1c = -sc.expm1(-x**c)
        return exm1c**a

    def _ppf(self, q, a, c):
        return (-sc.log1p(-q**(1.0/a)))**np.asarray(1.0/c)
exponweib = exponweib_gen(a=0.0, name='exponweib')


class exponpow_gen(rv_continuous):
    """An exponential power continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `exponpow` is::

        exponpow.pdf(x, b) = b * x**(b-1) * exp(1 + x**b - exp(x**b))

    for ``x >= 0``, ``b > 0``.  Note that this is a different distribution
    from the exponential power distribution that is also known under the names
    "generalized normal" or "generalized Gaussian".

    `exponpow` takes ``b`` as a shape parameter.

    %(after_notes)s

    References
    ----------
    http://www.math.wm.edu/~leemis/chart/UDR/PDFs/Exponentialpower.pdf

    %(example)s

    """
    def _pdf(self, x, b):
        return np.exp(self._logpdf(x, b))

    def _logpdf(self, x, b):
        xb = x**b
        f = 1 + np.log(b) + sc.xlogy(b - 1.0, x) + xb - np.exp(xb)
        return f

    def _cdf(self, x, b):
        return -sc.expm1(-sc.expm1(x**b))

    def _sf(self, x, b):
        return np.exp(-sc.expm1(x**b))

    def _isf(self, x, b):
        return (sc.log1p(-np.log(x)))**(1./b)

    def _ppf(self, q, b):
        return pow(sc.log1p(-sc.log1p(-q)), 1.0/b)
exponpow = exponpow_gen(a=0.0, name='exponpow')


class fatiguelife_gen(rv_continuous):
    """A fatigue-life (Birnbaum-Saunders) continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `fatiguelife` is::

        fatiguelife.pdf(x, c) =
            (x+1) / (2*c*sqrt(2*pi*x**3)) * exp(-(x-1)**2/(2*x*c**2))

    for ``x > 0``.

    `fatiguelife` takes ``c`` as a shape parameter.

    %(after_notes)s

    References
    ----------
    .. [1] "Birnbaum-Saunders distribution",
           http://en.wikipedia.org/wiki/Birnbaum-Saunders_distribution

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _rvs(self, c):
        z = self._random_state.standard_normal(self._size)
        x = 0.5*c*z
        x2 = x*x
        t = 1.0 + 2*x2 + 2*x*np.sqrt(1 + x2)
        return t

    def _pdf(self, x, c):
        return np.exp(self._logpdf(x, c))

    def _logpdf(self, x, c):
        return (np.log(x+1) - (x-1)**2 / (2.0*x*c**2) - np.log(2*c) -
                0.5*(np.log(2*np.pi) + 3*np.log(x)))

    def _cdf(self, x, c):
        return _norm_cdf(1.0 / c * (np.sqrt(x) - 1.0/np.sqrt(x)))

    def _ppf(self, q, c):
        tmp = c*sc.ndtri(q)
        return 0.25 * (tmp + np.sqrt(tmp**2 + 4))**2

    def _stats(self, c):
        # NB: the formula for kurtosis in wikipedia seems to have an error:
        # it's 40, not 41. At least it disagrees with the one from Wolfram
        # Alpha.  And the latter one, below, passes the tests, while the wiki
        # one doesn't So far I didn't have the guts to actually check the
        # coefficients from the expressions for the raw moments.
        c2 = c*c
        mu = c2 / 2.0 + 1.0
        den = 5.0 * c2 + 4.0
        mu2 = c2*den / 4.0
        g1 = 4 * c * (11*c2 + 6.0) / np.power(den, 1.5)
        g2 = 6 * c2 * (93*c2 + 40.0) / den**2.0
        return mu, mu2, g1, g2
fatiguelife = fatiguelife_gen(a=0.0, name='fatiguelife')


class foldcauchy_gen(rv_continuous):
    """A folded Cauchy continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `foldcauchy` is::

        foldcauchy.pdf(x, c) = 1/(pi*(1+(x-c)**2)) + 1/(pi*(1+(x+c)**2))

    for ``x >= 0``.

    `foldcauchy` takes ``c`` as a shape parameter.

    %(example)s

    """
    def _rvs(self, c):
        return abs(cauchy.rvs(loc=c, size=self._size,
                              random_state=self._random_state))

    def _pdf(self, x, c):
        return 1.0/np.pi*(1.0/(1+(x-c)**2) + 1.0/(1+(x+c)**2))

    def _cdf(self, x, c):
        return 1.0/np.pi*(np.arctan(x-c) + np.arctan(x+c))

    def _stats(self, c):
        return np.inf, np.inf, np.nan, np.nan
foldcauchy = foldcauchy_gen(a=0.0, name='foldcauchy')


class f_gen(rv_continuous):
    """An F continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `f` is::

                             df2**(df2/2) * df1**(df1/2) * x**(df1/2-1)
        F.pdf(x, df1, df2) = --------------------------------------------
                             (df2+df1*x)**((df1+df2)/2) * B(df1/2, df2/2)

    for ``x > 0``.

    `f` takes ``dfn`` and ``dfd`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self, dfn, dfd):
        return self._random_state.f(dfn, dfd, self._size)

    def _pdf(self, x, dfn, dfd):
        return np.exp(self._logpdf(x, dfn, dfd))

    def _logpdf(self, x, dfn, dfd):
        n = 1.0 * dfn
        m = 1.0 * dfd
        lPx = m/2 * np.log(m) + n/2 * np.log(n) + (n/2 - 1) * np.log(x)
        lPx -= ((n+m)/2) * np.log(m + n*x) + sc.betaln(n/2, m/2)
        return lPx

    def _cdf(self, x, dfn, dfd):
        return sc.fdtr(dfn, dfd, x)

    def _sf(self, x, dfn, dfd):
        return sc.fdtrc(dfn, dfd, x)

    def _ppf(self, q, dfn, dfd):
        return sc.fdtri(dfn, dfd, q)

    def _stats(self, dfn, dfd):
        v1, v2 = 1. * dfn, 1. * dfd
        v2_2, v2_4, v2_6, v2_8 = v2 - 2., v2 - 4., v2 - 6., v2 - 8.

        mu = _lazywhere(
            v2 > 2, (v2, v2_2),
            lambda v2, v2_2: v2 / v2_2,
            np.inf)

        mu2 = _lazywhere(
            v2 > 4, (v1, v2, v2_2, v2_4),
            lambda v1, v2, v2_2, v2_4:
            2 * v2 * v2 * (v1 + v2_2) / (v1 * v2_2**2 * v2_4),
            np.inf)

        g1 = _lazywhere(
            v2 > 6, (v1, v2_2, v2_4, v2_6),
            lambda v1, v2_2, v2_4, v2_6:
            (2 * v1 + v2_2) / v2_6 * np.sqrt(v2_4 / (v1 * (v1 + v2_2))),
            np.nan)
        g1 *= np.sqrt(8.)

        g2 = _lazywhere(
            v2 > 8, (g1, v2_6, v2_8),
            lambda g1, v2_6, v2_8: (8 + g1 * g1 * v2_6) / v2_8,
            np.nan)
        g2 *= 3. / 2.

        return mu, mu2, g1, g2
f = f_gen(a=0.0, name='f')


## Folded Normal
##   abs(Z) where (Z is normal with mu=L and std=S so that c=abs(L)/S)
##
##  note: regress docs have scale parameter correct, but first parameter
##    he gives is a shape parameter A = c * scale

##  Half-normal is folded normal with shape-parameter c=0.

class foldnorm_gen(rv_continuous):
    """A folded normal continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `foldnorm` is::

        foldnormal.pdf(x, c) = sqrt(2/pi) * cosh(c*x) * exp(-(x**2+c**2)/2)

    for ``c >= 0``.

    `foldnorm` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, c):
        return c >= 0

    def _rvs(self, c):
        return abs(self._random_state.standard_normal(self._size) + c)

    def _pdf(self, x, c):
        return _norm_pdf(x + c) + _norm_pdf(x-c)

    def _cdf(self, x, c):
        return _norm_cdf(x-c) + _norm_cdf(x+c) - 1.0

    def _stats(self, c):
        # Regina C. Elandt, Technometrics 3, 551 (1961)
        # http://www.jstor.org/stable/1266561
        #
        c2 = c*c
        expfac = np.exp(-0.5*c2) / np.sqrt(2.*np.pi)

        mu = 2.*expfac + c * sc.erf(c/np.sqrt(2))
        mu2 = c2 + 1 - mu*mu

        g1 = 2. * (mu*mu*mu - c2*mu - expfac)
        g1 /= np.power(mu2, 1.5)

        g2 = c2 * (c2 + 6.) + 3 + 8.*expfac*mu
        g2 += (2. * (c2 - 3.) - 3. * mu**2) * mu**2
        g2 = g2 / mu2**2.0 - 3.

        return mu, mu2, g1, g2
foldnorm = foldnorm_gen(a=0.0, name='foldnorm')


## Extreme Value Type II or Frechet
## (defined in Regress+ documentation as Extreme LB) as
##   a limiting value distribution.
##
class frechet_r_gen(rv_continuous):
    """A Frechet right (or Weibull minimum) continuous random variable.

    %(before_notes)s

    See Also
    --------
    weibull_min : The same distribution as `frechet_r`.
    frechet_l, weibull_max

    Notes
    -----
    The probability density function for `frechet_r` is::

        frechet_r.pdf(x, c) = c * x**(c-1) * exp(-x**c)

    for ``x > 0``, ``c > 0``.

    `frechet_r` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """

    def _pdf(self, x, c):
        return c*pow(x, c-1)*np.exp(-pow(x, c))

    def _logpdf(self, x, c):
        return np.log(c) + sc.xlogy(c - 1, x) - pow(x, c)

    def _cdf(self, x, c):
        return -sc.expm1(-pow(x, c))

    def _sf(self, x, c):
        return np.exp(-pow(x, c))

    def _logsf(self, x, c):
        return -pow(x, c)

    def _ppf(self, q, c):
        return pow(-sc.log1p(-q), 1.0/c)

    def _munp(self, n, c):
        return sc.gamma(1.0+n*1.0/c)

    def _entropy(self, c):
        return -_EULER / c - np.log(c) + _EULER + 1
frechet_r = frechet_r_gen(a=0.0, name='frechet_r')
weibull_min = frechet_r_gen(a=0.0, name='weibull_min')


class frechet_l_gen(rv_continuous):
    """A Frechet left (or Weibull maximum) continuous random variable.

    %(before_notes)s

    See Also
    --------
    weibull_max : The same distribution as `frechet_l`.
    frechet_r, weibull_min

    Notes
    -----
    The probability density function for `frechet_l` is::

        frechet_l.pdf(x, c) = c * (-x)**(c-1) * exp(-(-x)**c)

    for ``x < 0``, ``c > 0``.

    `frechet_l` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x, c):
        return c*pow(-x, c-1)*np.exp(-pow(-x, c))

    def _logpdf(self, x, c):
        return np.log(c) + sc.xlogy(c-1, -x) - pow(-x, c)

    def _cdf(self, x, c):
        return np.exp(-pow(-x, c))

    def _logcdf(self, x, c):
        return -pow(-x, c)

    def _sf(self, x, c):
        return -sc.expm1(-pow(-x, c))

    def _ppf(self, q, c):
        return -pow(-np.log(q), 1.0/c)

    def _munp(self, n, c):
        val = sc.gamma(1.0+n*1.0/c)
        if int(n) % 2:
            sgn = -1
        else:
            sgn = 1
        return sgn * val

    def _entropy(self, c):
        return -_EULER / c - np.log(c) + _EULER + 1
frechet_l = frechet_l_gen(b=0.0, name='frechet_l')
weibull_max = frechet_l_gen(b=0.0, name='weibull_max')


class genlogistic_gen(rv_continuous):
    """A generalized logistic continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `genlogistic` is::

        genlogistic.pdf(x, c) = c * exp(-x) / (1 + exp(-x))**(c+1)

    for ``x > 0``, ``c > 0``.

    `genlogistic` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x, c):
        return np.exp(self._logpdf(x, c))

    def _logpdf(self, x, c):
        return np.log(c) - x - (c+1.0)*sc.log1p(np.exp(-x))

    def _cdf(self, x, c):
        Cx = (1+np.exp(-x))**(-c)
        return Cx

    def _ppf(self, q, c):
        vals = -np.log(pow(q, -1.0/c)-1)
        return vals

    def _stats(self, c):
        mu = _EULER + sc.psi(c)
        mu2 = np.pi*np.pi/6.0 + sc.zeta(2, c)
        g1 = -2*sc.zeta(3, c) + 2*_ZETA3
        g1 /= np.power(mu2, 1.5)
        g2 = np.pi**4/15.0 + 6*sc.zeta(4, c)
        g2 /= mu2**2.0
        return mu, mu2, g1, g2
genlogistic = genlogistic_gen(name='genlogistic')


class genpareto_gen(rv_continuous):
    """A generalized Pareto continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `genpareto` is::

        genpareto.pdf(x, c) = (1 + c * x)**(-1 - 1/c)

    defined for ``x >= 0`` if ``c >=0``, and for
    ``0 <= x <= -1/c`` if ``c < 0``.

    `genpareto` takes ``c`` as a shape parameter.

    For ``c == 0``, `genpareto` reduces to the exponential
    distribution, `expon`::

        genpareto.pdf(x, c=0) = exp(-x)

    For ``c == -1``, `genpareto` is uniform on ``[0, 1]``::

        genpareto.cdf(x, c=-1) = x

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, c):
        c = np.asarray(c)
        self.b = _lazywhere(c < 0, (c,),
                            lambda c: -1. / c,
                            np.inf)
        return True

    def _pdf(self, x, c):
        return np.exp(self._logpdf(x, c))

    def _logpdf(self, x, c):
        return _lazywhere((x == x) & (c != 0), (x, c),
                          lambda x, c: -sc.xlog1py(c + 1., c*x) / c,
                          -x)

    def _cdf(self, x, c):
        return -sc.inv_boxcox1p(-x, -c)

    def _sf(self, x, c):
        return sc.inv_boxcox(-x, -c)

    def _logsf(self, x, c):
        return _lazywhere((x == x) & (c != 0), (x, c),
                          lambda x, c: -sc.log1p(c*x) / c,
                          -x)

    def _ppf(self, q, c):
        return -sc.boxcox1p(-q, -c)

    def _isf(self, q, c):
        return -sc.boxcox(q, -c)

    def _munp(self, n, c):
        def __munp(n, c):
            val = 0.0
            k = np.arange(0, n + 1)
            for ki, cnk in zip(k, sc.comb(n, k)):
                val = val + cnk * (-1) ** ki / (1.0 - c * ki)
            return np.where(c * n < 1, val * (-1.0 / c) ** n, np.inf)
        return _lazywhere(c != 0, (c,),
                          lambda c: __munp(n, c),
                          sc.gamma(n + 1))

    def _entropy(self, c):
        return 1. + c
genpareto = genpareto_gen(a=0.0, name='genpareto')


class genexpon_gen(rv_continuous):
    """A generalized exponential continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `genexpon` is::

        genexpon.pdf(x, a, b, c) = (a + b * (1 - exp(-c*x))) * \
                                   exp(-a*x - b*x + b/c * (1-exp(-c*x)))

    for ``x >= 0``, ``a, b, c > 0``.

    `genexpon` takes ``a``, ``b`` and ``c`` as shape parameters.

    %(after_notes)s

    References
    ----------
    H.K. Ryu, "An Extension of Marshall and Olkin's Bivariate Exponential
    Distribution", Journal of the American Statistical Association, 1993.

    N. Balakrishnan, "The Exponential Distribution: Theory, Methods and
    Applications", Asit P. Basu.

    %(example)s

    """
    def _pdf(self, x, a, b, c):
        return (a + b*(-sc.expm1(-c*x)))*np.exp((-a-b)*x +
                                                b*(-sc.expm1(-c*x))/c)

    def _cdf(self, x, a, b, c):
        return -sc.expm1((-a-b)*x + b*(-sc.expm1(-c*x))/c)

    def _logpdf(self, x, a, b, c):
        return np.log(a+b*(-sc.expm1(-c*x))) + (-a-b)*x+b*(-sc.expm1(-c*x))/c
genexpon = genexpon_gen(a=0.0, name='genexpon')


class genextreme_gen(rv_continuous):
    """A generalized extreme value continuous random variable.

    %(before_notes)s

    See Also
    --------
    gumbel_r

    Notes
    -----
    For ``c=0``, `genextreme` is equal to `gumbel_r`.
    The probability density function for `genextreme` is::

        genextreme.pdf(x, c) =
            exp(-exp(-x))*exp(-x),                    for c==0
            exp(-(1-c*x)**(1/c))*(1-c*x)**(1/c-1),    for x <= 1/c, c > 0

    Note that several sources and software packages use the opposite
    convention for the sign of the shape parameter ``c``.

    `genextreme` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, c):
        self.b = np.where(c > 0, 1.0 / np.maximum(c, _XMIN), np.inf)
        self.a = np.where(c < 0, 1.0 / np.minimum(c, -_XMIN), -np.inf)
        return np.where(abs(c) == np.inf, 0, 1)

    def _loglogcdf(self, x, c):
        return _lazywhere((x == x) & (c != 0), (x, c),
                          lambda x, c: sc.log1p(-c*x)/c, -x)

    def _pdf(self, x, c):
        return np.exp(self._logpdf(x, c))

    def _logpdf(self, x, c):
        cx = _lazywhere((x == x) & (c != 0), (x, c), lambda x, c: c*x, 0.0)
        logex2 = sc.log1p(-cx)
        logpex2 = self._loglogcdf(x, c)
        pex2 = np.exp(logpex2)
        # Handle special cases
        np.putmask(logpex2, (c == 0) & (x == -np.inf), 0.0)
        logpdf = np.where((cx == 1) | (cx == -np.inf),
                          -np.inf,
                          -pex2+logpex2-logex2)
        np.putmask(logpdf, (c == 1) & (x == 1), 0.0)
        return logpdf

    def _logcdf(self, x, c):
        return -np.exp(self._loglogcdf(x, c))

    def _cdf(self, x, c):
        return np.exp(self._logcdf(x, c))

    def _sf(self, x, c):
        return -sc.expm1(self._logcdf(x, c))

    def _ppf(self, q, c):
        x = -np.log(-np.log(q))
        return _lazywhere((x == x) & (c != 0), (x, c),
                          lambda x, c: -sc.expm1(-c * x) / c, x)

    def _isf(self, q, c):
        x = -np.log(-sc.log1p(-q))
        return _lazywhere((x == x) & (c != 0), (x, c),
                          lambda x, c: -sc.expm1(-c * x) / c, x)

    def _stats(self, c):
        g = lambda n: sc.gamma(n*c + 1)
        g1 = g(1)
        g2 = g(2)
        g3 = g(3)
        g4 = g(4)
        g2mg12 = np.where(abs(c) < 1e-7, (c*np.pi)**2.0/6.0, g2-g1**2.0)
        gam2k = np.where(abs(c) < 1e-7, np.pi**2.0/6.0,
                         sc.expm1(sc.gammaln(2.0*c+1.0)-2*sc.gammaln(c + 1.0))/c**2.0)
        eps = 1e-14
        gamk = np.where(abs(c) < eps, -_EULER, sc.expm1(sc.gammaln(c + 1))/c)

        m = np.where(c < -1.0, np.nan, -gamk)
        v = np.where(c < -0.5, np.nan, g1**2.0*gam2k)

        # skewness
        sk1 = np.where(c < -1./3, np.nan,
                       np.sign(c)*(-g3+(g2+2*g2mg12)*g1)/((g2mg12)**(3./2.)))
        sk = np.where(abs(c) <= eps**0.29, 12*np.sqrt(6)*_ZETA3/np.pi**3, sk1)

        # kurtosis
        ku1 = np.where(c < -1./4, np.nan,
                       (g4+(-4*g3+3*(g2+g2mg12)*g1)*g1)/((g2mg12)**2))
        ku = np.where(abs(c) <= (eps)**0.23, 12.0/5.0, ku1-3.0)
        return m, v, sk, ku

    def _fitstart(self, data):
        # This is better than the default shape of (1,).
        g = _skew(data)
        if g < 0:
            a = 0.5
        else:
            a = -0.5
        return super(genextreme_gen, self)._fitstart(data, args=(a,))

    def _munp(self, n, c):
        k = np.arange(0, n+1)
        vals = 1.0/c**n * np.sum(
            sc.comb(n, k) * (-1)**k * sc.gamma(c*k + 1),
            axis=0)
        return np.where(c*n > -1, vals, np.inf)

    def _entropy(self, c):
        return _EULER*(1 - c) + 1

genextreme = genextreme_gen(name='genextreme')


def _digammainv(y):
    # Inverse of the digamma function (real positive arguments only).
    # This function is used in the `fit` method of `gamma_gen`.
    # The function uses either optimize.fsolve or optimize.newton
    # to solve `sc.digamma(x) - y = 0`.  There is probably room for
    # improvement, but currently it works over a wide range of y:
    #    >>> y = 64*np.random.randn(1000000)
    #    >>> y.min(), y.max()
    #    (-311.43592651416662, 351.77388222276869)
    #    x = [_digammainv(t) for t in y]
    #    np.abs(sc.digamma(x) - y).max()
    #    1.1368683772161603e-13
    #
    _em = 0.5772156649015328606065120
    func = lambda x: sc.digamma(x) - y
    if y > -0.125:
        x0 = np.exp(y) + 0.5
        if y < 10:
            # Some experimentation shows that newton reliably converges
            # must faster than fsolve in this y range.  For larger y,
            # newton sometimes fails to converge.
            value = optimize.newton(func, x0, tol=1e-10)
            return value
    elif y > -3:
        x0 = np.exp(y/2.332) + 0.08661
    else:
        x0 = 1.0 / (-y - _em)

    value, info, ier, mesg = optimize.fsolve(func, x0, xtol=1e-11,
                                             full_output=True)
    if ier != 1:
        raise RuntimeError("_digammainv: fsolve failed, y = %r" % y)

    return value[0]


## Gamma (Use MATLAB and MATHEMATICA (b=theta=scale, a=alpha=shape) definition)

## gamma(a, loc, scale)  with a an integer is the Erlang distribution
## gamma(1, loc, scale)  is the Exponential distribution
## gamma(df/2, 0, 2) is the chi2 distribution with df degrees of freedom.

class gamma_gen(rv_continuous):
    """A gamma continuous random variable.

    %(before_notes)s

    See Also
    --------
    erlang, expon

    Notes
    -----
    The probability density function for `gamma` is::

        gamma.pdf(x, a) = x**(a-1) * exp(-x) / gamma(a)

    for ``x >= 0``, ``a > 0``. Here ``gamma(a)`` refers to the gamma function.

    `gamma` has a shape parameter `a` which needs to be set explicitly.

    When ``a`` is an integer, `gamma` reduces to the Erlang
    distribution, and when ``a=1`` to the exponential distribution.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self, a):
        return self._random_state.standard_gamma(a, self._size)

    def _pdf(self, x, a):
        return np.exp(self._logpdf(x, a))

    def _logpdf(self, x, a):
        return sc.xlogy(a-1.0, x) - x - sc.gammaln(a)

    def _cdf(self, x, a):
        return sc.gammainc(a, x)

    def _sf(self, x, a):
        return sc.gammaincc(a, x)

    def _ppf(self, q, a):
        return sc.gammaincinv(a, q)

    def _stats(self, a):
        return a, a, 2.0/np.sqrt(a), 6.0/a

    def _entropy(self, a):
        return sc.psi(a)*(1-a) + a + sc.gammaln(a)

    def _fitstart(self, data):
        # The skewness of the gamma distribution is `4 / np.sqrt(a)`.
        # We invert that to estimate the shape `a` using the skewness
        # of the data.  The formula is regularized with 1e-8 in the
        # denominator to allow for degenerate data where the skewness
        # is close to 0.
        a = 4 / (1e-8 + _skew(data)**2)
        return super(gamma_gen, self)._fitstart(data, args=(a,))

    @inherit_docstring_from(rv_continuous)
    def fit(self, data, *args, **kwds):
        f0 = (kwds.get('f0', None) or kwds.get('fa', None) or
              kwds.get('fix_a', None))
        floc = kwds.get('floc', None)
        fscale = kwds.get('fscale', None)

        if floc is None:
            # loc is not fixed.  Use the default fit method.
            return super(gamma_gen, self).fit(data, *args, **kwds)

        # Special case: loc is fixed.

        if f0 is not None and fscale is not None:
            # This check is for consistency with `rv_continuous.fit`.
            # Without this check, this function would just return the
            # parameters that were given.
            raise ValueError("All parameters fixed. There is nothing to "
                             "optimize.")

        # Fixed location is handled by shifting the data.
        data = np.asarray(data)
        if np.any(data <= floc):
            raise FitDataError("gamma", lower=floc, upper=np.inf)
        if floc != 0:
            # Don't do the subtraction in-place, because `data` might be a
            # view of the input array.
            data = data - floc
        xbar = data.mean()

        # Three cases to handle:
        # * shape and scale both free
        # * shape fixed, scale free
        # * shape free, scale fixed

        if fscale is None:
            # scale is free
            if f0 is not None:
                # shape is fixed
                a = f0
            else:
                # shape and scale are both free.
                # The MLE for the shape parameter `a` is the solution to:
                # np.log(a) - sc.digamma(a) - np.log(xbar) + np.log(data.mean) = 0
                s = np.log(xbar) - np.log(data).mean()
                func = lambda a: np.log(a) - sc.digamma(a) - s
                aest = (3-s + np.sqrt((s-3)**2 + 24*s)) / (12*s)
                xa = aest*(1-0.4)
                xb = aest*(1+0.4)
                a = optimize.brentq(func, xa, xb, disp=0)

            # The MLE for the scale parameter is just the data mean
            # divided by the shape parameter.
            scale = xbar / a
        else:
            # scale is fixed, shape is free
            # The MLE for the shape parameter `a` is the solution to:
            # sc.digamma(a) - np.log(data).mean() + np.log(fscale) = 0
            c = np.log(data).mean() - np.log(fscale)
            a = _digammainv(c)
            scale = fscale

        return a, floc, scale

gamma = gamma_gen(a=0.0, name='gamma')


class erlang_gen(gamma_gen):
    """An Erlang continuous random variable.

    %(before_notes)s

    See Also
    --------
    gamma

    Notes
    -----
    The Erlang distribution is a special case of the Gamma distribution, with
    the shape parameter `a` an integer.  Note that this restriction is not
    enforced by `erlang`. It will, however, generate a warning the first time
    a non-integer value is used for the shape parameter.

    Refer to `gamma` for examples.

    """

    def _argcheck(self, a):
        allint = np.all(np.floor(a) == a)
        allpos = np.all(a > 0)
        if not allint:
            # An Erlang distribution shouldn't really have a non-integer
            # shape parameter, so warn the user.
            warnings.warn(
                'The shape parameter of the erlang distribution '
                'has been given a non-integer value %r.' % (a,),
                RuntimeWarning)
        return allpos

    def _fitstart(self, data):
        # Override gamma_gen_fitstart so that an integer initial value is
        # used.  (Also regularize the division, to avoid issues when
        # _skew(data) is 0 or close to 0.)
        a = int(4.0 / (1e-8 + _skew(data)**2))
        return super(gamma_gen, self)._fitstart(data, args=(a,))

    # Trivial override of the fit method, so we can monkey-patch its
    # docstring.
    def fit(self, data, *args, **kwds):
        return super(erlang_gen, self).fit(data, *args, **kwds)

    if fit.__doc__ is not None:
        fit.__doc__ = (rv_continuous.fit.__doc__ +
            """
            Notes
            -----
            The Erlang distribution is generally defined to have integer values
            for the shape parameter.  This is not enforced by the `erlang` class.
            When fitting the distribution, it will generally return a non-integer
            value for the shape parameter.  By using the keyword argument
            `f0=<integer>`, the fit method can be constrained to fit the data to
            a specific integer shape parameter.
            """)
erlang = erlang_gen(a=0.0, name='erlang')


class gengamma_gen(rv_continuous):
    """A generalized gamma continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `gengamma` is::

        gengamma.pdf(x, a, c) = abs(c) * x**(c*a-1) * exp(-x**c) / gamma(a)

    for ``x >= 0``, ``a > 0``, and ``c != 0``.

    `gengamma` takes ``a`` and ``c`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, a, c):
        return (a > 0) & (c != 0)

    def _pdf(self, x, a, c):
        return np.exp(self._logpdf(x, a, c))

    def _logpdf(self, x, a, c):
        return np.log(abs(c)) + sc.xlogy(c*a - 1, x) - x**c - sc.gammaln(a)

    def _cdf(self, x, a, c):
        xc = x**c
        val1 = sc.gammainc(a, xc)
        val2 = sc.gammaincc(a, xc)
        return np.where(c > 0, val1, val2)

    def _sf(self, x, a, c):
        xc = x**c
        val1 = sc.gammainc(a, xc)
        val2 = sc.gammaincc(a, xc)
        return np.where(c > 0, val2, val1)

    def _ppf(self, q, a, c):
        val1 = sc.gammaincinv(a, q)
        val2 = sc.gammainccinv(a, q)
        return np.where(c > 0, val1, val2)**(1.0/c)

    def _isf(self, q, a, c):
        val1 = sc.gammaincinv(a, q)
        val2 = sc.gammainccinv(a, q)
        return np.where(c > 0, val2, val1)**(1.0/c)

    def _munp(self, n, a, c):
        # Pochhammer symbol: sc.pocha,n) = gamma(a+n)/gamma(a)
        return sc.poch(a, n*1.0/c)

    def _entropy(self, a, c):
        val = sc.psi(a)
        return a*(1-val) + 1.0/c*val + sc.gammaln(a) - np.log(abs(c))
gengamma = gengamma_gen(a=0.0, name='gengamma')


class genhalflogistic_gen(rv_continuous):
    """A generalized half-logistic continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `genhalflogistic` is::

        genhalflogistic.pdf(x, c) =
            2 * (1-c*x)**(1/c-1) / (1+(1-c*x)**(1/c))**2

    for ``0 <= x <= 1/c``, and ``c > 0``.

    `genhalflogistic` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, c):
        self.b = 1.0 / c
        return c > 0

    def _pdf(self, x, c):
        limit = 1.0/c
        tmp = np.asarray(1-c*x)
        tmp0 = tmp**(limit-1)
        tmp2 = tmp0*tmp
        return 2*tmp0 / (1+tmp2)**2

    def _cdf(self, x, c):
        limit = 1.0/c
        tmp = np.asarray(1-c*x)
        tmp2 = tmp**(limit)
        return (1.0-tmp2) / (1+tmp2)

    def _ppf(self, q, c):
        return 1.0/c*(1-((1.0-q)/(1.0+q))**c)

    def _entropy(self, c):
        return 2 - (2*c+1)*np.log(2)
genhalflogistic = genhalflogistic_gen(a=0.0, name='genhalflogistic')


class gompertz_gen(rv_continuous):
    """A Gompertz (or truncated Gumbel) continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `gompertz` is::

        gompertz.pdf(x, c) = c * exp(x) * exp(-c*(exp(x)-1))

    for ``x >= 0``, ``c > 0``.

    `gompertz` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x, c):
        return np.exp(self._logpdf(x, c))

    def _logpdf(self, x, c):
        return np.log(c) + x - c * sc.expm1(x)

    def _cdf(self, x, c):
        return -sc.expm1(-c * sc.expm1(x))

    def _ppf(self, q, c):
        return sc.log1p(-1.0 / c * sc.log1p(-q))

    def _entropy(self, c):
        return 1.0 - np.log(c) - np.exp(c)*sc.expn(1, c)
gompertz = gompertz_gen(a=0.0, name='gompertz')


class gumbel_r_gen(rv_continuous):
    """A right-skewed Gumbel continuous random variable.

    %(before_notes)s

    See Also
    --------
    gumbel_l, gompertz, genextreme

    Notes
    -----
    The probability density function for `gumbel_r` is::

        gumbel_r.pdf(x) = exp(-(x + exp(-x)))

    The Gumbel distribution is sometimes referred to as a type I Fisher-Tippett
    distribution.  It is also related to the extreme value distribution,
    log-Weibull and Gompertz distributions.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x):
        return np.exp(self._logpdf(x))

    def _logpdf(self, x):
        return -x - np.exp(-x)

    def _cdf(self, x):
        return np.exp(-np.exp(-x))

    def _logcdf(self, x):
        return -np.exp(-x)

    def _ppf(self, q):
        return -np.log(-np.log(q))

    def _stats(self):
        return _EULER, np.pi*np.pi/6.0, 12*np.sqrt(6)/np.pi**3 * _ZETA3, 12.0/5

    def _entropy(self):
        # http://en.wikipedia.org/wiki/Gumbel_distribution
        return _EULER + 1.
gumbel_r = gumbel_r_gen(name='gumbel_r')


class gumbel_l_gen(rv_continuous):
    """A left-skewed Gumbel continuous random variable.

    %(before_notes)s

    See Also
    --------
    gumbel_r, gompertz, genextreme

    Notes
    -----
    The probability density function for `gumbel_l` is::

        gumbel_l.pdf(x) = exp(x - exp(x))

    The Gumbel distribution is sometimes referred to as a type I Fisher-Tippett
    distribution.  It is also related to the extreme value distribution,
    log-Weibull and Gompertz distributions.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x):
        return np.exp(self._logpdf(x))

    def _logpdf(self, x):
        return x - np.exp(x)

    def _cdf(self, x):
        return -sc.expm1(-np.exp(x))

    def _ppf(self, q):
        return np.log(-sc.log1p(-q))

    def _logsf(self, x):
        return -np.exp(x)

    def _sf(self, x):
        return np.exp(-np.exp(x))

    def _isf(self, x):
        return np.log(-np.log(x))

    def _stats(self):
        return -_EULER, np.pi*np.pi/6.0, \
               -12*np.sqrt(6)/np.pi**3 * _ZETA3, 12.0/5

    def _entropy(self):
        return _EULER + 1.
gumbel_l = gumbel_l_gen(name='gumbel_l')


class halfcauchy_gen(rv_continuous):
    """A Half-Cauchy continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `halfcauchy` is::

        halfcauchy.pdf(x) = 2 / (pi * (1 + x**2))

    for ``x >= 0``.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x):
        return 2.0/np.pi/(1.0+x*x)

    def _logpdf(self, x):
        return np.log(2.0/np.pi) - sc.log1p(x*x)

    def _cdf(self, x):
        return 2.0/np.pi*np.arctan(x)

    def _ppf(self, q):
        return np.tan(np.pi/2*q)

    def _stats(self):
        return np.inf, np.inf, np.nan, np.nan

    def _entropy(self):
        return np.log(2*np.pi)
halfcauchy = halfcauchy_gen(a=0.0, name='halfcauchy')


class halflogistic_gen(rv_continuous):
    """A half-logistic continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `halflogistic` is::

        halflogistic.pdf(x) = 2 * exp(-x) / (1+exp(-x))**2 = 1/2 * sech(x/2)**2

    for ``x >= 0``.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x):
        return np.exp(self._logpdf(x))

    def _logpdf(self, x):
        return np.log(2) - x - 2. * sc.log1p(np.exp(-x))

    def _cdf(self, x):
        return np.tanh(x/2.0)

    def _ppf(self, q):
        return 2*np.arctanh(q)

    def _munp(self, n):
        if n == 1:
            return 2*np.log(2)
        if n == 2:
            return np.pi*np.pi/3.0
        if n == 3:
            return 9*_ZETA3
        if n == 4:
            return 7*np.pi**4 / 15.0
        return 2*(1-pow(2.0, 1-n))*sc.gamma(n+1)*sc.zeta(n, 1)

    def _entropy(self):
        return 2-np.log(2)
halflogistic = halflogistic_gen(a=0.0, name='halflogistic')


class halfnorm_gen(rv_continuous):
    """A half-normal continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `halfnorm` is::

        halfnorm.pdf(x) = sqrt(2/pi) * exp(-x**2/2)

    for ``x > 0``.

    `halfnorm` is a special case of `chi` with ``df == 1``.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self):
        return abs(self._random_state.standard_normal(size=self._size))

    def _pdf(self, x):
        return np.sqrt(2.0/np.pi)*np.exp(-x*x/2.0)

    def _logpdf(self, x):
        return 0.5 * np.log(2.0/np.pi) - x*x/2.0

    def _cdf(self, x):
        return _norm_cdf(x)*2-1.0

    def _ppf(self, q):
        return sc.ndtri((1+q)/2.0)

    def _stats(self):
        return (np.sqrt(2.0/np.pi), 1-2.0/np.pi, np.sqrt(2)*(4-np.pi)/(np.pi-2)**1.5,
                8*(np.pi-3)/(np.pi-2)**2)

    def _entropy(self):
        return 0.5*np.log(np.pi/2.0)+0.5
halfnorm = halfnorm_gen(a=0.0, name='halfnorm')


class hypsecant_gen(rv_continuous):
    """A hyperbolic secant continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `hypsecant` is::

        hypsecant.pdf(x) = 1/pi * sech(x)

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x):
        return 1.0/(np.pi*np.cosh(x))

    def _cdf(self, x):
        return 2.0/np.pi*np.arctan(np.exp(x))

    def _ppf(self, q):
        return np.log(np.tan(np.pi*q/2.0))

    def _stats(self):
        return 0, np.pi*np.pi/4, 0, 2

    def _entropy(self):
        return np.log(2*np.pi)
hypsecant = hypsecant_gen(name='hypsecant')


class gausshyper_gen(rv_continuous):
    """A Gauss hypergeometric continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `gausshyper` is::

        gausshyper.pdf(x, a, b, c, z) =
            C * x**(a-1) * (1-x)**(b-1) * (1+z*x)**(-c)

    for ``0 <= x <= 1``, ``a > 0``, ``b > 0``, and
    ``C = 1 / (B(a, b) F[2, 1](c, a; a+b; -z))``

    `gausshyper` takes ``a``, ``b``, ``c`` and ``z`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, a, b, c, z):
        return (a > 0) & (b > 0) & (c == c) & (z == z)

    def _pdf(self, x, a, b, c, z):
        Cinv = sc.gamma(a)*sc.gamma(b)/sc.gamma(a+b)*sc.hyp2f1(c, a, a+b, -z)
        return 1.0/Cinv * x**(a-1.0) * (1.0-x)**(b-1.0) / (1.0+z*x)**c

    def _munp(self, n, a, b, c, z):
        fac = sc.beta(n+a, b) / sc.beta(a, b)
        num = sc.hyp2f1(c, a+n, a+b+n, -z)
        den = sc.hyp2f1(c, a, a+b, -z)
        return fac*num / den
gausshyper = gausshyper_gen(a=0.0, b=1.0, name='gausshyper')


class invgamma_gen(rv_continuous):
    """An inverted gamma continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `invgamma` is::

        invgamma.pdf(x, a) = x**(-a-1) / gamma(a) * exp(-1/x)

    for x > 0, a > 0.

    `invgamma` takes ``a`` as a shape parameter.

    `invgamma` is a special case of `gengamma` with ``c == -1``.

    %(after_notes)s

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _pdf(self, x, a):
        return np.exp(self._logpdf(x, a))

    def _logpdf(self, x, a):
        return -(a+1) * np.log(x) - sc.gammaln(a) - 1.0/x

    def _cdf(self, x, a):
        return sc.gammaincc(a, 1.0 / x)

    def _ppf(self, q, a):
        return 1.0 / sc.gammainccinv(a, q)

    def _sf(self, x, a):
        return sc.gammainc(a, 1.0 / x)

    def _isf(self, q, a):
        return 1.0 / sc.gammaincinv(a, q)

    def _stats(self, a, moments='mvsk'):
        m1 = _lazywhere(a > 1, (a,), lambda x: 1. / (x - 1.), np.inf)
        m2 = _lazywhere(a > 2, (a,), lambda x: 1. / (x - 1.)**2 / (x - 2.),
                        np.inf)

        g1, g2 = None, None
        if 's' in moments:
            g1 = _lazywhere(
                a > 3, (a,),
                lambda x: 4. * np.sqrt(x - 2.) / (x - 3.), np.nan)
        if 'k' in moments:
            g2 = _lazywhere(
                a > 4, (a,),
                lambda x: 6. * (5. * x - 11.) / (x - 3.) / (x - 4.), np.nan)
        return m1, m2, g1, g2

    def _entropy(self, a):
        return a - (a+1.0) * sc.psi(a) + sc.gammaln(a)
invgamma = invgamma_gen(a=0.0, name='invgamma')


# scale is gamma from DATAPLOT and B from Regress
class invgauss_gen(rv_continuous):
    """An inverse Gaussian continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `invgauss` is::

        invgauss.pdf(x, mu) = 1 / sqrt(2*pi*x**3) * exp(-(x-mu)**2/(2*x*mu**2))

    for ``x > 0``.

    `invgauss` takes ``mu`` as a shape parameter.

    %(after_notes)s

    When `mu` is too small, evaluating the cumulative distribution function will be
    inaccurate due to ``cdf(mu -> 0) = inf * 0``.
    NaNs are returned for ``mu <= 0.0028``.

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _rvs(self, mu):
        return self._random_state.wald(mu, 1.0, size=self._size)

    def _pdf(self, x, mu):
        return 1.0/np.sqrt(2*np.pi*x**3.0)*np.exp(-1.0/(2*x)*((x-mu)/mu)**2)

    def _logpdf(self, x, mu):
        return -0.5*np.log(2*np.pi) - 1.5*np.log(x) - ((x-mu)/mu)**2/(2*x)

    def _cdf(self, x, mu):
        fac = np.sqrt(1.0/x)
        # Numerical accuracy for small `mu` is bad.  See #869.
        C1 = _norm_cdf(fac*(x-mu)/mu)
        C1 += np.exp(1.0/mu) * _norm_cdf(-fac*(x+mu)/mu) * np.exp(1.0/mu)
        return C1

    def _stats(self, mu):
        return mu, mu**3.0, 3*np.sqrt(mu), 15*mu
invgauss = invgauss_gen(a=0.0, name='invgauss')


class invweibull_gen(rv_continuous):
    """An inverted Weibull continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `invweibull` is::

        invweibull.pdf(x, c) = c * x**(-c-1) * exp(-x**(-c))

    for ``x > 0``, ``c > 0``.

    `invweibull` takes ``c`` as a shape parameter.

    %(after_notes)s

    References
    ----------
    F.R.S. de Gusmao, E.M.M Ortega and G.M. Cordeiro, "The generalized inverse
    Weibull distribution", Stat. Papers, vol. 52, pp. 591-619, 2011.

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _pdf(self, x, c):
        xc1 = np.power(x, -c - 1.0)
        xc2 = np.power(x, -c)
        xc2 = np.exp(-xc2)
        return c * xc1 * xc2

    def _cdf(self, x, c):
        xc1 = np.power(x, -c)
        return np.exp(-xc1)

    def _ppf(self, q, c):
        return np.power(-np.log(q), -1.0/c)

    def _munp(self, n, c):
        return sc.gamma(1 - n / c)

    def _entropy(self, c):
        return 1+_EULER + _EULER / c - np.log(c)
invweibull = invweibull_gen(a=0, name='invweibull')


class johnsonsb_gen(rv_continuous):
    """A Johnson SB continuous random variable.

    %(before_notes)s

    See Also
    --------
    johnsonsu

    Notes
    -----
    The probability density function for `johnsonsb` is::

        johnsonsb.pdf(x, a, b) = b / (x*(1-x)) * phi(a + b * log(x/(1-x)))

    for ``0 < x < 1`` and ``a, b > 0``, and ``phi`` is the normal pdf.

    `johnsonsb` takes ``a`` and ``b`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _argcheck(self, a, b):
        return (b > 0) & (a == a)

    def _pdf(self, x, a, b):
        trm = _norm_pdf(a + b*np.log(x/(1.0-x)))
        return b*1.0/(x*(1-x))*trm

    def _cdf(self, x, a, b):
        return _norm_cdf(a + b*np.log(x/(1.0-x)))

    def _ppf(self, q, a, b):
        return 1.0 / (1 + np.exp(-1.0 / b * (_norm_ppf(q) - a)))
johnsonsb = johnsonsb_gen(a=0.0, b=1.0, name='johnsonsb')


class johnsonsu_gen(rv_continuous):
    """A Johnson SU continuous random variable.

    %(before_notes)s

    See Also
    --------
    johnsonsb

    Notes
    -----
    The probability density function for `johnsonsu` is::

        johnsonsu.pdf(x, a, b) = b / sqrt(x**2 + 1) *
                                 phi(a + b * log(x + sqrt(x**2 + 1)))

    for all ``x, a, b > 0``, and `phi` is the normal pdf.

    `johnsonsu` takes ``a`` and ``b`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, a, b):
        return (b > 0) & (a == a)

    def _pdf(self, x, a, b):
        x2 = x*x
        trm = _norm_pdf(a + b * np.log(x + np.sqrt(x2+1)))
        return b*1.0/np.sqrt(x2+1.0)*trm

    def _cdf(self, x, a, b):
        return _norm_cdf(a + b * np.log(x + np.sqrt(x*x + 1)))

    def _ppf(self, q, a, b):
        return np.sinh((_norm_ppf(q) - a) / b)
johnsonsu = johnsonsu_gen(name='johnsonsu')


class laplace_gen(rv_continuous):
    """A Laplace continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `laplace` is::

        laplace.pdf(x) = 1/2 * exp(-abs(x))

    %(after_notes)s

    %(example)s

    """
    def _rvs(self):
        return self._random_state.laplace(0, 1, size=self._size)

    def _pdf(self, x):
        return 0.5*np.exp(-abs(x))

    def _cdf(self, x):
        return np.where(x > 0, 1.0-0.5*np.exp(-x), 0.5*np.exp(x))

    def _ppf(self, q):
        return np.where(q > 0.5, -np.log(2*(1-q)), np.log(2*q))

    def _stats(self):
        return 0, 2, 0, 3

    def _entropy(self):
        return np.log(2)+1
laplace = laplace_gen(name='laplace')


class levy_gen(rv_continuous):
    """A Levy continuous random variable.

    %(before_notes)s

    See Also
    --------
    levy_stable, levy_l

    Notes
    -----
    The probability density function for `levy` is::

        levy.pdf(x) = 1 / (x * sqrt(2*pi*x)) * exp(-1/(2*x))

    for ``x > 0``.

    This is the same as the Levy-stable distribution with a=1/2 and b=1.

    %(after_notes)s

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _pdf(self, x):
        return 1 / np.sqrt(2*np.pi*x) / x * np.exp(-1/(2*x))

    def _cdf(self, x):
        # Equivalent to 2*norm.sf(np.sqrt(1/x))
        return sc.erfc(np.sqrt(0.5 / x))

    def _ppf(self, q):
        # Equivalent to 1.0/(norm.isf(q/2)**2) or 0.5/(erfcinv(q)**2)
        val = -sc.ndtri(q/2)
        return 1.0 / (val * val)

    def _stats(self):
        return np.inf, np.inf, np.nan, np.nan
levy = levy_gen(a=0.0, name="levy")


class levy_l_gen(rv_continuous):
    """A left-skewed Levy continuous random variable.

    %(before_notes)s

    See Also
    --------
    levy, levy_stable

    Notes
    -----
    The probability density function for `levy_l` is::

        levy_l.pdf(x) = 1 / (abs(x) * sqrt(2*pi*abs(x))) * exp(-1/(2*abs(x)))

    for ``x < 0``.

    This is the same as the Levy-stable distribution with a=1/2 and b=-1.

    %(after_notes)s

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _pdf(self, x):
        ax = abs(x)
        return 1/np.sqrt(2*np.pi*ax)/ax*np.exp(-1/(2*ax))

    def _cdf(self, x):
        ax = abs(x)
        return 2 * _norm_cdf(1 / np.sqrt(ax)) - 1

    def _ppf(self, q):
        val = _norm_ppf((q + 1.0) / 2)
        return -1.0 / (val * val)

    def _stats(self):
        return np.inf, np.inf, np.nan, np.nan
levy_l = levy_l_gen(b=0.0, name="levy_l")


class levy_stable_gen(rv_continuous):
    """A Levy-stable continuous random variable.

    %(before_notes)s

    See Also
    --------
    levy, levy_l

    Notes
    -----
    Levy-stable distribution (only random variates available -- ignore other
    docs)

    %(after_notes)s

    %(example)s

    """

    def _rvs(self, alpha, beta):

        def alpha1func(alpha, beta, TH, aTH, bTH, cosTH, tanTH, W):
            return (2/np.pi*(np.pi/2 + bTH)*tanTH -
                    beta*np.log((np.pi/2*W*cosTH)/(np.pi/2 + bTH)))

        def beta0func(alpha, beta, TH, aTH, bTH, cosTH, tanTH, W):
            return (W/(cosTH/np.tan(aTH) + np.sin(TH)) *
                    ((np.cos(aTH) + np.sin(aTH)*tanTH)/W)**(1.0/alpha))

        def otherwise(alpha, beta, TH, aTH, bTH, cosTH, tanTH, W):
            # alpha is not 1 and beta is not 0
            val0 = beta*np.tan(np.pi*alpha/2)
            th0 = np.arctan(val0)/alpha
            val3 = W/(cosTH/np.tan(alpha*(th0 + TH)) + np.sin(TH))
            res3 = val3*((np.cos(aTH) + np.sin(aTH)*tanTH -
                          val0*(np.sin(aTH) - np.cos(aTH)*tanTH))/W)**(1.0/alpha)
            return res3

        def alphanot1func(alpha, beta, TH, aTH, bTH, cosTH, tanTH, W):
            res = _lazywhere(beta == 0,
                             (alpha, beta, TH, aTH, bTH, cosTH, tanTH, W),
                             beta0func, f2=otherwise)
            return res

        sz = self._size
        alpha = broadcast_to(alpha, sz)
        beta = broadcast_to(beta, sz)
        TH = uniform.rvs(loc=-np.pi/2.0, scale=np.pi, size=sz,
                         random_state=self._random_state)
        W = expon.rvs(size=sz, random_state=self._random_state)
        aTH = alpha*TH
        bTH = beta*TH
        cosTH = np.cos(TH)
        tanTH = np.tan(TH)
        res = _lazywhere(alpha == 1, (alpha, beta, TH, aTH, bTH, cosTH, tanTH, W),
                         alpha1func, f2=alphanot1func)
        return res

    def _argcheck(self, alpha, beta):
        return (alpha > 0) & (alpha <= 2) & (beta <= 1) & (beta >= -1)

    def _pdf(self, x, alpha, beta):
        raise NotImplementedError
levy_stable = levy_stable_gen(name='levy_stable')


class logistic_gen(rv_continuous):
    """A logistic (or Sech-squared) continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `logistic` is::

        logistic.pdf(x) = exp(-x) / (1+exp(-x))**2

    `logistic` is a special case of `genlogistic` with ``c == 1``.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self):
        return self._random_state.logistic(size=self._size)

    def _pdf(self, x):
        return np.exp(self._logpdf(x))

    def _logpdf(self, x):
        return -x - 2. * sc.log1p(np.exp(-x))

    def _cdf(self, x):
        return sc.expit(x)

    def _ppf(self, q):
        return sc.logit(q)

    def _sf(self, x):
        return sc.expit(-x)

    def _isf(self, q):
        return -sc.logit(q)

    def _stats(self):
        return 0, np.pi*np.pi/3.0, 0, 6.0/5.0

    def _entropy(self):
        # http://en.wikipedia.org/wiki/Logistic_distribution
        return 2.0
logistic = logistic_gen(name='logistic')


class loggamma_gen(rv_continuous):
    """A log gamma continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `loggamma` is::

        loggamma.pdf(x, c) = exp(c*x-exp(x)) / gamma(c)

    for all ``x, c > 0``.

    `loggamma` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self, c):
        return np.log(self._random_state.gamma(c, size=self._size))

    def _pdf(self, x, c):
        return np.exp(c*x-np.exp(x)-sc.gammaln(c))

    def _cdf(self, x, c):
        return sc.gammainc(c, np.exp(x))

    def _ppf(self, q, c):
        return np.log(sc.gammaincinv(c, q))

    def _stats(self, c):
        # See, for example, "A Statistical Study of Log-Gamma Distribution", by
        # Ping Shing Chan (thesis, McMaster University, 1993).
        mean = sc.digamma(c)
        var = sc.polygamma(1, c)
        skewness = sc.polygamma(2, c) / np.power(var, 1.5)
        excess_kurtosis = sc.polygamma(3, c) / (var*var)
        return mean, var, skewness, excess_kurtosis

loggamma = loggamma_gen(name='loggamma')


class loglaplace_gen(rv_continuous):
    """A log-Laplace continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `loglaplace` is::

        loglaplace.pdf(x, c) = c / 2 * x**(c-1),   for 0 < x < 1
                             = c / 2 * x**(-c-1),  for x >= 1

    for ``c > 0``.

    `loglaplace` takes ``c`` as a shape parameter.

    %(after_notes)s

    References
    ----------
    T.J. Kozubowski and K. Podgorski, "A log-Laplace growth rate model",
    The Mathematical Scientist, vol. 28, pp. 49-60, 2003.

    %(example)s

    """
    def _pdf(self, x, c):
        cd2 = c/2.0
        c = np.where(x < 1, c, -c)
        return cd2*x**(c-1)

    def _cdf(self, x, c):
        return np.where(x < 1, 0.5*x**c, 1-0.5*x**(-c))

    def _ppf(self, q, c):
        return np.where(q < 0.5, (2.0*q)**(1.0/c), (2*(1.0-q))**(-1.0/c))

    def _munp(self, n, c):
        return c**2 / (c**2 - n**2)

    def _entropy(self, c):
        return np.log(2.0/c) + 1.0
loglaplace = loglaplace_gen(a=0.0, name='loglaplace')


def _lognorm_logpdf(x, s):
    return _lazywhere(x != 0, (x, s),
                      lambda x, s: -np.log(x)**2 / (2*s**2) - np.log(s*x*np.sqrt(2*np.pi)),
                      -np.inf)


class lognorm_gen(rv_continuous):
    """A lognormal continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `lognorm` is::

        lognorm.pdf(x, s) = 1 / (s*x*sqrt(2*pi)) * exp(-1/2*(log(x)/s)**2)

    for ``x > 0``, ``s > 0``.

    `lognorm` takes ``s`` as a shape parameter.

    %(after_notes)s

    A common parametrization for a lognormal random variable ``Y`` is in
    terms of the mean, ``mu``, and standard deviation, ``sigma``, of the
    unique normally distributed random variable ``X`` such that exp(X) = Y.
    This parametrization corresponds to setting ``s = sigma`` and ``scale =
    exp(mu)``.

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _rvs(self, s):
        return np.exp(s * self._random_state.standard_normal(self._size))

    def _pdf(self, x, s):
        return np.exp(self._logpdf(x, s))

    def _logpdf(self, x, s):
        return _lognorm_logpdf(x, s)

    def _cdf(self, x, s):
        return _norm_cdf(np.log(x) / s)

    def _logcdf(self, x, s):
        return _norm_logcdf(np.log(x) / s)

    def _ppf(self, q, s):
        return np.exp(s * _norm_ppf(q))

    def _sf(self, x, s):
        return _norm_sf(np.log(x) / s)

    def _logsf(self, x, s):
        return _norm_logsf(np.log(x) / s)

    def _stats(self, s):
        p = np.exp(s*s)
        mu = np.sqrt(p)
        mu2 = p*(p-1)
        g1 = np.sqrt((p-1))*(2+p)
        g2 = np.polyval([1, 2, 3, 0, -6.0], p)
        return mu, mu2, g1, g2

    def _entropy(self, s):
        return 0.5 * (1 + np.log(2*np.pi) + 2 * np.log(s))
lognorm = lognorm_gen(a=0.0, name='lognorm')


class gilbrat_gen(rv_continuous):
    """A Gilbrat continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `gilbrat` is::

        gilbrat.pdf(x) = 1/(x*sqrt(2*pi)) * exp(-1/2*(log(x))**2)

    `gilbrat` is a special case of `lognorm` with ``s = 1``.

    %(after_notes)s

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _rvs(self):
        return np.exp(self._random_state.standard_normal(self._size))

    def _pdf(self, x):
        return np.exp(self._logpdf(x))

    def _logpdf(self, x):
        return _lognorm_logpdf(x, 1.0)

    def _cdf(self, x):
        return _norm_cdf(np.log(x))

    def _ppf(self, q):
        return np.exp(_norm_ppf(q))

    def _stats(self):
        p = np.e
        mu = np.sqrt(p)
        mu2 = p * (p - 1)
        g1 = np.sqrt((p - 1)) * (2 + p)
        g2 = np.polyval([1, 2, 3, 0, -6.0], p)
        return mu, mu2, g1, g2

    def _entropy(self):
        return 0.5 * np.log(2 * np.pi) + 0.5
gilbrat = gilbrat_gen(a=0.0, name='gilbrat')


class maxwell_gen(rv_continuous):
    """A Maxwell continuous random variable.

    %(before_notes)s

    Notes
    -----
    A special case of a `chi` distribution,  with ``df = 3``, ``loc = 0.0``,
    and given ``scale = a``, where ``a`` is the parameter used in the
    Mathworld description [1]_.

    The probability density function for `maxwell` is::

        maxwell.pdf(x) = sqrt(2/pi)x**2 * exp(-x**2/2)

    for ``x > 0``.

    %(after_notes)s

    References
    ----------
    .. [1] http://mathworld.wolfram.com/MaxwellDistribution.html

    %(example)s
    """
    def _rvs(self):
        return chi.rvs(3.0, size=self._size, random_state=self._random_state)

    def _pdf(self, x):
        return np.sqrt(2.0/np.pi)*x*x*np.exp(-x*x/2.0)

    def _cdf(self, x):
        return sc.gammainc(1.5, x*x/2.0)

    def _ppf(self, q):
        return np.sqrt(2*sc.gammaincinv(1.5, q))

    def _stats(self):
        val = 3*np.pi-8
        return (2*np.sqrt(2.0/np.pi),
                3-8/np.pi,
                np.sqrt(2)*(32-10*np.pi)/val**1.5,
                (-12*np.pi*np.pi + 160*np.pi - 384) / val**2.0)

    def _entropy(self):
        return _EULER + 0.5*np.log(2*np.pi)-0.5
maxwell = maxwell_gen(a=0.0, name='maxwell')


class mielke_gen(rv_continuous):
    """A Mielke's Beta-Kappa continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `mielke` is::

        mielke.pdf(x, k, s) = k * x**(k-1) / (1+x**s)**(1+k/s)

    for ``x > 0``.

    `mielke` takes ``k`` and ``s`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x, k, s):
        return k*x**(k-1.0) / (1.0+x**s)**(1.0+k*1.0/s)

    def _cdf(self, x, k, s):
        return x**k / (1.0+x**s)**(k*1.0/s)

    def _ppf(self, q, k, s):
        qsk = pow(q, s*1.0/k)
        return pow(qsk/(1.0-qsk), 1.0/s)
mielke = mielke_gen(a=0.0, name='mielke')


class kappa4_gen(rv_continuous):
    """Kappa 4 parameter distribution.

    %(before_notes)s

    Notes
    -----
    The probability density function for kappa4 is::

        kappa4.pdf(x, h, k) = (1.0 - k*x)**(1.0/k - 1.0)*
                              (1.0 - h*(1.0 - k*x)**(1.0/k))**(1.0/h-1)

    if ``h`` and ``k`` are not equal to 0.

    If ``h`` or ``k`` are zero then the pdf can be simplified:

    h = 0 and k != 0::

        kappa4.pdf(x, h, k) = (1.0 - k*x)**(1.0/k - 1.0)*
                              exp(-(1.0 - k*x)**(1.0/k))

    h != 0 and k = 0::

        kappa4.pdf(x, h, k) = exp(-x)*(1.0 - h*exp(-x))**(1.0/h - 1.0)

    h = 0 and k = 0::

        kappa4.pdf(x, h, k) = exp(-x)*exp(-exp(-x))

    kappa4 takes ``h`` and ``k`` as shape parameters.

    The kappa4 distribution returns other distributions when certain
    ``h`` and ``k`` values are used.

    +------+-------------+----------------+------------------+
    | h    | k=0.0       | k=1.0          | -inf<=k<=inf     |
    +======+=============+================+==================+
    | -1.0 | Logistic    |                | Generalized      |
    |      |             |                | Logistic(1)      |
    |      |             |                |                  |
    |      | logistic(x) |                |                  |
    +------+-------------+----------------+------------------+
    |  0.0 | Gumbel      | Reverse        | Generalized      |
    |      |             | Exponential(2) | Extreme Value    |
    |      |             |                |                  |
    |      | gumbel_r(x) |                | genextreme(x, k) |
    +------+-------------+----------------+------------------+
    |  1.0 | Exponential | Uniform        | Generalized      |
    |      |             |                | Pareto           |
    |      |             |                |                  |
    |      | expon(x)    | uniform(x)     | genpareto(x, -k) |
    +------+-------------+----------------+------------------+

    (1) There are at least five generalized logistic distributions.
        Four are described here:
        https://en.wikipedia.org/wiki/Generalized_logistic_distribution
        The "fifth" one is the one kappa4 should match which currently
        isn't implemented in scipy:
        https://en.wikipedia.org/wiki/Talk:Generalized_logistic_distribution
        http://www.mathwave.com/help/easyfit/html/analyses/distributions/gen_logistic.html
    (2) This distribution is currently not in scipy.

    References
    ----------
    J.C. Finney, "Optimization of a Skewed Logistic Distribution With Respect
    to the Kolmogorov-Smirnov Test", A Dissertation Submitted to the Graduate
    Faculty of the Louisiana State University and Agricultural and Mechanical
    College, (August, 2004),
    http://etd.lsu.edu/docs/available/etd-05182004-144851/unrestricted/Finney_dis.pdf

    J.R.M. Hosking, "The four-parameter kappa distribution". IBM J. Res.
    Develop. 38 (3), 25 1-258 (1994).

    B. Kumphon, A. Kaew-Man, P. Seenoi, "A Rainfall Distribution for the Lampao
    Site in the Chi River Basin, Thailand", Journal of Water Resource and
    Protection, vol. 4, 866-869, (2012).
    http://file.scirp.org/pdf/JWARP20121000009_14676002.pdf

    C. Winchester, "On Estimation of the Four-Parameter Kappa Distribution", A
    Thesis Submitted to Dalhousie University, Halifax, Nova Scotia, (March
    2000).
    http://www.nlc-bnc.ca/obj/s4/f2/dsk2/ftp01/MQ57336.pdf

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, h, k):
        condlist = [np.logical_and(h > 0, k > 0),
                    np.logical_and(h > 0, k == 0),
                    np.logical_and(h > 0, k < 0),
                    np.logical_and(h <= 0, k > 0),
                    np.logical_and(h <= 0, k == 0),
                    np.logical_and(h <= 0, k < 0)]

        def f0(h, k):
            return (1.0 - h**(-k))/k

        def f1(h, k):
            return np.log(h)

        def f3(h, k):
            a = np.empty(np.shape(h))
            a[:] = -np.inf
            return a

        def f5(h, k):
            return 1.0/k

        self.a = _lazyselect(condlist,
                             [f0, f1, f0, f3, f3, f5],
                             [h, k],
                             default=np.nan)

        def f0(h, k):
            return 1.0/k

        def f1(h, k):
            a = np.empty(np.shape(h))
            a[:] = np.inf
            return a

        self.b = _lazyselect(condlist,
                             [f0, f1, f1, f0, f1, f1],
                             [h, k],
                             default=np.nan)
        return h == h

    def _pdf(self, x, h, k):
        return np.exp(self._logpdf(x, h, k))

    def _logpdf(self, x, h, k):
        condlist = [np.logical_and(h != 0, k != 0),
                    np.logical_and(h == 0, k != 0),
                    np.logical_and(h != 0, k == 0),
                    np.logical_and(h == 0, k == 0)]

        def f0(x, h, k):
            '''pdf = (1.0 - k*x)**(1.0/k - 1.0)*(
                      1.0 - h*(1.0 - k*x)**(1.0/k))**(1.0/h-1.0)
               logpdf = ...
            '''
            return (sc.xlog1py(1.0/k - 1.0, -k*x) +
                    sc.xlog1py(1.0/h - 1.0, -h*(1.0 - k*x)**(1.0/k)))

        def f1(x, h, k):
            '''pdf = (1.0 - k*x)**(1.0/k - 1.0)*np.exp(-(
                      1.0 - k*x)**(1.0/k))
               logpdf = ...
            '''
            return sc.xlog1py(1.0/k - 1.0, -k*x) - (1.0 - k*x)**(1.0/k)

        def f2(x, h, k):
            '''pdf = np.exp(-x)*(1.0 - h*np.exp(-x))**(1.0/h - 1.0)
               logpdf = ...
            '''
            return -x + sc.xlog1py(1.0/h - 1.0, -h*np.exp(-x))

        def f3(x, h, k):
            '''pdf = np.exp(-x-np.exp(-x))
               logpdf = ...
            '''
            return -x - np.exp(-x)

        return _lazyselect(condlist,
                           [f0, f1, f2, f3],
                           [x, h, k],
                           default=np.nan)

    def _cdf(self, x, h, k):
        return np.exp(self._logcdf(x, h, k))

    def _logcdf(self, x, h, k):
        condlist = [np.logical_and(h != 0, k != 0),
                    np.logical_and(h == 0, k != 0),
                    np.logical_and(h != 0, k == 0),
                    np.logical_and(h == 0, k == 0)]

        def f0(x, h, k):
            '''cdf = (1.0 - h*(1.0 - k*x)**(1.0/k))**(1.0/h)
               logcdf = ...
            '''
            return (1.0/h)*sc.log1p(-h*(1.0 - k*x)**(1.0/k))

        def f1(x, h, k):
            '''cdf = np.exp(-(1.0 - k*x)**(1.0/k))
               logcdf = ...
            '''
            return -(1.0 - k*x)**(1.0/k)

        def f2(x, h, k):
            '''cdf = (1.0 - h*np.exp(-x))**(1.0/h)
               logcdf = ...
            '''
            return (1.0/h)*sc.log1p(-h*np.exp(-x))

        def f3(x, h, k):
            '''cdf = np.exp(-np.exp(-x))
               logcdf = ...
            '''
            return -np.exp(-x)

        return _lazyselect(condlist,
                           [f0, f1, f2, f3],
                           [x, h, k],
                           default=np.nan)

    def _ppf(self, q, h, k):
        condlist = [np.logical_and(h != 0, k != 0),
                    np.logical_and(h == 0, k != 0),
                    np.logical_and(h != 0, k == 0),
                    np.logical_and(h == 0, k == 0)]

        def f0(q, h, k):
            return 1.0/k*(1.0 - ((1.0 - (q**h))/h)**k)

        def f1(q, h, k):
            return 1.0/k*(1.0 - (-np.log(q))**k)

        def f2(q, h, k):
            '''ppf = -np.log((1.0 - (q**h))/h)
            '''
            return -sc.log1p(-(q**h)) + np.log(h)

        def f3(q, h, k):
            return -np.log(-np.log(q))

        return _lazyselect(condlist,
                           [f0, f1, f2, f3],
                           [q, h, k],
                           default=np.nan)

    def _stats(self, h, k):
        if h >= 0 and k >= 0:
            maxr = 5
        elif h < 0 and k >= 0:
            maxr = int(-1.0/h*k)
        elif k < 0:
            maxr = int(-1.0/k)
        else:
            maxr = 5

        outputs = [None if r < maxr else np.nan for r in range(1, 5)]
        return outputs[:]
kappa4 = kappa4_gen(name='kappa4')


class kappa3_gen(rv_continuous):
    """Kappa 3 parameter distribution.

    %(before_notes)s

    Notes
    -----
    The probability density function for `kappa` is::

        kappa3.pdf(x, a) =
            a*[a + x**a]**(-(a + 1)/a),     for ``x > 0``
            0.0,                            for ``x <= 0``

    `kappa3` takes ``a`` as a shape parameter and ``a > 0``.

    References
    ----------
    P.W. Mielke and E.S. Johnson, "Three-Parameter Kappa Distribution Maximum
    Likelihood and Likelihood Ratio Tests", Methods in Weather Research,
    701-707, (September, 1973),
    http://docs.lib.noaa.gov/rescue/mwr/101/mwr-101-09-0701.pdf

    B. Kumphon, "Maximum Entropy and Maximum Likelihood Estimation for the
    Three-Parameter Kappa Distribution", Open Journal of Statistics, vol 2,
    415-419 (2012)
    http://file.scirp.org/pdf/OJS20120400011_95789012.pdf

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, a):
        return a > 0

    def _pdf(self, x, a):
        return a*(a + x**a)**(-1.0/a-1)

    def _cdf(self, x, a):
        return x*(a + x**a)**(-1.0/a)

    def _ppf(self, q, a):
        return (a/(q**-a - 1.0))**(1.0/a)

    def _stats(self, a):
        outputs = [None if i < a else np.nan for i in range(1, 5)]
        return outputs[:]
kappa3 = kappa3_gen(a=0.0, name='kappa3')


class nakagami_gen(rv_continuous):
    """A Nakagami continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `nakagami` is::

        nakagami.pdf(x, nu) = 2 * nu**nu / gamma(nu) *
                              x**(2*nu-1) * exp(-nu*x**2)

    for ``x > 0``, ``nu > 0``.

    `nakagami` takes ``nu`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x, nu):
        return 2*nu**nu/sc.gamma(nu)*(x**(2*nu-1.0))*np.exp(-nu*x*x)

    def _cdf(self, x, nu):
        return sc.gammainc(nu, nu*x*x)

    def _ppf(self, q, nu):
        return np.sqrt(1.0/nu*sc.gammaincinv(nu, q))

    def _stats(self, nu):
        mu = sc.gamma(nu+0.5)/sc.gamma(nu)/np.sqrt(nu)
        mu2 = 1.0-mu*mu
        g1 = mu * (1 - 4*nu*mu2) / 2.0 / nu / np.power(mu2, 1.5)
        g2 = -6*mu**4*nu + (8*nu-2)*mu**2-2*nu + 1
        g2 /= nu*mu2**2.0
        return mu, mu2, g1, g2
nakagami = nakagami_gen(a=0.0, name="nakagami")


class ncx2_gen(rv_continuous):
    """A non-central chi-squared continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `ncx2` is::

        ncx2.pdf(x, df, nc) = exp(-(nc+x)/2) * 1/2 * (x/nc)**((df-2)/4)
                              * I[(df-2)/2](sqrt(nc*x))

    for ``x > 0``.

    `ncx2` takes ``df`` and ``nc`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self, df, nc):
        return self._random_state.noncentral_chisquare(df, nc, self._size)

    def _logpdf(self, x, df, nc):
        return _ncx2_log_pdf(x, df, nc)

    def _pdf(self, x, df, nc):
        return _ncx2_pdf(x, df, nc)

    def _cdf(self, x, df, nc):
        return _ncx2_cdf(x, df, nc)

    def _ppf(self, q, df, nc):
        return sc.chndtrix(q, df, nc)

    def _stats(self, df, nc):
        val = df + 2.0*nc
        return (df + nc,
                2*val,
                np.sqrt(8)*(val+nc)/val**1.5,
                12.0*(val+2*nc)/val**2.0)
ncx2 = ncx2_gen(a=0.0, name='ncx2')


class ncf_gen(rv_continuous):
    """A non-central F distribution continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `ncf` is::

        ncf.pdf(x, df1, df2, nc) = exp(nc/2 + nc*df1*x/(2*(df1*x+df2))) *
                    df1**(df1/2) * df2**(df2/2) * x**(df1/2-1) *
                    (df2+df1*x)**(-(df1+df2)/2) *
                    gamma(df1/2)*gamma(1+df2/2) *
                    L^{v1/2-1}^{v2/2}(-nc*v1*x/(2*(v1*x+v2))) /
                    (B(v1/2, v2/2) * gamma((v1+v2)/2))

    for ``df1, df2, nc > 0``.

    `ncf` takes ``df1``, ``df2`` and ``nc`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self, dfn, dfd, nc):
        return self._random_state.noncentral_f(dfn, dfd, nc, self._size)

    def _pdf_skip(self, x, dfn, dfd, nc):
        n1, n2 = dfn, dfd
        term = -nc/2+nc*n1*x/(2*(n2+n1*x)) + sc.gammaln(n1/2.)+sc.gammaln(1+n2/2.)
        term -= sc.gammaln((n1+n2)/2.0)
        Px = np.exp(term)
        Px *= n1**(n1/2) * n2**(n2/2) * x**(n1/2-1)
        Px *= (n2+n1*x)**(-(n1+n2)/2)
        Px *= sc.assoc_laguerre(-nc*n1*x/(2.0*(n2+n1*x)), n2/2, n1/2-1)
        Px /= sc.beta(n1/2, n2/2)
        # This function does not have a return.  Drop it for now, the generic
        # function seems to work OK.

    def _cdf(self, x, dfn, dfd, nc):
        return sc.ncfdtr(dfn, dfd, nc, x)

    def _ppf(self, q, dfn, dfd, nc):
        return sc.ncfdtri(dfn, dfd, nc, q)

    def _munp(self, n, dfn, dfd, nc):
        val = (dfn * 1.0/dfd)**n
        term = sc.gammaln(n+0.5*dfn) + sc.gammaln(0.5*dfd-n) - sc.gammaln(dfd*0.5)
        val *= np.exp(-nc / 2.0+term)
        val *= sc.hyp1f1(n+0.5*dfn, 0.5*dfn, 0.5*nc)
        return val

    def _stats(self, dfn, dfd, nc):
        mu = np.where(dfd <= 2, np.inf, dfd / (dfd-2.0)*(1+nc*1.0/dfn))
        mu2 = np.where(dfd <= 4, np.inf, 2*(dfd*1.0/dfn)**2.0 *
                       ((dfn+nc/2.0)**2.0 + (dfn+nc)*(dfd-2.0)) /
                       ((dfd-2.0)**2.0 * (dfd-4.0)))
        return mu, mu2, None, None
ncf = ncf_gen(a=0.0, name='ncf')


class t_gen(rv_continuous):
    """A Student's T continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `t` is::

                                       gamma((df+1)/2)
        t.pdf(x, df) = ---------------------------------------------------
                       sqrt(pi*df) * gamma(df/2) * (1+x**2/df)**((df+1)/2)

    for ``df > 0``.

    `t` takes ``df`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _rvs(self, df):
        return self._random_state.standard_t(df, size=self._size)

    def _pdf(self, x, df):
        r = np.asarray(df*1.0)
        Px = np.exp(sc.gammaln((r+1)/2)-sc.gammaln(r/2))
        Px /= np.sqrt(r*np.pi)*(1+(x**2)/r)**((r+1)/2)
        return Px

    def _logpdf(self, x, df):
        r = df*1.0
        lPx = sc.gammaln((r+1)/2)-sc.gammaln(r/2)
        lPx -= 0.5*np.log(r*np.pi) + (r+1)/2*np.log(1+(x**2)/r)
        return lPx

    def _cdf(self, x, df):
        return sc.stdtr(df, x)

    def _sf(self, x, df):
        return sc.stdtr(df, -x)

    def _ppf(self, q, df):
        return sc.stdtrit(df, q)

    def _isf(self, q, df):
        return -sc.stdtrit(df, q)

    def _stats(self, df):
        mu2 = _lazywhere(df > 2, (df,),
                         lambda df: df / (df-2.0),
                         np.inf)
        g1 = np.where(df > 3, 0.0, np.nan)
        g2 = _lazywhere(df > 4, (df,),
                        lambda df: 6.0 / (df-4.0),
                        np.nan)
        return 0, mu2, g1, g2
t = t_gen(name='t')


class nct_gen(rv_continuous):
    """A non-central Student's T continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `nct` is::

                                            df**(df/2) * gamma(df+1)
        nct.pdf(x, df, nc) = ----------------------------------------------------
                             2**df*exp(nc**2/2) * (df+x**2)**(df/2) * gamma(df/2)

    for ``df > 0``.

    `nct` takes ``df`` and ``nc`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, df, nc):
        return (df > 0) & (nc == nc)

    def _rvs(self, df, nc):
        sz, rndm = self._size, self._random_state
        n = norm.rvs(loc=nc, size=sz, random_state=rndm)
        c2 = chi2.rvs(df, size=sz, random_state=rndm)
        return n * np.sqrt(df) / np.sqrt(c2)

    def _pdf(self, x, df, nc):
        n = df*1.0
        nc = nc*1.0
        x2 = x*x
        ncx2 = nc*nc*x2
        fac1 = n + x2
        trm1 = n/2.*np.log(n) + sc.gammaln(n+1)
        trm1 -= n*np.log(2)+nc*nc/2.+(n/2.)*np.log(fac1)+sc.gammaln(n/2.)
        Px = np.exp(trm1)
        valF = ncx2 / (2*fac1)
        trm1 = np.sqrt(2)*nc*x*sc.hyp1f1(n/2+1, 1.5, valF)
        trm1 /= np.asarray(fac1*sc.gamma((n+1)/2))
        trm2 = sc.hyp1f1((n+1)/2, 0.5, valF)
        trm2 /= np.asarray(np.sqrt(fac1)*sc.gamma(n/2+1))
        Px *= trm1+trm2
        return Px

    def _cdf(self, x, df, nc):
        return sc.nctdtr(df, nc, x)

    def _ppf(self, q, df, nc):
        return sc.nctdtrit(df, nc, q)

    def _stats(self, df, nc, moments='mv'):
        #
        # See D. Hogben, R.S. Pinkham, and M.B. Wilk,
        # 'The moments of the non-central t-distribution'
        # Biometrika 48, p. 465 (2961).
        # e.g. http://www.jstor.org/stable/2332772 (gated)
        #
        mu, mu2, g1, g2 = None, None, None, None

        gfac = sc.gamma(df/2.-0.5) / sc.gamma(df/2.)
        c11 = np.sqrt(df/2.) * gfac
        c20 = df / (df-2.)
        c22 = c20 - c11*c11
        mu = np.where(df > 1, nc*c11, np.inf)
        mu2 = np.where(df > 2, c22*nc*nc + c20, np.inf)
        if 's' in moments:
            c33t = df * (7.-2.*df) / (df-2.) / (df-3.) + 2.*c11*c11
            c31t = 3.*df / (df-2.) / (df-3.)
            mu3 = (c33t*nc*nc + c31t) * c11*nc
            g1 = np.where(df > 3, mu3 / np.power(mu2, 1.5), np.nan)
        #kurtosis
        if 'k' in moments:
            c44 = df*df / (df-2.) / (df-4.)
            c44 -= c11*c11 * 2.*df*(5.-df) / (df-2.) / (df-3.)
            c44 -= 3.*c11**4
            c42 = df / (df-4.) - c11*c11 * (df-1.) / (df-3.)
            c42 *= 6.*df / (df-2.)
            c40 = 3.*df*df / (df-2.) / (df-4.)

            mu4 = c44 * nc**4 + c42*nc**2 + c40
            g2 = np.where(df > 4, mu4/mu2**2 - 3., np.nan)
        return mu, mu2, g1, g2
nct = nct_gen(name="nct")


class pareto_gen(rv_continuous):
    """A Pareto continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `pareto` is::

        pareto.pdf(x, b) = b / x**(b+1)

    for ``x >= 1``, ``b > 0``.

    `pareto` takes ``b`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x, b):
        return b * x**(-b-1)

    def _cdf(self, x, b):
        return 1 - x**(-b)

    def _ppf(self, q, b):
        return pow(1-q, -1.0/b)

    def _stats(self, b, moments='mv'):
        mu, mu2, g1, g2 = None, None, None, None
        if 'm' in moments:
            mask = b > 1
            bt = np.extract(mask, b)
            mu = valarray(np.shape(b), value=np.inf)
            np.place(mu, mask, bt / (bt-1.0))
        if 'v' in moments:
            mask = b > 2
            bt = np.extract(mask, b)
            mu2 = valarray(np.shape(b), value=np.inf)
            np.place(mu2, mask, bt / (bt-2.0) / (bt-1.0)**2)
        if 's' in moments:
            mask = b > 3
            bt = np.extract(mask, b)
            g1 = valarray(np.shape(b), value=np.nan)
            vals = 2 * (bt + 1.0) * np.sqrt(bt - 2.0) / ((bt - 3.0) * np.sqrt(bt))
            np.place(g1, mask, vals)
        if 'k' in moments:
            mask = b > 4
            bt = np.extract(mask, b)
            g2 = valarray(np.shape(b), value=np.nan)
            vals = (6.0*np.polyval([1.0, 1.0, -6, -2], bt) /
                    np.polyval([1.0, -7.0, 12.0, 0.0], bt))
            np.place(g2, mask, vals)
        return mu, mu2, g1, g2

    def _entropy(self, c):
        return 1 + 1.0/c - np.log(c)
pareto = pareto_gen(a=1.0, name="pareto")


class lomax_gen(rv_continuous):
    """A Lomax (Pareto of the second kind) continuous random variable.

    %(before_notes)s

    Notes
    -----
    The Lomax distribution is a special case of the Pareto distribution, with
    (loc=-1.0).

    The probability density function for `lomax` is::

        lomax.pdf(x, c) = c / (1+x)**(c+1)

    for ``x >= 0``, ``c > 0``.

    `lomax` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x, c):
        return c*1.0/(1.0+x)**(c+1.0)

    def _logpdf(self, x, c):
        return np.log(c) - (c+1)*sc.log1p(x)

    def _cdf(self, x, c):
        return -sc.expm1(-c*sc.log1p(x))

    def _sf(self, x, c):
        return np.exp(-c*sc.log1p(x))

    def _logsf(self, x, c):
        return -c*sc.log1p(x)

    def _ppf(self, q, c):
        return sc.expm1(-sc.log1p(-q)/c)

    def _stats(self, c):
        mu, mu2, g1, g2 = pareto.stats(c, loc=-1.0, moments='mvsk')
        return mu, mu2, g1, g2

    def _entropy(self, c):
        return 1+1.0/c-np.log(c)
lomax = lomax_gen(a=0.0, name="lomax")


class pearson3_gen(rv_continuous):
    """A pearson type III continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `pearson3` is::

        pearson3.pdf(x, skew) = abs(beta) / gamma(alpha) *
            (beta * (x - zeta))**(alpha - 1) * exp(-beta*(x - zeta))

    where::

            beta = 2 / (skew * stddev)
            alpha = (stddev * beta)**2
            zeta = loc - alpha / beta

    `pearson3` takes ``skew`` as a shape parameter.

    %(after_notes)s

    %(example)s

    References
    ----------
    R.W. Vogel and D.E. McMartin, "Probability Plot Goodness-of-Fit and
    Skewness Estimation Procedures for the Pearson Type 3 Distribution", Water
    Resources Research, Vol.27, 3149-3158 (1991).

    L.R. Salvosa, "Tables of Pearson's Type III Function", Ann. Math. Statist.,
    Vol.1, 191-198 (1930).

    "Using Modern Computing Tools to Fit the Pearson Type III Distribution to
    Aviation Loads Data", Office of Aviation Research (2003).

    """
    def _preprocess(self, x, skew):
        # The real 'loc' and 'scale' are handled in the calling pdf(...). The
        # local variables 'loc' and 'scale' within pearson3._pdf are set to
        # the defaults just to keep them as part of the equations for
        # documentation.
        loc = 0.0
        scale = 1.0

        # If skew is small, return _norm_pdf. The divide between pearson3
        # and norm was found by brute force and is approximately a skew of
        # 0.000016.  No one, I hope, would actually use a skew value even
        # close to this small.
        norm2pearson_transition = 0.000016

        ans, x, skew = np.broadcast_arrays([1.0], x, skew)
        ans = ans.copy()

        # mask is True where skew is small enough to use the normal approx.
        mask = np.absolute(skew) < norm2pearson_transition
        invmask = ~mask

        beta = 2.0 / (skew[invmask] * scale)
        alpha = (scale * beta)**2
        zeta = loc - alpha / beta

        transx = beta * (x[invmask] - zeta)
        return ans, x, transx, mask, invmask, beta, alpha, zeta

    def _argcheck(self, skew):
        # The _argcheck function in rv_continuous only allows positive
        # arguments.  The skew argument for pearson3 can be zero (which I want
        # to handle inside pearson3._pdf) or negative.  So just return True
        # for all skew args.
        return np.ones(np.shape(skew), dtype=bool)

    def _stats(self, skew):
        _, _, _, _, _, beta, alpha, zeta = (
            self._preprocess([1], skew))
        m = zeta + alpha / beta
        v = alpha / (beta**2)
        s = 2.0 / (alpha**0.5) * np.sign(beta)
        k = 6.0 / alpha
        return m, v, s, k

    def _pdf(self, x, skew):
        # Do the calculation in _logpdf since helps to limit
        # overflow/underflow problems
        ans = np.exp(self._logpdf(x, skew))
        if ans.ndim == 0:
            if np.isnan(ans):
                return 0.0
            return ans
        ans[np.isnan(ans)] = 0.0
        return ans

    def _logpdf(self, x, skew):
        #   PEARSON3 logpdf                           GAMMA logpdf
        #   np.log(abs(beta))
        # + (alpha - 1)*np.log(beta*(x - zeta))          + (a - 1)*np.log(x)
        # - beta*(x - zeta)                           - x
        # - sc.gammalnalpha)                              - sc.gammalna)
        ans, x, transx, mask, invmask, beta, alpha, _ = (
            self._preprocess(x, skew))

        ans[mask] = np.log(_norm_pdf(x[mask]))
        ans[invmask] = np.log(abs(beta)) + gamma._logpdf(transx, alpha)
        return ans

    def _cdf(self, x, skew):
        ans, x, transx, mask, invmask, _, alpha, _ = (
            self._preprocess(x, skew))

        ans[mask] = _norm_cdf(x[mask])
        ans[invmask] = gamma._cdf(transx, alpha)
        return ans

    def _rvs(self, skew):
        skew = broadcast_to(skew, self._size)
        ans, _, _, mask, invmask, beta, alpha, zeta = (
            self._preprocess([0], skew))

        nsmall = mask.sum()
        nbig = mask.size - nsmall
        ans[mask] = self._random_state.standard_normal(nsmall)
        ans[invmask] = (self._random_state.standard_gamma(alpha, nbig)/beta +
                        zeta)

        if self._size == ():
            ans = ans[0]
        return ans

    def _ppf(self, q, skew):
        ans, q, _, mask, invmask, beta, alpha, zeta = (
            self._preprocess(q, skew))
        ans[mask] = _norm_ppf(q[mask])
        ans[invmask] = sc.gammaincinv(alpha, q[invmask])/beta + zeta
        return ans
pearson3 = pearson3_gen(name="pearson3")


class powerlaw_gen(rv_continuous):
    """A power-function continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `powerlaw` is::

        powerlaw.pdf(x, a) = a * x**(a-1)

    for ``0 <= x <= 1``, ``a > 0``.

    `powerlaw` takes ``a`` as a shape parameter.

    %(after_notes)s

    `powerlaw` is a special case of `beta` with ``b == 1``.

    %(example)s

    """
    def _pdf(self, x, a):
        return a*x**(a-1.0)

    def _logpdf(self, x, a):
        return np.log(a) + sc.xlogy(a - 1, x)

    def _cdf(self, x, a):
        return x**(a*1.0)

    def _logcdf(self, x, a):
        return a*np.log(x)

    def _ppf(self, q, a):
        return pow(q, 1.0/a)

    def _stats(self, a):
        return (a / (a + 1.0),
                a / (a + 2.0) / (a + 1.0) ** 2,
                -2.0 * ((a - 1.0) / (a + 3.0)) * np.sqrt((a + 2.0) / a),
                6 * np.polyval([1, -1, -6, 2], a) / (a * (a + 3.0) * (a + 4)))

    def _entropy(self, a):
        return 1 - 1.0/a - np.log(a)
powerlaw = powerlaw_gen(a=0.0, b=1.0, name="powerlaw")


class powerlognorm_gen(rv_continuous):
    """A power log-normal continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `powerlognorm` is::

        powerlognorm.pdf(x, c, s) = c / (x*s) * phi(log(x)/s) *
                                                (Phi(-log(x)/s))**(c-1),

    where ``phi`` is the normal pdf, and ``Phi`` is the normal cdf,
    and ``x > 0``, ``s, c > 0``.

    `powerlognorm` takes ``c`` and ``s`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _pdf(self, x, c, s):
        return (c/(x*s) * _norm_pdf(np.log(x)/s) *
                pow(_norm_cdf(-np.log(x)/s), c*1.0-1.0))

    def _cdf(self, x, c, s):
        return 1.0 - pow(_norm_cdf(-np.log(x)/s), c*1.0)

    def _ppf(self, q, c, s):
        return np.exp(-s * _norm_ppf(pow(1.0 - q, 1.0 / c)))
powerlognorm = powerlognorm_gen(a=0.0, name="powerlognorm")


class powernorm_gen(rv_continuous):
    """A power normal continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `powernorm` is::

        powernorm.pdf(x, c) = c * phi(x) * (Phi(-x))**(c-1)

    where ``phi`` is the normal pdf, and ``Phi`` is the normal cdf,
    and ``x > 0``, ``c > 0``.

    `powernorm` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x, c):
        return c*_norm_pdf(x) * (_norm_cdf(-x)**(c-1.0))

    def _logpdf(self, x, c):
        return np.log(c) + _norm_logpdf(x) + (c-1)*_norm_logcdf(-x)

    def _cdf(self, x, c):
        return 1.0-_norm_cdf(-x)**(c*1.0)

    def _ppf(self, q, c):
        return -_norm_ppf(pow(1.0 - q, 1.0 / c))
powernorm = powernorm_gen(name='powernorm')


class rdist_gen(rv_continuous):
    """An R-distributed continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `rdist` is::

        rdist.pdf(x, c) = (1-x**2)**(c/2-1) / B(1/2, c/2)

    for ``-1 <= x <= 1``, ``c > 0``.

    `rdist` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x, c):
        return np.power((1.0 - x**2), c / 2.0 - 1) / sc.beta(0.5, c / 2.0)

    def _cdf(self, x, c):
        term1 = x / sc.beta(0.5, c / 2.0)
        res = 0.5 + term1 * sc.hyp2f1(0.5, 1 - c / 2.0, 1.5, x**2)
        # There's an issue with hyp2f1, it returns nans near x = +-1, c > 100.
        # Use the generic implementation in that case.  See gh-1285 for
        # background.
        if np.any(np.isnan(res)):
            return rv_continuous._cdf(self, x, c)
        return res

    def _munp(self, n, c):
        numerator = (1 - (n % 2)) * sc.beta((n + 1.0) / 2, c / 2.0)
        return numerator / sc.beta(1. / 2, c / 2.)
rdist = rdist_gen(a=-1.0, b=1.0, name="rdist")


class rayleigh_gen(rv_continuous):
    """A Rayleigh continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `rayleigh` is::

        rayleigh.pdf(r) = r * exp(-r**2/2)

    for ``x >= 0``.

    `rayleigh` is a special case of `chi` with ``df == 2``.

    %(after_notes)s

    %(example)s

    """
    _support_mask = rv_continuous._open_support_mask

    def _rvs(self):
        return chi.rvs(2, size=self._size, random_state=self._random_state)

    def _pdf(self, r):
        return np.exp(self._logpdf(r))

    def _logpdf(self, r):
        return np.log(r) - 0.5 * r * r

    def _cdf(self, r):
        return -sc.expm1(-0.5 * r**2)

    def _ppf(self, q):
        return np.sqrt(-2 * sc.log1p(-q))

    def _sf(self, r):
        return np.exp(self._logsf(r))

    def _logsf(self, r):
        return -0.5 * r * r

    def _isf(self, q):
        return np.sqrt(-2 * np.log(q))

    def _stats(self):
        val = 4 - np.pi
        return (np.sqrt(np.pi/2),
                val/2,
                2*(np.pi-3)*np.sqrt(np.pi)/val**1.5,
                6*np.pi/val-16/val**2)

    def _entropy(self):
        return _EULER/2.0 + 1 - 0.5*np.log(2)
rayleigh = rayleigh_gen(a=0.0, name="rayleigh")


class reciprocal_gen(rv_continuous):
    """A reciprocal continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `reciprocal` is::

        reciprocal.pdf(x, a, b) = 1 / (x*log(b/a))

    for ``a <= x <= b``, ``a, b > 0``.

    `reciprocal` takes ``a`` and ``b`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, a, b):
        self.a = a
        self.b = b
        self.d = np.log(b*1.0 / a)
        return (a > 0) & (b > 0) & (b > a)

    def _pdf(self, x, a, b):
        return 1.0 / (x * self.d)

    def _logpdf(self, x, a, b):
        return -np.log(x) - np.log(self.d)

    def _cdf(self, x, a, b):
        return (np.log(x)-np.log(a)) / self.d

    def _ppf(self, q, a, b):
        return a*pow(b*1.0/a, q)

    def _munp(self, n, a, b):
        return 1.0/self.d / n * (pow(b*1.0, n) - pow(a*1.0, n))

    def _entropy(self, a, b):
        return 0.5*np.log(a*b)+np.log(np.log(b/a))
reciprocal = reciprocal_gen(name="reciprocal")


class rice_gen(rv_continuous):
    """A Rice continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `rice` is::

        rice.pdf(x, b) = x * exp(-(x**2+b**2)/2) * I[0](x*b)

    for ``x > 0``, ``b > 0``.

    `rice` takes ``b`` as a shape parameter.

    %(after_notes)s

    The Rice distribution describes the length, ``r``, of a 2-D vector
    with components ``(U+u, V+v)``, where ``U, V`` are constant, ``u, v``
    are independent Gaussian random variables with standard deviation
    ``s``.  Let ``R = (U**2 + V**2)**0.5``. Then the pdf of ``r`` is
    ``rice.pdf(x, R/s, scale=s)``.

    %(example)s

    """
    def _argcheck(self, b):
        return b >= 0

    def _rvs(self, b):
        # http://en.wikipedia.org/wiki/Rice_distribution
        t = b/np.sqrt(2) + self._random_state.standard_normal(size=(2,) +
                                                              self._size)
        return np.sqrt((t*t).sum(axis=0))

    def _cdf(self, x, b):
        return sc.chndtr(np.square(x), 2, np.square(b))

    def _ppf(self, q, b):
        return np.sqrt(sc.chndtrix(q, 2, np.square(b)))

    def _pdf(self, x, b):
        # We use (x**2 + b**2)/2 = ((x-b)**2)/2 + xb.
        # The factor of np.exp(-xb) is then included in the i0e function
        # in place of the modified Bessel function, i0, improving
        # numerical stability for large values of xb.
        return x * np.exp(-(x-b)*(x-b)/2.0) * sc.i0e(x*b)

    def _munp(self, n, b):
        nd2 = n/2.0
        n1 = 1 + nd2
        b2 = b*b/2.0
        return (2.0**(nd2) * np.exp(-b2) * sc.gamma(n1) *
                sc.hyp1f1(n1, 1, b2))
rice = rice_gen(a=0.0, name="rice")


# FIXME: PPF does not work.
class recipinvgauss_gen(rv_continuous):
    """A reciprocal inverse Gaussian continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `recipinvgauss` is::

        recipinvgauss.pdf(x, mu) = 1/sqrt(2*pi*x) * exp(-(1-mu*x)**2/(2*x*mu**2))

    for ``x >= 0``.

    `recipinvgauss` takes ``mu`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """

    def _pdf(self, x, mu):
        return 1.0/np.sqrt(2*np.pi*x)*np.exp(-(1-mu*x)**2.0 / (2*x*mu**2.0))

    def _logpdf(self, x, mu):
        return -(1-mu*x)**2.0 / (2*x*mu**2.0) - 0.5*np.log(2*np.pi*x)

    def _cdf(self, x, mu):
        trm1 = 1.0/mu - x
        trm2 = 1.0/mu + x
        isqx = 1.0/np.sqrt(x)
        return 1.0-_norm_cdf(isqx*trm1)-np.exp(2.0/mu)*_norm_cdf(-isqx*trm2)

    def _rvs(self, mu):
        return 1.0/self._random_state.wald(mu, 1.0, size=self._size)
recipinvgauss = recipinvgauss_gen(a=0.0, name='recipinvgauss')


class semicircular_gen(rv_continuous):
    """A semicircular continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `semicircular` is::

        semicircular.pdf(x) = 2/pi * sqrt(1-x**2)

    for ``-1 <= x <= 1``.

    %(after_notes)s

    %(example)s

    """
    def _pdf(self, x):
        return 2.0/np.pi*np.sqrt(1-x*x)

    def _cdf(self, x):
        return 0.5+1.0/np.pi*(x*np.sqrt(1-x*x) + np.arcsin(x))

    def _stats(self):
        return 0, 0.25, 0, -1.0

    def _entropy(self):
        return 0.64472988584940017414
semicircular = semicircular_gen(a=-1.0, b=1.0, name="semicircular")


class skew_norm_gen(rv_continuous):
    """A skew-normal random variable.

    %(before_notes)s

    Notes
    -----
    The pdf is::

        skewnorm.pdf(x, a) = 2*norm.pdf(x)*norm.cdf(ax)

    `skewnorm` takes ``a`` as a skewness parameter
    When a=0 the distribution is identical to a normal distribution.
    rvs implements the method of [1]_.

    %(after_notes)s

    %(example)s


    References
    ----------

    .. [1] A. Azzalini and A. Capitanio (1999). Statistical applications of the
        multivariate skew-normal distribution. J. Roy. Statist. Soc., B 61, 579-602.
        http://azzalini.stat.unipd.it/SN/faq-r.html
    """

    def _argcheck(self, a):
        return np.isfinite(a)

    def _pdf(self, x, a):
        return 2.*_norm_pdf(x)*_norm_cdf(a*x)

    def _rvs(self, a):
        u0 = self._random_state.normal(size=self._size)
        v = self._random_state.normal(size=self._size)
        d = a/np.sqrt(1 + a**2)
        u1 = d*u0 + v*np.sqrt(1 - d**2)
        return np.where(u0 >= 0, u1, -u1)

    def _stats(self, a, moments='mvsk'):
        output = [None, None, None, None]
        const = np.sqrt(2/np.pi) * a/np.sqrt(1 + a**2)

        if 'm' in moments:
            output[0] = const
        if 'v' in moments:
            output[1] = 1 - const**2
        if 's' in moments:
            output[2] = ((4 - np.pi)/2) * (const/np.sqrt(1 - const**2))**3
        if 'k' in moments:
            output[3] = (2*(np.pi - 3)) * (const**4/(1 - const**2)**2)

        return output

skewnorm = skew_norm_gen(name='skewnorm')


class trapz_gen(rv_continuous):
    """A trapezoidal continuous random variable.

    %(before_notes)s

    Notes
    -----
    The trapezoidal distribution can be represented with an up-sloping line
    from ``loc`` to ``(loc + c*scale)``, then constant to ``(loc + d*scale)``
    and then downsloping from ``(loc + d*scale)`` to ``(loc+scale)``.

    `trapz` takes ``c`` and ``d`` as shape parameters.

    %(after_notes)s

    The standard form is in the range [0, 1] with c the mode.
    The location parameter shifts the start to `loc`.
    The scale parameter changes the width from 1 to `scale`.

    %(example)s

    """
    def _argcheck(self, c, d):
        return (c >= 0) & (c <= 1) & (d >= 0) & (d <= 1) & (d >= c)

    def _pdf(self, x, c, d):
        u = 2 / (d - c + 1)

        condlist = [x < c, x <= d, x > d]
        choicelist = [u * x / c, u, u * (1 - x) / (1 - d)]
        return np.select(condlist, choicelist)

    def _cdf(self, x, c, d):
        condlist = [x < c, x <= d, x > d]
        choicelist = [x**2 / c / (d - c + 1),
                      (c + 2 * (x - c)) / (d - c + 1),
                      1 - ((1 - x)**2 / (d - c + 1) / (1 - d))]
        return np.select(condlist, choicelist)

    def _ppf(self, q, c, d):
        qc, qd = self._cdf(c, c, d), self._cdf(d, c, d)
        condlist = [q < qc, q <= qd, q > qd]
        choicelist = [np.sqrt(q * c * (1 + d - c)),
                      0.5 * q * (1 + d - c) + 0.5 * c,
                      1 - np.sqrt((1 - q) * (d - c + 1) * (1 - d))]
        return np.select(condlist, choicelist)
trapz = trapz_gen(a=0.0, b=1.0, name="trapz")


class triang_gen(rv_continuous):
    """A triangular continuous random variable.

    %(before_notes)s

    Notes
    -----
    The triangular distribution can be represented with an up-sloping line from
    ``loc`` to ``(loc + c*scale)`` and then downsloping for ``(loc + c*scale)``
    to ``(loc+scale)``.

    `triang` takes ``c`` as a shape parameter.

    %(after_notes)s

    The standard form is in the range [0, 1] with c the mode.
    The location parameter shifts the start to `loc`.
    The scale parameter changes the width from 1 to `scale`.

    %(example)s

    """
    def _rvs(self, c):
        return self._random_state.triangular(0, c, 1, self._size)

    def _argcheck(self, c):
        return (c >= 0) & (c <= 1)

    def _pdf(self, x, c):
        return np.where(x < c, 2*x/c, 2*(1-x)/(1-c))

    def _cdf(self, x, c):
        return np.where(x < c, x*x/c, (x*x-2*x+c)/(c-1))

    def _ppf(self, q, c):
        return np.where(q < c, np.sqrt(c*q), 1-np.sqrt((1-c)*(1-q)))

    def _stats(self, c):
        return ((c+1.0)/3.0,
                (1.0-c+c*c)/18,
                np.sqrt(2)*(2*c-1)*(c+1)*(c-2) / (5*np.power((1.0-c+c*c), 1.5)),
                -3.0/5.0)

    def _entropy(self, c):
        return 0.5-np.log(2)
triang = triang_gen(a=0.0, b=1.0, name="triang")


class truncexpon_gen(rv_continuous):
    """A truncated exponential continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `truncexpon` is::

        truncexpon.pdf(x, b) = exp(-x) / (1-exp(-b))

    for ``0 < x < b``.

    `truncexpon` takes ``b`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, b):
        self.b = b
        return b > 0

    def _pdf(self, x, b):
        return np.exp(-x)/(-sc.expm1(-b))

    def _logpdf(self, x, b):
        return -x - np.log(-sc.expm1(-b))

    def _cdf(self, x, b):
        return sc.expm1(-x)/sc.expm1(-b)

    def _ppf(self, q, b):
        return -sc.log1p(q*sc.expm1(-b))

    def _munp(self, n, b):
        # wrong answer with formula, same as in continuous.pdf
        # return sc.gamman+1)-sc.gammainc1+n, b)
        if n == 1:
            return (1-(b+1)*np.exp(-b))/(-sc.expm1(-b))
        elif n == 2:
            return 2*(1-0.5*(b*b+2*b+2)*np.exp(-b))/(-sc.expm1(-b))
        else:
            # return generic for higher moments
            # return rv_continuous._mom1_sc(self, n, b)
            return self._mom1_sc(n, b)

    def _entropy(self, b):
        eB = np.exp(b)
        return np.log(eB-1)+(1+eB*(b-1.0))/(1.0-eB)
truncexpon = truncexpon_gen(a=0.0, name='truncexpon')


class truncnorm_gen(rv_continuous):
    """A truncated normal continuous random variable.

    %(before_notes)s

    Notes
    -----
    The standard form of this distribution is a standard normal truncated to
    the range [a, b] --- notice that a and b are defined over the domain of the
    standard normal.  To convert clip values for a specific mean and standard
    deviation, use::

        a, b = (myclip_a - my_mean) / my_std, (myclip_b - my_mean) / my_std

    `truncnorm` takes ``a`` and ``b`` as shape parameters.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, a, b):
        self.a = a
        self.b = b
        self._nb = _norm_cdf(b)
        self._na = _norm_cdf(a)
        self._sb = _norm_sf(b)
        self._sa = _norm_sf(a)
        self._delta = np.where(self.a > 0,
                               -(self._sb - self._sa),
                               self._nb - self._na)
        self._logdelta = np.log(self._delta)
        return a != b

    def _pdf(self, x, a, b):
        return _norm_pdf(x) / self._delta

    def _logpdf(self, x, a, b):
        return _norm_logpdf(x) - self._logdelta

    def _cdf(self, x, a, b):
        return (_norm_cdf(x) - self._na) / self._delta

    def _ppf(self, q, a, b):
        # XXX Use _lazywhere...
        ppf = np.where(self.a > 0,
                       _norm_isf(q*self._sb + self._sa*(1.0-q)),
                       _norm_ppf(q*self._nb + self._na*(1.0-q)))
        return ppf

    def _stats(self, a, b):
        nA, nB = self._na, self._nb
        d = nB - nA
        pA, pB = _norm_pdf(a), _norm_pdf(b)
        mu = (pA - pB) / d   # correction sign
        mu2 = 1 + (a*pA - b*pB) / d - mu*mu
        return mu, mu2, None, None
truncnorm = truncnorm_gen(name='truncnorm')


# FIXME: RVS does not work.
class tukeylambda_gen(rv_continuous):
    """A Tukey-Lamdba continuous random variable.

    %(before_notes)s

    Notes
    -----
    A flexible distribution, able to represent and interpolate between the
    following distributions:

        - Cauchy                (lam=-1)
        - logistic              (lam=0.0)
        - approx Normal         (lam=0.14)
        - u-shape               (lam = 0.5)
        - uniform from -1 to 1  (lam = 1)

    `tukeylambda` takes ``lam`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, lam):
        return np.ones(np.shape(lam), dtype=bool)

    def _pdf(self, x, lam):
        Fx = np.asarray(sc.tklmbda(x, lam))
        Px = Fx**(lam-1.0) + (np.asarray(1-Fx))**(lam-1.0)
        Px = 1.0/np.asarray(Px)
        return np.where((lam <= 0) | (abs(x) < 1.0/np.asarray(lam)), Px, 0.0)

    def _cdf(self, x, lam):
        return sc.tklmbda(x, lam)

    def _ppf(self, q, lam):
        return sc.boxcox(q, lam) - sc.boxcox1p(-q, lam)

    def _stats(self, lam):
        return 0, _tlvar(lam), 0, _tlkurt(lam)

    def _entropy(self, lam):
        def integ(p):
            return np.log(pow(p, lam-1)+pow(1-p, lam-1))
        return integrate.quad(integ, 0, 1)[0]
tukeylambda = tukeylambda_gen(name='tukeylambda')


class uniform_gen(rv_continuous):
    """A uniform continuous random variable.

    This distribution is constant between `loc` and ``loc + scale``.

    %(before_notes)s

    %(example)s

    """
    def _rvs(self):
        return self._random_state.uniform(0.0, 1.0, self._size)

    def _pdf(self, x):
        return 1.0*(x == x)

    def _cdf(self, x):
        return x

    def _ppf(self, q):
        return q

    def _stats(self):
        return 0.5, 1.0/12, 0, -1.2

    def _entropy(self):
        return 0.0
uniform = uniform_gen(a=0.0, b=1.0, name='uniform')


class vonmises_gen(rv_continuous):
    """A Von Mises continuous random variable.

    %(before_notes)s

    Notes
    -----
    If `x` is not in range or `loc` is not in range it assumes they are angles
    and converts them to [-pi, pi] equivalents.

    The probability density function for `vonmises` is::

        vonmises.pdf(x, kappa) = exp(kappa * cos(x)) / (2*pi*I[0](kappa))

    for ``-pi <= x <= pi``, ``kappa > 0``.

    `vonmises` takes ``kappa`` as a shape parameter.

    %(after_notes)s

    See Also
    --------
    vonmises_line : The same distribution, defined on a [-pi, pi] segment
                    of the real line.

    %(example)s

    """
    def _rvs(self, kappa):
        return self._random_state.vonmises(0.0, kappa, size=self._size)

    def _pdf(self, x, kappa):
        return np.exp(kappa * np.cos(x)) / (2*np.pi*sc.i0(kappa))

    def _cdf(self, x, kappa):
        return _stats.von_mises_cdf(kappa, x)

    def _stats_skip(self, kappa):
        return 0, None, 0, None

    def _entropy(self, kappa):
        return (-kappa * sc.i1(kappa) / sc.i0(kappa) +
                np.log(2 * np.pi * sc.i0(kappa)))
vonmises = vonmises_gen(name='vonmises')
vonmises_line = vonmises_gen(a=-np.pi, b=np.pi, name='vonmises_line')


class wald_gen(invgauss_gen):
    """A Wald continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `wald` is::

        wald.pdf(x) = 1/sqrt(2*pi*x**3) * exp(-(x-1)**2/(2*x))

    for ``x > 0``.

    `wald` is a special case of `invgauss` with ``mu == 1``.

    %(after_notes)s

    %(example)s
    """
    _support_mask = rv_continuous._open_support_mask

    def _rvs(self):
        return self._random_state.wald(1.0, 1.0, size=self._size)

    def _pdf(self, x):
        return invgauss._pdf(x, 1.0)

    def _logpdf(self, x):
        return invgauss._logpdf(x, 1.0)

    def _cdf(self, x):
        return invgauss._cdf(x, 1.0)

    def _stats(self):
        return 1.0, 1.0, 3.0, 15.0
wald = wald_gen(a=0.0, name="wald")


class wrapcauchy_gen(rv_continuous):
    """A wrapped Cauchy continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `wrapcauchy` is::

        wrapcauchy.pdf(x, c) = (1-c**2) / (2*pi*(1+c**2-2*c*cos(x)))

    for ``0 <= x <= 2*pi``, ``0 < c < 1``.

    `wrapcauchy` takes ``c`` as a shape parameter.

    %(after_notes)s

    %(example)s

    """
    def _argcheck(self, c):
        return (c > 0) & (c < 1)

    def _pdf(self, x, c):
        return (1.0-c*c)/(2*np.pi*(1+c*c-2*c*np.cos(x)))

    def _cdf(self, x, c):
        output = np.zeros(x.shape, dtype=x.dtype)
        val = (1.0+c)/(1.0-c)
        c1 = x < np.pi
        c2 = 1-c1
        xp = np.extract(c1, x)
        xn = np.extract(c2, x)
        if np.any(xn):
            valn = np.extract(c2, np.ones_like(x)*val)
            xn = 2*np.pi - xn
            yn = np.tan(xn/2.0)
            on = 1.0-1.0/np.pi*np.arctan(valn*yn)
            np.place(output, c2, on)
        if np.any(xp):
            valp = np.extract(c1, np.ones_like(x)*val)
            yp = np.tan(xp/2.0)
            op = 1.0/np.pi*np.arctan(valp*yp)
            np.place(output, c1, op)
        return output

    def _ppf(self, q, c):
        val = (1.0-c)/(1.0+c)
        rcq = 2*np.arctan(val*np.tan(np.pi*q))
        rcmq = 2*np.pi-2*np.arctan(val*np.tan(np.pi*(1-q)))
        return np.where(q < 1.0/2, rcq, rcmq)

    def _entropy(self, c):
        return np.log(2*np.pi*(1-c*c))
wrapcauchy = wrapcauchy_gen(a=0.0, b=2*np.pi, name='wrapcauchy')


class gennorm_gen(rv_continuous):
    """A generalized normal continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `gennorm` is [1]_::

                                     beta
        gennorm.pdf(x, beta) =  ---------------  exp(-|x|**beta)
                                2 gamma(1/beta)

    `gennorm` takes ``beta`` as a shape parameter.
    For ``beta = 1``, it is identical to a Laplace distribution.
    For ``beta = 2``, it is identical to a normal distribution
    (with ``scale=1/sqrt(2)``).

    See Also
    --------
    laplace : Laplace distribution
    norm : normal distribution

    References
    ----------

    .. [1] "Generalized normal distribution, Version 1",
           https://en.wikipedia.org/wiki/Generalized_normal_distribution#Version_1

    %(example)s

    """

    def _pdf(self, x, beta):
        return np.exp(self._logpdf(x, beta))

    def _logpdf(self, x, beta):
        return np.log(0.5*beta) - sc.gammaln(1.0/beta) - abs(x)**beta

    def _cdf(self, x, beta):
        c = 0.5 * np.sign(x)
        # evaluating (.5 + c) first prevents numerical cancellation
        return (0.5 + c) - c * sc.gammaincc(1.0/beta, abs(x)**beta)

    def _ppf(self, x, beta):
        c = np.sign(x - 0.5)
        # evaluating (1. + c) first prevents numerical cancellation
        return c * sc.gammainccinv(1.0/beta, (1.0 + c) - 2.0*c*x)**(1.0/beta)

    def _sf(self, x, beta):
        return self._cdf(-x, beta)

    def _isf(self, x, beta):
        return -self._ppf(x, beta)

    def _stats(self, beta):
        c1, c3, c5 = sc.gammaln([1.0/beta, 3.0/beta, 5.0/beta])
        return 0., np.exp(c3 - c1), 0., np.exp(c5 + c1 - 2.0*c3) - 3.

    def _entropy(self, beta):
        return 1. / beta - np.log(.5 * beta) + sc.gammaln(1. / beta)
gennorm = gennorm_gen(name='gennorm')


class halfgennorm_gen(rv_continuous):
    """The upper half of a generalized normal continuous random variable.

    %(before_notes)s

    Notes
    -----
    The probability density function for `halfgennorm` is::

                                        beta
        halfgennorm.pdf(x, beta) =  -------------  exp(-|x|**beta)
                                    gamma(1/beta)

    `gennorm` takes ``beta`` as a shape parameter.
    For ``beta = 1``, it is identical to an exponential distribution.
    For ``beta = 2``, it is identical to a half normal distribution
    (with ``scale=1/sqrt(2)``).

    See Also
    --------
    gennorm : generalized normal distribution
    expon : exponential distribution
    halfnorm : half normal distribution

    References
    ----------

    .. [1] "Generalized normal distribution, Version 1",
           https://en.wikipedia.org/wiki/Generalized_normal_distribution#Version_1

    %(example)s

    """

    def _pdf(self, x, beta):
        return np.exp(self._logpdf(x, beta))

    def _logpdf(self, x, beta):
        return np.log(beta) - sc.gammaln(1.0/beta) - x**beta

    def _cdf(self, x, beta):
        return sc.gammainc(1.0/beta, x**beta)

    def _ppf(self, x, beta):
        return sc.gammaincinv(1.0/beta, x)**(1.0/beta)

    def _sf(self, x, beta):
        return sc.gammaincc(1.0/beta, x**beta)

    def _isf(self, x, beta):
        return sc.gammainccinv(1.0/beta, x)**(1.0/beta)

    def _entropy(self, beta):
        return 1.0/beta - np.log(beta) + sc.gammaln(1.0/beta)
halfgennorm = halfgennorm_gen(a=0, name='halfgennorm')


def _argus_phi(chi):
    """
    Utility function for the argus distribution
    used in the CDF and norm of the Argus Funktion
    """
    return _norm_cdf(chi) - chi * _norm_pdf(chi) - 0.5


class argus_gen(rv_continuous):
    """
    Argus distribution

    %(before_notes)s

    Notes
    -----
    The probability density function for `argus` is::

        argus.pdf(x, chi) = chi**3 / (sqrt(2*pi) * Psi(chi)) * x * sqrt(1-x**2) * exp(- 0.5 * chi**2 * (1 - x**2))

        where:
                 Psi(chi) = Phi(chi) - chi * phi(chi) - 1/2
                 with Phi and phi being the CDF and PDF of a standard normal distribution, respectively.

    `argus` takes ``chi`` as shape a parameter.

    References
    ----------

    .. [1] "ARGUS distribution",
           https://en.wikipedia.org/wiki/ARGUS_distribution

    %(after_notes)s
    
    .. versionadded:: 0.19.0

    %(example)s
    """
    def _pdf(self, x, chi):
        """
        Return PDF of the argus function
        """
        y = 1.0 - x**2
        return chi**3 / (_norm_pdf_C * _argus_phi(chi)) * x * np.sqrt(y) * np.exp(-chi**2 * y / 2)

    def _cdf(self, x, chi):
        """
        Return CDF of the argus function
        """
        return 1.0 - self._sf(x, chi)

    def _sf(self, x, chi):
        """
        Return survival function of the argus function
        """
        return _argus_phi(chi * np.sqrt(1 - x**2)) / _argus_phi(chi)
argus = argus_gen(name='argus', longname="An Argus Function", a=0.0, b=1.0)


class rv_histogram(rv_continuous):
    """
    Generates a distribution given by a histogram.
    This is useful to generate a template distribution from a binned
    datasample.

    As a subclass of the `rv_continuous` class, `rv_histogram` inherits from it
    a collection of generic methods (see `rv_continuous` for the full list),
    and implements them based on the properties of the provided binned
    datasample.
        
    Parameters
    ----------
    histogram : tuple of array_like
      Tuple containing two array_like objects
      The first containing the content of n bins
      The second containing the (n+1) bin boundaries
      In particular the return value np.histogram is accepted

    Notes
    -----
    There are no additional shape parameters except for the loc and scale.
    The pdf is defined as a stepwise function from the provided histogram
    The cdf is a linear interpolation of the pdf.

    .. versionadded:: 0.19.0

    Examples
    --------

    Create a scipy.stats distribution from a numpy histogram

    >>> import scipy.stats
    >>> import numpy as np
    >>> data = scipy.stats.norm.rvs(size=100000, loc=0, scale=1.5, random_state=123)
    >>> hist = np.histogram(data, bins=100)
    >>> hist_dist = scipy.stats.rv_histogram(hist)

    Behaves like an ordinary scipy rv_continuous distribution

    >>> hist_dist.pdf(1.0)
    0.20538577847618705
    >>> hist_dist.cdf(2.0)
    0.90818568543056499

    PDF is zero above (below) the highest (lowest) bin of the histogram,
    defined by the max (min) of the original dataset

    >>> hist_dist.pdf(np.max(data))
    0.0
    >>> hist_dist.cdf(np.max(data))
    1.0
    >>> hist_dist.pdf(np.min(data))
    7.7591907244498314e-05
    >>> hist_dist.cdf(np.min(data))
    0.0

    PDF and CDF follow the histogram

    >>> import matplotlib.pyplot as plt
    >>> X = np.linspace(-5.0, 5.0, 100)
    >>> plt.title("PDF from Template")
    >>> plt.hist(data, normed=True, bins=100)
    >>> plt.plot(X, hist_dist.pdf(X), label='PDF')
    >>> plt.plot(X, hist_dist.cdf(X), label='CDF')
    >>> plt.show()

    """
    _support_mask = rv_continuous._support_mask

    def __init__(self, histogram, *args, **kwargs):
        """
        Create a new distribution using the given histogram

        Parameters
        ----------
        histogram : tuple of array_like
          Tuple containing two array_like objects
          The first containing the content of n bins
          The second containing the (n+1) bin boundaries
          In particular the return value np.histogram is accepted
        """
        self._histogram = histogram
        if len(histogram) != 2:
            raise ValueError("Expected length 2 for parameter histogram")
        self._hpdf = np.asarray(histogram[0])
        self._hbins = np.asarray(histogram[1])
        if len(self._hpdf) + 1 != len(self._hbins):
            raise ValueError("Number of elements in histogram content "
                             "and histogram boundaries do not match, "
                             "expected n and n+1.")
        self._hbin_widths = self._hbins[1:] - self._hbins[:-1]
        self._hpdf = self._hpdf / float(np.sum(self._hpdf * self._hbin_widths))
        self._hcdf = np.cumsum(self._hpdf * self._hbin_widths)
        self._hpdf = np.hstack([0.0, self._hpdf, 0.0])
        self._hcdf = np.hstack([0.0, self._hcdf])
        # Set support
        kwargs['a'] = self._hbins[0]
        kwargs['b'] = self._hbins[-1]
        super(rv_histogram, self).__init__(*args, **kwargs)

    def _pdf(self, x):
        """
        PDF of the histogram
        """
        return self._hpdf[np.searchsorted(self._hbins, x, side='right')]

    def _cdf(self, x):
        """
        CDF calculated from the histogram
        """
        return np.interp(x, self._hbins, self._hcdf)

    def _ppf(self, x):
        """
        Percentile function calculated from the histogram
        """
        return np.interp(x, self._hcdf, self._hbins)

    def _munp(self, n):
        """Compute the n-th non-central moment."""
        integrals = (self._hbins[1:]**(n+1) - self._hbins[:-1]**(n+1)) / (n+1)
        return np.sum(self._hpdf[1:-1] * integrals)

    def _entropy(self):
        """Compute entropy of distribution"""
        res = _lazywhere(self._hpdf[1:-1] > 0.0,
                         (self._hpdf[1:-1],),
                         np.log,
                         0.0)
        return -np.sum(self._hpdf[1:-1] * res * self._hbin_widths)

    def _updated_ctor_param(self):
        """
        Set the histogram as additional constructor argument
        """
        dct = super(rv_histogram, self)._updated_ctor_param()
        dct['histogram'] = self._histogram
        return dct


# Collect names of classes and objects in this module.
pairs = list(globals().items())
_distn_names, _distn_gen_names = get_distribution_names(pairs, rv_continuous)

__all__ = _distn_names + _distn_gen_names + ['rv_histogram']