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r"""
=================
Nonlinear solvers
=================
.. currentmodule:: scipy.optimize
This is a collection of general-purpose nonlinear multidimensional
solvers. These solvers find *x* for which *F(x) = 0*. Both *x*
and *F* can be multidimensional.
Routines
========
Large-scale nonlinear solvers:
.. autosummary::
newton_krylov
anderson
General nonlinear solvers:
.. autosummary::
broyden1
broyden2
Simple iterations:
.. autosummary::
excitingmixing
linearmixing
diagbroyden
Examples
========
Small problem
-------------
>>> def F(x):
... return np.cos(x) + x[::-1] - [1, 2, 3, 4]
>>> import scipy.optimize
>>> x = scipy.optimize.broyden1(F, [1,1,1,1], f_tol=1e-14)
>>> x
array([ 4.04674914, 3.91158389, 2.71791677, 1.61756251])
>>> np.cos(x) + x[::-1]
array([ 1., 2., 3., 4.])
Large problem
-------------
Suppose that we needed to solve the following integrodifferential
equation on the square :math:`[0,1]\times[0,1]`:
.. math::
\nabla^2 P = 10 \left(\int_0^1\int_0^1\cosh(P)\,dx\,dy\right)^2
with :math:`P(x,1) = 1` and :math:`P=0` elsewhere on the boundary of
the square.
The solution can be found using the `newton_krylov` solver:
.. plot::
import numpy as np
from scipy.optimize import newton_krylov
from numpy import cosh, zeros_like, mgrid, zeros
# parameters
nx, ny = 75, 75
hx, hy = 1./(nx-1), 1./(ny-1)
P_left, P_right = 0, 0
P_top, P_bottom = 1, 0
def residual(P):
d2x = zeros_like(P)
d2y = zeros_like(P)
d2x[1:-1] = (P[2:] - 2*P[1:-1] + P[:-2]) / hx/hx
d2x[0] = (P[1] - 2*P[0] + P_left)/hx/hx
d2x[-1] = (P_right - 2*P[-1] + P[-2])/hx/hx
d2y[:,1:-1] = (P[:,2:] - 2*P[:,1:-1] + P[:,:-2])/hy/hy
d2y[:,0] = (P[:,1] - 2*P[:,0] + P_bottom)/hy/hy
d2y[:,-1] = (P_top - 2*P[:,-1] + P[:,-2])/hy/hy
return d2x + d2y - 10*cosh(P).mean()**2
# solve
guess = zeros((nx, ny), float)
sol = newton_krylov(residual, guess, method='lgmres', verbose=1)
print('Residual: %g' % abs(residual(sol)).max())
# visualize
import matplotlib.pyplot as plt
x, y = mgrid[0:1:(nx*1j), 0:1:(ny*1j)]
plt.pcolor(x, y, sol)
plt.colorbar()
plt.show()
"""
# Copyright (C) 2009, Pauli Virtanen <pav@iki.fi>
# Distributed under the same license as Scipy.
from __future__ import division, print_function, absolute_import
import sys
import numpy as np
from scipy._lib.six import callable, exec_
from scipy._lib.six import xrange
from scipy.linalg import norm, solve, inv, qr, svd, LinAlgError
from numpy import asarray, dot, vdot
import scipy.sparse.linalg
import scipy.sparse
from scipy.linalg import get_blas_funcs
import inspect
from .linesearch import scalar_search_wolfe1, scalar_search_armijo
__all__ = [
'broyden1', 'broyden2', 'anderson', 'linearmixing',
'diagbroyden', 'excitingmixing', 'newton_krylov']
#------------------------------------------------------------------------------
# Utility functions
#------------------------------------------------------------------------------
class NoConvergence(Exception):
pass
def maxnorm(x):
return np.absolute(x).max()
def _as_inexact(x):
"""Return `x` as an array, of either floats or complex floats"""
x = asarray(x)
if not np.issubdtype(x.dtype, np.inexact):
return asarray(x, dtype=np.float_)
return x
def _array_like(x, x0):
"""Return ndarray `x` as same array subclass and shape as `x0`"""
x = np.reshape(x, np.shape(x0))
wrap = getattr(x0, '__array_wrap__', x.__array_wrap__)
return wrap(x)
def _safe_norm(v):
if not np.isfinite(v).all():
return np.array(np.inf)
return norm(v)
#------------------------------------------------------------------------------
# Generic nonlinear solver machinery
#------------------------------------------------------------------------------
_doc_parts = dict(
params_basic="""
F : function(x) -> f
Function whose root to find; should take and return an array-like
object.
x0 : array_like
Initial guess for the solution
""".strip(),
params_extra="""
iter : int, optional
Number of iterations to make. If omitted (default), make as many
as required to meet tolerances.
verbose : bool, optional
Print status to stdout on every iteration.
maxiter : int, optional
Maximum number of iterations to make. If more are needed to
meet convergence, `NoConvergence` is raised.
f_tol : float, optional
Absolute tolerance (in max-norm) for the residual.
If omitted, default is 6e-6.
f_rtol : float, optional
Relative tolerance for the residual. If omitted, not used.
x_tol : float, optional
Absolute minimum step size, as determined from the Jacobian
approximation. If the step size is smaller than this, optimization
is terminated as successful. If omitted, not used.
x_rtol : float, optional
Relative minimum step size. If omitted, not used.
tol_norm : function(vector) -> scalar, optional
Norm to use in convergence check. Default is the maximum norm.
line_search : {None, 'armijo' (default), 'wolfe'}, optional
Which type of a line search to use to determine the step size in the
direction given by the Jacobian approximation. Defaults to 'armijo'.
callback : function, optional
Optional callback function. It is called on every iteration as
``callback(x, f)`` where `x` is the current solution and `f`
the corresponding residual.
Returns
-------
sol : ndarray
An array (of similar array type as `x0`) containing the final solution.
Raises
------
NoConvergence
When a solution was not found.
""".strip()
)
def _set_doc(obj):
if obj.__doc__:
obj.__doc__ = obj.__doc__ % _doc_parts
def nonlin_solve(F, x0, jacobian='krylov', iter=None, verbose=False,
maxiter=None, f_tol=None, f_rtol=None, x_tol=None, x_rtol=None,
tol_norm=None, line_search='armijo', callback=None,
full_output=False, raise_exception=True):
"""
Find a root of a function, in a way suitable for large-scale problems.
Parameters
----------
%(params_basic)s
jacobian : Jacobian
A Jacobian approximation: `Jacobian` object or something that
`asjacobian` can transform to one. Alternatively, a string specifying
which of the builtin Jacobian approximations to use:
krylov, broyden1, broyden2, anderson
diagbroyden, linearmixing, excitingmixing
%(params_extra)s
full_output : bool
If true, returns a dictionary `info` containing convergence
information.
raise_exception : bool
If True, a `NoConvergence` exception is raise if no solution is found.
See Also
--------
asjacobian, Jacobian
Notes
-----
This algorithm implements the inexact Newton method, with
backtracking or full line searches. Several Jacobian
approximations are available, including Krylov and Quasi-Newton
methods.
References
----------
.. [KIM] C. T. Kelley, \"Iterative Methods for Linear and Nonlinear
Equations\". Society for Industrial and Applied Mathematics. (1995)
http://www.siam.org/books/kelley/
"""
condition = TerminationCondition(f_tol=f_tol, f_rtol=f_rtol,
x_tol=x_tol, x_rtol=x_rtol,
iter=iter, norm=tol_norm)
x0 = _as_inexact(x0)
func = lambda z: _as_inexact(F(_array_like(z, x0))).flatten()
x = x0.flatten()
dx = np.inf
Fx = func(x)
Fx_norm = norm(Fx)
jacobian = asjacobian(jacobian)
jacobian.setup(x.copy(), Fx, func)
if maxiter is None:
if iter is not None:
maxiter = iter + 1
else:
maxiter = 100*(x.size+1)
if line_search is True:
line_search = 'armijo'
elif line_search is False:
line_search = None
if line_search not in (None, 'armijo', 'wolfe'):
raise ValueError("Invalid line search")
# Solver tolerance selection
gamma = 0.9
eta_max = 0.9999
eta_treshold = 0.1
eta = 1e-3
for n in xrange(maxiter):
status = condition.check(Fx, x, dx)
if status:
break
# The tolerance, as computed for scipy.sparse.linalg.* routines
tol = min(eta, eta*Fx_norm)
dx = -jacobian.solve(Fx, tol=tol)
if norm(dx) == 0:
raise ValueError("Jacobian inversion yielded zero vector. "
"This indicates a bug in the Jacobian "
"approximation.")
# Line search, or Newton step
if line_search:
s, x, Fx, Fx_norm_new = _nonlin_line_search(func, x, Fx, dx,
line_search)
else:
s = 1.0
x = x + dx
Fx = func(x)
Fx_norm_new = norm(Fx)
jacobian.update(x.copy(), Fx)
if callback:
callback(x, Fx)
# Adjust forcing parameters for inexact methods
eta_A = gamma * Fx_norm_new**2 / Fx_norm**2
if gamma * eta**2 < eta_treshold:
eta = min(eta_max, eta_A)
else:
eta = min(eta_max, max(eta_A, gamma*eta**2))
Fx_norm = Fx_norm_new
# Print status
if verbose:
sys.stdout.write("%d: |F(x)| = %g; step %g; tol %g\n" % (
n, norm(Fx), s, eta))
sys.stdout.flush()
else:
if raise_exception:
raise NoConvergence(_array_like(x, x0))
else:
status = 2
if full_output:
info = {'nit': condition.iteration,
'fun': Fx,
'status': status,
'success': status == 1,
'message': {1: 'A solution was found at the specified '
'tolerance.',
2: 'The maximum number of iterations allowed '
'has been reached.'
}[status]
}
return _array_like(x, x0), info
else:
return _array_like(x, x0)
_set_doc(nonlin_solve)
def _nonlin_line_search(func, x, Fx, dx, search_type='armijo', rdiff=1e-8,
smin=1e-2):
tmp_s = [0]
tmp_Fx = [Fx]
tmp_phi = [norm(Fx)**2]
s_norm = norm(x) / norm(dx)
def phi(s, store=True):
if s == tmp_s[0]:
return tmp_phi[0]
xt = x + s*dx
v = func(xt)
p = _safe_norm(v)**2
if store:
tmp_s[0] = s
tmp_phi[0] = p
tmp_Fx[0] = v
return p
def derphi(s):
ds = (abs(s) + s_norm + 1) * rdiff
return (phi(s+ds, store=False) - phi(s)) / ds
if search_type == 'wolfe':
s, phi1, phi0 = scalar_search_wolfe1(phi, derphi, tmp_phi[0],
xtol=1e-2, amin=smin)
elif search_type == 'armijo':
s, phi1 = scalar_search_armijo(phi, tmp_phi[0], -tmp_phi[0],
amin=smin)
if s is None:
# XXX: No suitable step length found. Take the full Newton step,
# and hope for the best.
s = 1.0
x = x + s*dx
if s == tmp_s[0]:
Fx = tmp_Fx[0]
else:
Fx = func(x)
Fx_norm = norm(Fx)
return s, x, Fx, Fx_norm
class TerminationCondition(object):
"""
Termination condition for an iteration. It is terminated if
- |F| < f_rtol*|F_0|, AND
- |F| < f_tol
AND
- |dx| < x_rtol*|x|, AND
- |dx| < x_tol
"""
def __init__(self, f_tol=None, f_rtol=None, x_tol=None, x_rtol=None,
iter=None, norm=maxnorm):
if f_tol is None:
f_tol = np.finfo(np.float_).eps ** (1./3)
if f_rtol is None:
f_rtol = np.inf
if x_tol is None:
x_tol = np.inf
if x_rtol is None:
x_rtol = np.inf
self.x_tol = x_tol
self.x_rtol = x_rtol
self.f_tol = f_tol
self.f_rtol = f_rtol
if norm is None:
self.norm = maxnorm
else:
self.norm = norm
self.iter = iter
self.f0_norm = None
self.iteration = 0
def check(self, f, x, dx):
self.iteration += 1
f_norm = self.norm(f)
x_norm = self.norm(x)
dx_norm = self.norm(dx)
if self.f0_norm is None:
self.f0_norm = f_norm
if f_norm == 0:
return 1
if self.iter is not None:
# backwards compatibility with Scipy 0.6.0
return 2 * (self.iteration > self.iter)
# NB: condition must succeed for rtol=inf even if norm == 0
return int((f_norm <= self.f_tol
and f_norm/self.f_rtol <= self.f0_norm)
and (dx_norm <= self.x_tol
and dx_norm/self.x_rtol <= x_norm))
#------------------------------------------------------------------------------
# Generic Jacobian approximation
#------------------------------------------------------------------------------
class Jacobian(object):
"""
Common interface for Jacobians or Jacobian approximations.
The optional methods come useful when implementing trust region
etc. algorithms that often require evaluating transposes of the
Jacobian.
Methods
-------
solve
Returns J^-1 * v
update
Updates Jacobian to point `x` (where the function has residual `Fx`)
matvec : optional
Returns J * v
rmatvec : optional
Returns A^H * v
rsolve : optional
Returns A^-H * v
matmat : optional
Returns A * V, where V is a dense matrix with dimensions (N,K).
todense : optional
Form the dense Jacobian matrix. Necessary for dense trust region
algorithms, and useful for testing.
Attributes
----------
shape
Matrix dimensions (M, N)
dtype
Data type of the matrix.
func : callable, optional
Function the Jacobian corresponds to
"""
def __init__(self, **kw):
names = ["solve", "update", "matvec", "rmatvec", "rsolve",
"matmat", "todense", "shape", "dtype"]
for name, value in kw.items():
if name not in names:
raise ValueError("Unknown keyword argument %s" % name)
if value is not None:
setattr(self, name, kw[name])
if hasattr(self, 'todense'):
self.__array__ = lambda: self.todense()
def aspreconditioner(self):
return InverseJacobian(self)
def solve(self, v, tol=0):
raise NotImplementedError
def update(self, x, F):
pass
def setup(self, x, F, func):
self.func = func
self.shape = (F.size, x.size)
self.dtype = F.dtype
if self.__class__.setup is Jacobian.setup:
# Call on the first point unless overridden
self.update(self, x, F)
class InverseJacobian(object):
def __init__(self, jacobian):
self.jacobian = jacobian
self.matvec = jacobian.solve
self.update = jacobian.update
if hasattr(jacobian, 'setup'):
self.setup = jacobian.setup
if hasattr(jacobian, 'rsolve'):
self.rmatvec = jacobian.rsolve
@property
def shape(self):
return self.jacobian.shape
@property
def dtype(self):
return self.jacobian.dtype
def asjacobian(J):
"""
Convert given object to one suitable for use as a Jacobian.
"""
spsolve = scipy.sparse.linalg.spsolve
if isinstance(J, Jacobian):
return J
elif inspect.isclass(J) and issubclass(J, Jacobian):
return J()
elif isinstance(J, np.ndarray):
if J.ndim > 2:
raise ValueError('array must have rank <= 2')
J = np.atleast_2d(np.asarray(J))
if J.shape[0] != J.shape[1]:
raise ValueError('array must be square')
return Jacobian(matvec=lambda v: dot(J, v),
rmatvec=lambda v: dot(J.conj().T, v),
solve=lambda v: solve(J, v),
rsolve=lambda v: solve(J.conj().T, v),
dtype=J.dtype, shape=J.shape)
elif scipy.sparse.isspmatrix(J):
if J.shape[0] != J.shape[1]:
raise ValueError('matrix must be square')
return Jacobian(matvec=lambda v: J*v,
rmatvec=lambda v: J.conj().T * v,
solve=lambda v: spsolve(J, v),
rsolve=lambda v: spsolve(J.conj().T, v),
dtype=J.dtype, shape=J.shape)
elif hasattr(J, 'shape') and hasattr(J, 'dtype') and hasattr(J, 'solve'):
return Jacobian(matvec=getattr(J, 'matvec'),
rmatvec=getattr(J, 'rmatvec'),
solve=J.solve,
rsolve=getattr(J, 'rsolve'),
update=getattr(J, 'update'),
setup=getattr(J, 'setup'),
dtype=J.dtype,
shape=J.shape)
elif callable(J):
# Assume it's a function J(x) that returns the Jacobian
class Jac(Jacobian):
def update(self, x, F):
self.x = x
def solve(self, v, tol=0):
m = J(self.x)
if isinstance(m, np.ndarray):
return solve(m, v)
elif scipy.sparse.isspmatrix(m):
return spsolve(m, v)
else:
raise ValueError("Unknown matrix type")
def matvec(self, v):
m = J(self.x)
if isinstance(m, np.ndarray):
return dot(m, v)
elif scipy.sparse.isspmatrix(m):
return m*v
else:
raise ValueError("Unknown matrix type")
def rsolve(self, v, tol=0):
m = J(self.x)
if isinstance(m, np.ndarray):
return solve(m.conj().T, v)
elif scipy.sparse.isspmatrix(m):
return spsolve(m.conj().T, v)
else:
raise ValueError("Unknown matrix type")
def rmatvec(self, v):
m = J(self.x)
if isinstance(m, np.ndarray):
return dot(m.conj().T, v)
elif scipy.sparse.isspmatrix(m):
return m.conj().T * v
else:
raise ValueError("Unknown matrix type")
return Jac()
elif isinstance(J, str):
return dict(broyden1=BroydenFirst,
broyden2=BroydenSecond,
anderson=Anderson,
diagbroyden=DiagBroyden,
linearmixing=LinearMixing,
excitingmixing=ExcitingMixing,
krylov=KrylovJacobian)[J]()
else:
raise TypeError('Cannot convert object to a Jacobian')
#------------------------------------------------------------------------------
# Broyden
#------------------------------------------------------------------------------
class GenericBroyden(Jacobian):
def setup(self, x0, f0, func):
Jacobian.setup(self, x0, f0, func)
self.last_f = f0
self.last_x = x0
if hasattr(self, 'alpha') and self.alpha is None:
# Autoscale the initial Jacobian parameter
# unless we have already guessed the solution.
normf0 = norm(f0)
if normf0:
self.alpha = 0.5*max(norm(x0), 1) / normf0
else:
self.alpha = 1.0
def _update(self, x, f, dx, df, dx_norm, df_norm):
raise NotImplementedError
def update(self, x, f):
df = f - self.last_f
dx = x - self.last_x
self._update(x, f, dx, df, norm(dx), norm(df))
self.last_f = f
self.last_x = x
class LowRankMatrix(object):
r"""
A matrix represented as
.. math:: \alpha I + \sum_{n=0}^{n=M} c_n d_n^\dagger
However, if the rank of the matrix reaches the dimension of the vectors,
full matrix representation will be used thereon.
"""
def __init__(self, alpha, n, dtype):
self.alpha = alpha
self.cs = []
self.ds = []
self.n = n
self.dtype = dtype
self.collapsed = None
@staticmethod
def _matvec(v, alpha, cs, ds):
axpy, scal, dotc = get_blas_funcs(['axpy', 'scal', 'dotc'],
cs[:1] + [v])
w = alpha * v
for c, d in zip(cs, ds):
a = dotc(d, v)
w = axpy(c, w, w.size, a)
return w
@staticmethod
def _solve(v, alpha, cs, ds):
"""Evaluate w = M^-1 v"""
if len(cs) == 0:
return v/alpha
# (B + C D^H)^-1 = B^-1 - B^-1 C (I + D^H B^-1 C)^-1 D^H B^-1
axpy, dotc = get_blas_funcs(['axpy', 'dotc'], cs[:1] + [v])
c0 = cs[0]
A = alpha * np.identity(len(cs), dtype=c0.dtype)
for i, d in enumerate(ds):
for j, c in enumerate(cs):
A[i,j] += dotc(d, c)
q = np.zeros(len(cs), dtype=c0.dtype)
for j, d in enumerate(ds):
q[j] = dotc(d, v)
q /= alpha
q = solve(A, q)
w = v/alpha
for c, qc in zip(cs, q):
w = axpy(c, w, w.size, -qc)
return w
def matvec(self, v):
"""Evaluate w = M v"""
if self.collapsed is not None:
return np.dot(self.collapsed, v)
return LowRankMatrix._matvec(v, self.alpha, self.cs, self.ds)
def rmatvec(self, v):
"""Evaluate w = M^H v"""
if self.collapsed is not None:
return np.dot(self.collapsed.T.conj(), v)
return LowRankMatrix._matvec(v, np.conj(self.alpha), self.ds, self.cs)
def solve(self, v, tol=0):
"""Evaluate w = M^-1 v"""
if self.collapsed is not None:
return solve(self.collapsed, v)
return LowRankMatrix._solve(v, self.alpha, self.cs, self.ds)
def rsolve(self, v, tol=0):
"""Evaluate w = M^-H v"""
if self.collapsed is not None:
return solve(self.collapsed.T.conj(), v)
return LowRankMatrix._solve(v, np.conj(self.alpha), self.ds, self.cs)
def append(self, c, d):
if self.collapsed is not None:
self.collapsed += c[:,None] * d[None,:].conj()
return
self.cs.append(c)
self.ds.append(d)
if len(self.cs) > c.size:
self.collapse()
def __array__(self):
if self.collapsed is not None:
return self.collapsed
Gm = self.alpha*np.identity(self.n, dtype=self.dtype)
for c, d in zip(self.cs, self.ds):
Gm += c[:,None]*d[None,:].conj()
return Gm
def collapse(self):
"""Collapse the low-rank matrix to a full-rank one."""
self.collapsed = np.array(self)
self.cs = None
self.ds = None
self.alpha = None
def restart_reduce(self, rank):
"""
Reduce the rank of the matrix by dropping all vectors.
"""
if self.collapsed is not None:
return
assert rank > 0
if len(self.cs) > rank:
del self.cs[:]
del self.ds[:]
def simple_reduce(self, rank):
"""
Reduce the rank of the matrix by dropping oldest vectors.
"""
if self.collapsed is not None:
return
assert rank > 0
while len(self.cs) > rank:
del self.cs[0]
del self.ds[0]
def svd_reduce(self, max_rank, to_retain=None):
"""
Reduce the rank of the matrix by retaining some SVD components.
This corresponds to the \"Broyden Rank Reduction Inverse\"
algorithm described in [1]_.
Note that the SVD decomposition can be done by solving only a
problem whose size is the effective rank of this matrix, which
is viable even for large problems.
Parameters
----------
max_rank : int
Maximum rank of this matrix after reduction.
to_retain : int, optional
Number of SVD components to retain when reduction is done
(ie. rank > max_rank). Default is ``max_rank - 2``.
References
----------
.. [1] B.A. van der Rotten, PhD thesis,
\"A limited memory Broyden method to solve high-dimensional
systems of nonlinear equations\". Mathematisch Instituut,
Universiteit Leiden, The Netherlands (2003).
http://www.math.leidenuniv.nl/scripties/Rotten.pdf
"""
if self.collapsed is not None:
return
p = max_rank
if to_retain is not None:
q = to_retain
else:
q = p - 2
if self.cs:
p = min(p, len(self.cs[0]))
q = max(0, min(q, p-1))
m = len(self.cs)
if m < p:
# nothing to do
return
C = np.array(self.cs).T
D = np.array(self.ds).T
D, R = qr(D, mode='economic')
C = dot(C, R.T.conj())
U, S, WH = svd(C, full_matrices=False, compute_uv=True)
C = dot(C, inv(WH))
D = dot(D, WH.T.conj())
for k in xrange(q):
self.cs[k] = C[:,k].copy()
self.ds[k] = D[:,k].copy()
del self.cs[q:]
del self.ds[q:]
_doc_parts['broyden_params'] = """
alpha : float, optional
Initial guess for the Jacobian is ``(-1/alpha)``.
reduction_method : str or tuple, optional
Method used in ensuring that the rank of the Broyden matrix
stays low. Can either be a string giving the name of the method,
or a tuple of the form ``(method, param1, param2, ...)``
that gives the name of the method and values for additional parameters.
Methods available:
- ``restart``: drop all matrix columns. Has no extra parameters.
- ``simple``: drop oldest matrix column. Has no extra parameters.
- ``svd``: keep only the most significant SVD components.
Takes an extra parameter, ``to_retain``, which determines the
number of SVD components to retain when rank reduction is done.
Default is ``max_rank - 2``.
max_rank : int, optional
Maximum rank for the Broyden matrix.
Default is infinity (ie., no rank reduction).
""".strip()
class BroydenFirst(GenericBroyden):
r"""
Find a root of a function, using Broyden's first Jacobian approximation.
This method is also known as \"Broyden's good method\".
Parameters
----------
%(params_basic)s
%(broyden_params)s
%(params_extra)s
Notes
-----
This algorithm implements the inverse Jacobian Quasi-Newton update
.. math:: H_+ = H + (dx - H df) dx^\dagger H / ( dx^\dagger H df)
which corresponds to Broyden's first Jacobian update
.. math:: J_+ = J + (df - J dx) dx^\dagger / dx^\dagger dx
References
----------
.. [1] B.A. van der Rotten, PhD thesis,
\"A limited memory Broyden method to solve high-dimensional
systems of nonlinear equations\". Mathematisch Instituut,
Universiteit Leiden, The Netherlands (2003).
http://www.math.leidenuniv.nl/scripties/Rotten.pdf
"""
def __init__(self, alpha=None, reduction_method='restart', max_rank=None):
GenericBroyden.__init__(self)
self.alpha = alpha
self.Gm = None
if max_rank is None:
max_rank = np.inf
self.max_rank = max_rank
if isinstance(reduction_method, str):
reduce_params = ()
else:
reduce_params = reduction_method[1:]
reduction_method = reduction_method[0]
reduce_params = (max_rank - 1,) + reduce_params
if reduction_method == 'svd':
self._reduce = lambda: self.Gm.svd_reduce(*reduce_params)
elif reduction_method == 'simple':
self._reduce = lambda: self.Gm.simple_reduce(*reduce_params)
elif reduction_method == 'restart':
self._reduce = lambda: self.Gm.restart_reduce(*reduce_params)
else:
raise ValueError("Unknown rank reduction method '%s'" %
reduction_method)
def setup(self, x, F, func):
GenericBroyden.setup(self, x, F, func)
self.Gm = LowRankMatrix(-self.alpha, self.shape[0], self.dtype)
def todense(self):
return inv(self.Gm)
def solve(self, f, tol=0):
r = self.Gm.matvec(f)
if not np.isfinite(r).all():
# singular; reset the Jacobian approximation
self.setup(self.last_x, self.last_f, self.func)
return self.Gm.matvec(f)
def matvec(self, f):
return self.Gm.solve(f)
def rsolve(self, f, tol=0):
return self.Gm.rmatvec(f)
def rmatvec(self, f):
return self.Gm.rsolve(f)
def _update(self, x, f, dx, df, dx_norm, df_norm):
self._reduce() # reduce first to preserve secant condition
v = self.Gm.rmatvec(dx)
c = dx - self.Gm.matvec(df)
d = v / vdot(df, v)
self.Gm.append(c, d)
class BroydenSecond(BroydenFirst):
"""
Find a root of a function, using Broyden\'s second Jacobian approximation.
This method is also known as \"Broyden's bad method\".
Parameters
----------
%(params_basic)s
%(broyden_params)s
%(params_extra)s
Notes
-----
This algorithm implements the inverse Jacobian Quasi-Newton update
.. math:: H_+ = H + (dx - H df) df^\dagger / ( df^\dagger df)
corresponding to Broyden's second method.
References
----------
.. [1] B.A. van der Rotten, PhD thesis,
\"A limited memory Broyden method to solve high-dimensional
systems of nonlinear equations\". Mathematisch Instituut,
Universiteit Leiden, The Netherlands (2003).
http://www.math.leidenuniv.nl/scripties/Rotten.pdf
"""
def _update(self, x, f, dx, df, dx_norm, df_norm):
self._reduce() # reduce first to preserve secant condition
v = df
c = dx - self.Gm.matvec(df)
d = v / df_norm**2
self.Gm.append(c, d)
#------------------------------------------------------------------------------
# Broyden-like (restricted memory)
#------------------------------------------------------------------------------
class Anderson(GenericBroyden):
"""
Find a root of a function, using (extended) Anderson mixing.
The Jacobian is formed by for a 'best' solution in the space
spanned by last `M` vectors. As a result, only a MxM matrix
inversions and MxN multiplications are required. [Ey]_
Parameters
----------
%(params_basic)s
alpha : float, optional
Initial guess for the Jacobian is (-1/alpha).
M : float, optional
Number of previous vectors to retain. Defaults to 5.
w0 : float, optional
Regularization parameter for numerical stability.
Compared to unity, good values of the order of 0.01.
%(params_extra)s
References
----------
.. [Ey] V. Eyert, J. Comp. Phys., 124, 271 (1996).
"""
# Note:
#
# Anderson method maintains a rank M approximation of the inverse Jacobian,
#
# J^-1 v ~ -v*alpha + (dX + alpha dF) A^-1 dF^H v
# A = W + dF^H dF
# W = w0^2 diag(dF^H dF)
#
# so that for w0 = 0 the secant condition applies for last M iterates, ie.,
#
# J^-1 df_j = dx_j
#
# for all j = 0 ... M-1.
#
# Moreover, (from Sherman-Morrison-Woodbury formula)
#
# J v ~ [ b I - b^2 C (I + b dF^H A^-1 C)^-1 dF^H ] v
# C = (dX + alpha dF) A^-1
# b = -1/alpha
#
# and after simplification
#
# J v ~ -v/alpha + (dX/alpha + dF) (dF^H dX - alpha W)^-1 dF^H v
#
def __init__(self, alpha=None, w0=0.01, M=5):
GenericBroyden.__init__(self)
self.alpha = alpha
self.M = M
self.dx = []
self.df = []
self.gamma = None
self.w0 = w0
def solve(self, f, tol=0):
dx = -self.alpha*f
n = len(self.dx)
if n == 0:
return dx
df_f = np.empty(n, dtype=f.dtype)
for k in xrange(n):
df_f[k] = vdot(self.df[k], f)
try:
gamma = solve(self.a, df_f)
except LinAlgError:
# singular; reset the Jacobian approximation
del self.dx[:]
del self.df[:]
return dx
for m in xrange(n):
dx += gamma[m]*(self.dx[m] + self.alpha*self.df[m])
return dx
def matvec(self, f):
dx = -f/self.alpha
n = len(self.dx)
if n == 0:
return dx
df_f = np.empty(n, dtype=f.dtype)
for k in xrange(n):
df_f[k] = vdot(self.df[k], f)
b = np.empty((n, n), dtype=f.dtype)
for i in xrange(n):
for j in xrange(n):
b[i,j] = vdot(self.df[i], self.dx[j])
if i == j and self.w0 != 0:
b[i,j] -= vdot(self.df[i], self.df[i])*self.w0**2*self.alpha
gamma = solve(b, df_f)
for m in xrange(n):
dx += gamma[m]*(self.df[m] + self.dx[m]/self.alpha)
return dx
def _update(self, x, f, dx, df, dx_norm, df_norm):
if self.M == 0:
return
self.dx.append(dx)
self.df.append(df)
while len(self.dx) > self.M:
self.dx.pop(0)
self.df.pop(0)
n = len(self.dx)
a = np.zeros((n, n), dtype=f.dtype)
for i in xrange(n):
for j in xrange(i, n):
if i == j:
wd = self.w0**2
else:
wd = 0
a[i,j] = (1+wd)*vdot(self.df[i], self.df[j])
a += np.triu(a, 1).T.conj()
self.a = a
#------------------------------------------------------------------------------
# Simple iterations
#------------------------------------------------------------------------------
class DiagBroyden(GenericBroyden):
"""
Find a root of a function, using diagonal Broyden Jacobian approximation.
The Jacobian approximation is derived from previous iterations, by
retaining only the diagonal of Broyden matrices.
.. warning::
This algorithm may be useful for specific problems, but whether
it will work may depend strongly on the problem.
Parameters
----------
%(params_basic)s
alpha : float, optional
Initial guess for the Jacobian is (-1/alpha).
%(params_extra)s
"""
def __init__(self, alpha=None):
GenericBroyden.__init__(self)
self.alpha = alpha
def setup(self, x, F, func):
GenericBroyden.setup(self, x, F, func)
self.d = np.ones((self.shape[0],), dtype=self.dtype) / self.alpha
def solve(self, f, tol=0):
return -f / self.d
def matvec(self, f):
return -f * self.d
def rsolve(self, f, tol=0):
return -f / self.d.conj()
def rmatvec(self, f):
return -f * self.d.conj()
def todense(self):
return np.diag(-self.d)
def _update(self, x, f, dx, df, dx_norm, df_norm):
self.d -= (df + self.d*dx)*dx/dx_norm**2
class LinearMixing(GenericBroyden):
"""
Find a root of a function, using a scalar Jacobian approximation.
.. warning::
This algorithm may be useful for specific problems, but whether
it will work may depend strongly on the problem.
Parameters
----------
%(params_basic)s
alpha : float, optional
The Jacobian approximation is (-1/alpha).
%(params_extra)s
"""
def __init__(self, alpha=None):
GenericBroyden.__init__(self)
self.alpha = alpha
def solve(self, f, tol=0):
return -f*self.alpha
def matvec(self, f):
return -f/self.alpha
def rsolve(self, f, tol=0):
return -f*np.conj(self.alpha)
def rmatvec(self, f):
return -f/np.conj(self.alpha)
def todense(self):
return np.diag(-np.ones(self.shape[0])/self.alpha)
def _update(self, x, f, dx, df, dx_norm, df_norm):
pass
class ExcitingMixing(GenericBroyden):
"""
Find a root of a function, using a tuned diagonal Jacobian approximation.
The Jacobian matrix is diagonal and is tuned on each iteration.
.. warning::
This algorithm may be useful for specific problems, but whether
it will work may depend strongly on the problem.
Parameters
----------
%(params_basic)s
alpha : float, optional
Initial Jacobian approximation is (-1/alpha).
alphamax : float, optional
The entries of the diagonal Jacobian are kept in the range
``[alpha, alphamax]``.
%(params_extra)s
"""
def __init__(self, alpha=None, alphamax=1.0):
GenericBroyden.__init__(self)
self.alpha = alpha
self.alphamax = alphamax
self.beta = None
def setup(self, x, F, func):
GenericBroyden.setup(self, x, F, func)
self.beta = self.alpha * np.ones((self.shape[0],), dtype=self.dtype)
def solve(self, f, tol=0):
return -f*self.beta
def matvec(self, f):
return -f/self.beta
def rsolve(self, f, tol=0):
return -f*self.beta.conj()
def rmatvec(self, f):
return -f/self.beta.conj()
def todense(self):
return np.diag(-1/self.beta)
def _update(self, x, f, dx, df, dx_norm, df_norm):
incr = f*self.last_f > 0
self.beta[incr] += self.alpha
self.beta[~incr] = self.alpha
np.clip(self.beta, 0, self.alphamax, out=self.beta)
#------------------------------------------------------------------------------
# Iterative/Krylov approximated Jacobians
#------------------------------------------------------------------------------
class KrylovJacobian(Jacobian):
r"""
Find a root of a function, using Krylov approximation for inverse Jacobian.
This method is suitable for solving large-scale problems.
Parameters
----------
%(params_basic)s
rdiff : float, optional
Relative step size to use in numerical differentiation.
method : {'lgmres', 'gmres', 'bicgstab', 'cgs', 'minres'} or function
Krylov method to use to approximate the Jacobian.
Can be a string, or a function implementing the same interface as
the iterative solvers in `scipy.sparse.linalg`.
The default is `scipy.sparse.linalg.lgmres`.
inner_M : LinearOperator or InverseJacobian
Preconditioner for the inner Krylov iteration.
Note that you can use also inverse Jacobians as (adaptive)
preconditioners. For example,
>>> from scipy.optimize.nonlin import BroydenFirst, KrylovJacobian
>>> from scipy.optimize.nonlin import InverseJacobian
>>> jac = BroydenFirst()
>>> kjac = KrylovJacobian(inner_M=InverseJacobian(jac))
If the preconditioner has a method named 'update', it will be called
as ``update(x, f)`` after each nonlinear step, with ``x`` giving
the current point, and ``f`` the current function value.
inner_tol, inner_maxiter, ...
Parameters to pass on to the \"inner\" Krylov solver.
See `scipy.sparse.linalg.gmres` for details.
outer_k : int, optional
Size of the subspace kept across LGMRES nonlinear iterations.
See `scipy.sparse.linalg.lgmres` for details.
%(params_extra)s
See Also
--------
scipy.sparse.linalg.gmres
scipy.sparse.linalg.lgmres
Notes
-----
This function implements a Newton-Krylov solver. The basic idea is
to compute the inverse of the Jacobian with an iterative Krylov
method. These methods require only evaluating the Jacobian-vector
products, which are conveniently approximated by a finite difference:
.. math:: J v \approx (f(x + \omega*v/|v|) - f(x)) / \omega
Due to the use of iterative matrix inverses, these methods can
deal with large nonlinear problems.
Scipy's `scipy.sparse.linalg` module offers a selection of Krylov
solvers to choose from. The default here is `lgmres`, which is a
variant of restarted GMRES iteration that reuses some of the
information obtained in the previous Newton steps to invert
Jacobians in subsequent steps.
For a review on Newton-Krylov methods, see for example [1]_,
and for the LGMRES sparse inverse method, see [2]_.
References
----------
.. [1] D.A. Knoll and D.E. Keyes, J. Comp. Phys. 193, 357 (2003).
.. [2] A.H. Baker and E.R. Jessup and T. Manteuffel,
SIAM J. Matrix Anal. Appl. 26, 962 (2005).
"""
def __init__(self, rdiff=None, method='lgmres', inner_maxiter=20,
inner_M=None, outer_k=10, **kw):
self.preconditioner = inner_M
self.rdiff = rdiff
self.method = dict(
bicgstab=scipy.sparse.linalg.bicgstab,
gmres=scipy.sparse.linalg.gmres,
lgmres=scipy.sparse.linalg.lgmres,
cgs=scipy.sparse.linalg.cgs,
minres=scipy.sparse.linalg.minres,
).get(method, method)
self.method_kw = dict(maxiter=inner_maxiter, M=self.preconditioner)
if self.method is scipy.sparse.linalg.gmres:
# Replace GMRES's outer iteration with Newton steps
self.method_kw['restrt'] = inner_maxiter
self.method_kw['maxiter'] = 1
elif self.method is scipy.sparse.linalg.lgmres:
self.method_kw['outer_k'] = outer_k
# Replace LGMRES's outer iteration with Newton steps
self.method_kw['maxiter'] = 1
# Carry LGMRES's `outer_v` vectors across nonlinear iterations
self.method_kw.setdefault('outer_v', [])
# But don't carry the corresponding Jacobian*v products, in case
# the Jacobian changes a lot in the nonlinear step
#
# XXX: some trust-region inspired ideas might be more efficient...
# See eg. Brown & Saad. But needs to be implemented separately
# since it's not an inexact Newton method.
self.method_kw.setdefault('store_outer_Av', False)
for key, value in kw.items():
if not key.startswith('inner_'):
raise ValueError("Unknown parameter %s" % key)
self.method_kw[key[6:]] = value
def _update_diff_step(self):
mx = abs(self.x0).max()
mf = abs(self.f0).max()
self.omega = self.rdiff * max(1, mx) / max(1, mf)
def matvec(self, v):
nv = norm(v)
if nv == 0:
return 0*v
sc = self.omega / nv
r = (self.func(self.x0 + sc*v) - self.f0) / sc
if not np.all(np.isfinite(r)) and np.all(np.isfinite(v)):
raise ValueError('Function returned non-finite results')
return r
def solve(self, rhs, tol=0):
if 'tol' in self.method_kw:
sol, info = self.method(self.op, rhs, **self.method_kw)
else:
sol, info = self.method(self.op, rhs, tol=tol, **self.method_kw)
return sol
def update(self, x, f):
self.x0 = x
self.f0 = f
self._update_diff_step()
# Update also the preconditioner, if possible
if self.preconditioner is not None:
if hasattr(self.preconditioner, 'update'):
self.preconditioner.update(x, f)
def setup(self, x, f, func):
Jacobian.setup(self, x, f, func)
self.x0 = x
self.f0 = f
self.op = scipy.sparse.linalg.aslinearoperator(self)
if self.rdiff is None:
self.rdiff = np.finfo(x.dtype).eps ** (1./2)
self._update_diff_step()
# Setup also the preconditioner, if possible
if self.preconditioner is not None:
if hasattr(self.preconditioner, 'setup'):
self.preconditioner.setup(x, f, func)
#------------------------------------------------------------------------------
# Wrapper functions
#------------------------------------------------------------------------------
def _nonlin_wrapper(name, jac):
"""
Construct a solver wrapper with given name and jacobian approx.
It inspects the keyword arguments of ``jac.__init__``, and allows to
use the same arguments in the wrapper function, in addition to the
keyword arguments of `nonlin_solve`
"""
import inspect
args, varargs, varkw, defaults = inspect.getargspec(jac.__init__)
kwargs = list(zip(args[-len(defaults):], defaults))
kw_str = ", ".join(["%s=%r" % (k, v) for k, v in kwargs])
if kw_str:
kw_str = ", " + kw_str
kwkw_str = ", ".join(["%s=%s" % (k, k) for k, v in kwargs])
if kwkw_str:
kwkw_str = kwkw_str + ", "
# Construct the wrapper function so that its keyword arguments
# are visible in pydoc.help etc.
wrapper = """
def %(name)s(F, xin, iter=None %(kw)s, verbose=False, maxiter=None,
f_tol=None, f_rtol=None, x_tol=None, x_rtol=None,
tol_norm=None, line_search='armijo', callback=None, **kw):
jac = %(jac)s(%(kwkw)s **kw)
return nonlin_solve(F, xin, jac, iter, verbose, maxiter,
f_tol, f_rtol, x_tol, x_rtol, tol_norm, line_search,
callback)
"""
wrapper = wrapper % dict(name=name, kw=kw_str, jac=jac.__name__,
kwkw=kwkw_str)
ns = {}
ns.update(globals())
exec_(wrapper, ns)
func = ns[name]
func.__doc__ = jac.__doc__
_set_doc(func)
return func
broyden1 = _nonlin_wrapper('broyden1', BroydenFirst)
broyden2 = _nonlin_wrapper('broyden2', BroydenSecond)
anderson = _nonlin_wrapper('anderson', Anderson)
linearmixing = _nonlin_wrapper('linearmixing', LinearMixing)
diagbroyden = _nonlin_wrapper('diagbroyden', DiagBroyden)
excitingmixing = _nonlin_wrapper('excitingmixing', ExcitingMixing)
newton_krylov = _nonlin_wrapper('newton_krylov', KrylovJacobian)
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