solvers.py

"""
This module contain solvers for all kinds of equations:

    - algebraic or transcendental, use solve()

    - recurrence, use rsolve()

    - differential, use dsolve()

    - nonlinear (numerically), use nsolve()
      (you will need a good starting point)

"""

from sympy import divisors, binomial, expand_func
from sympy.core.assumptions import check_assumptions
from sympy.core.compatibility import (iterable, is_sequence, ordered,
    default_sort_key)
from sympy.core.sympify import sympify
from sympy.core import (S, Add, Symbol, Equality, Dummy, Expr, Mul,
    Pow, Unequality, Wild)
from sympy.core.exprtools import factor_terms
from sympy.core.function import (expand_mul, expand_log,
                          Derivative, AppliedUndef, UndefinedFunction, nfloat,
                          Function, expand_power_exp, _mexpand, expand)
from sympy.integrals.integrals import Integral
from sympy.core.numbers import ilcm, Float, Rational
from sympy.core.relational import Relational
from sympy.core.logic import fuzzy_not
from sympy.core.power import integer_log
from sympy.logic.boolalg import And, Or, BooleanAtom
from sympy.core.basic import preorder_traversal

from sympy.functions import (log, exp, LambertW, cos, sin, tan, acos, asin, atan,
                             Abs, re, im, arg, sqrt, atan2)
from sympy.functions.elementary.trigonometric import (TrigonometricFunction,
                                                      HyperbolicFunction)
from sympy.simplify import (simplify, collect, powsimp, posify,  # type: ignore
    powdenest, nsimplify, denom, logcombine, sqrtdenest, fraction,
    separatevars)
from sympy.simplify.sqrtdenest import sqrt_depth
from sympy.simplify.fu import TR1, TR2i
from sympy.matrices.common import NonInvertibleMatrixError
from sympy.matrices import Matrix, zeros
from sympy.polys import roots, cancel, factor, Poly
from sympy.polys.polyerrors import GeneratorsNeeded, PolynomialError

from sympy.polys.solvers import sympy_eqs_to_ring, solve_lin_sys
from sympy.functions.elementary.piecewise import piecewise_fold, Piecewise

from sympy.utilities.lambdify import lambdify
from sympy.utilities.misc import filldedent
from sympy.utilities.iterables import (cartes, connected_components, flatten,
    generate_bell, uniq, sift)
from sympy.utilities.decorator import conserve_mpmath_dps

from mpmath import findroot

from sympy.solvers.polysys import solve_poly_system
from sympy.solvers.inequalities import reduce_inequalities

from types import GeneratorType
from collections import defaultdict
import warnings


def recast_to_symbols(eqs, symbols):
    """
    Return (e, s, d) where e and s are versions of *eqs* and
    *symbols* in which any non-Symbol objects in *symbols* have
    been replaced with generic Dummy symbols and d is a dictionary
    that can be used to restore the original expressions.

    Examples
    ========

    >>> from sympy.solvers.solvers import recast_to_symbols
    >>> from sympy import symbols, Function
    >>> x, y = symbols('x y')
    >>> fx = Function('f')(x)
    >>> eqs, syms = [fx + 1, x, y], [fx, y]
    >>> e, s, d = recast_to_symbols(eqs, syms); (e, s, d)
    ([_X0 + 1, x, y], [_X0, y], {_X0: f(x)})

    The original equations and symbols can be restored using d:

    >>> assert [i.xreplace(d) for i in eqs] == eqs
    >>> assert [d.get(i, i) for i in s] == syms

    """
    if not iterable(eqs) and iterable(symbols):
        raise ValueError('Both eqs and symbols must be iterable')
    new_symbols = list(symbols)
    swap_sym = {}
    for i, s in enumerate(symbols):
        if not isinstance(s, Symbol) and s not in swap_sym:
            swap_sym[s] = Dummy('X%d' % i)
            new_symbols[i] = swap_sym[s]
    new_f = []
    for i in eqs:
        isubs = getattr(i, 'subs', None)
        if isubs is not None:
            new_f.append(isubs(swap_sym))
        else:
            new_f.append(i)
    swap_sym = {v: k for k, v in swap_sym.items()}
    return new_f, new_symbols, swap_sym


def _ispow(e):
    """Return True if e is a Pow or is exp."""
    return isinstance(e, Expr) and (e.is_Pow or isinstance(e, exp))


def _simple_dens(f, symbols):
    # when checking if a denominator is zero, we can just check the
    # base of powers with nonzero exponents since if the base is zero
    # the power will be zero, too. To keep it simple and fast, we
    # limit simplification to exponents that are Numbers
    dens = set()
    for d in denoms(f, symbols):
        if d.is_Pow and d.exp.is_Number:
            if d.exp.is_zero:
                continue  # foo**0 is never 0
            d = d.base
        dens.add(d)
    return dens


def denoms(eq, *symbols):
    """
    Return (recursively) set of all denominators that appear in *eq*
    that contain any symbol in *symbols*; if *symbols* are not
    provided then all denominators will be returned.

    Examples
    ========

    >>> from sympy.solvers.solvers import denoms
    >>> from sympy.abc import x, y, z

    >>> denoms(x/y)
    {y}

    >>> denoms(x/(y*z))
    {y, z}

    >>> denoms(3/x + y/z)
    {x, z}

    >>> denoms(x/2 + y/z)
    {2, z}

    If *symbols* are provided then only denominators containing
    those symbols will be returned:

    >>> denoms(1/x + 1/y + 1/z, y, z)
    {y, z}

    """

    pot = preorder_traversal(eq)
    dens = set()
    for p in pot:
        # Here p might be Tuple or Relational
        # Expr subtrees (e.g. lhs and rhs) will be traversed after by pot
        if not isinstance(p, Expr):
            continue
        den = denom(p)
        if den is S.One:
            continue
        for d in Mul.make_args(den):
            dens.add(d)
    if not symbols:
        return dens
    elif len(symbols) == 1:
        if iterable(symbols[0]):
            symbols = symbols[0]
    rv = []
    for d in dens:
        free = d.free_symbols
        if any(s in free for s in symbols):
            rv.append(d)
    return set(rv)


def checksol(f, symbol, sol=None, **flags):
    """
    Checks whether sol is a solution of equation f == 0.

    Explanation
    ===========

    Input can be either a single symbol and corresponding value
    or a dictionary of symbols and values. When given as a dictionary
    and flag ``simplify=True``, the values in the dictionary will be
    simplified. *f* can be a single equation or an iterable of equations.
    A solution must satisfy all equations in *f* to be considered valid;
    if a solution does not satisfy any equation, False is returned; if one or
    more checks are inconclusive (and none are False) then None is returned.

    Examples
    ========

    >>> from sympy import symbols
    >>> from sympy.solvers import checksol
    >>> x, y = symbols('x,y')
    >>> checksol(x**4 - 1, x, 1)
    True
    >>> checksol(x**4 - 1, x, 0)
    False
    >>> checksol(x**2 + y**2 - 5**2, {x: 3, y: 4})
    True

    To check if an expression is zero using ``checksol()``, pass it
    as *f* and send an empty dictionary for *symbol*:

    >>> checksol(x**2 + x - x*(x + 1), {})
    True

    None is returned if ``checksol()`` could not conclude.

    flags:
        'numerical=True (default)'
           do a fast numerical check if ``f`` has only one symbol.
        'minimal=True (default is False)'
           a very fast, minimal testing.
        'warn=True (default is False)'
           show a warning if checksol() could not conclude.
        'simplify=True (default)'
           simplify solution before substituting into function and
           simplify the function before trying specific simplifications
        'force=True (default is False)'
           make positive all symbols without assumptions regarding sign.

    """
    from sympy.physics.units import Unit

    minimal = flags.get('minimal', False)

    if sol is not None:
        sol = {symbol: sol}
    elif isinstance(symbol, dict):
        sol = symbol
    else:
        msg = 'Expecting (sym, val) or ({sym: val}, None) but got (%s, %s)'
        raise ValueError(msg % (symbol, sol))

    if iterable(f):
        if not f:
            raise ValueError('no functions to check')
        rv = True
        for fi in f:
            check = checksol(fi, sol, **flags)
            if check:
                continue
            if check is False:
                return False
            rv = None  # don't return, wait to see if there's a False
        return rv

    if isinstance(f, Poly):
        f = f.as_expr()
    elif isinstance(f, (Equality, Unequality)):
        if f.rhs in (S.true, S.false):
            f = f.reversed
        B, E = f.args
        if isinstance(B, BooleanAtom):
            f = f.subs(sol)
            if not f.is_Boolean:
                return
        else:
            f = f.rewrite(Add, evaluate=False)

    if isinstance(f, BooleanAtom):
        return bool(f)
    elif not f.is_Relational and not f:
        return True

    if sol and not f.free_symbols & set(sol.keys()):
        # if f(y) == 0, x=3 does not set f(y) to zero...nor does it not
        return None

    illegal = {S.NaN,
               S.ComplexInfinity,
               S.Infinity,
               S.NegativeInfinity}
    if any(sympify(v).atoms() & illegal for k, v in sol.items()):
        return False

    was = f
    attempt = -1
    numerical = flags.get('numerical', True)
    while 1:
        attempt += 1
        if attempt == 0:
            val = f.subs(sol)
            if isinstance(val, Mul):
                val = val.as_independent(Unit)[0]
            if val.atoms() & illegal:
                return False
        elif attempt == 1:
            if not val.is_number:
                if not val.is_constant(*list(sol.keys()), simplify=not minimal):
                    return False
                # there are free symbols -- simple expansion might work
                _, val = val.as_content_primitive()
                val = _mexpand(val.as_numer_denom()[0], recursive=True)
        elif attempt == 2:
            if minimal:
                return
            if flags.get('simplify', True):
                for k in sol:
                    sol[k] = simplify(sol[k])
            # start over without the failed expanded form, possibly
            # with a simplified solution
            val = simplify(f.subs(sol))
            if flags.get('force', True):
                val, reps = posify(val)
                # expansion may work now, so try again and check
                exval = _mexpand(val, recursive=True)
                if exval.is_number:
                    # we can decide now
                    val = exval
        else:
            # if there are no radicals and no functions then this can't be
            # zero anymore -- can it?
            pot = preorder_traversal(expand_mul(val))
            seen = set()
            saw_pow_func = False
            for p in pot:
                if p in seen:
                    continue
                seen.add(p)
                if p.is_Pow and not p.exp.is_Integer:
                    saw_pow_func = True
                elif p.is_Function:
                    saw_pow_func = True
                elif isinstance(p, UndefinedFunction):
                    saw_pow_func = True
                if saw_pow_func:
                    break
            if saw_pow_func is False:
                return False
            if flags.get('force', True):
                # don't do a zero check with the positive assumptions in place
                val = val.subs(reps)
            nz = fuzzy_not(val.is_zero)
            if nz is not None:
                # issue 5673: nz may be True even when False
                # so these are just hacks to keep a false positive
                # from being returned

                # HACK 1: LambertW (issue 5673)
                if val.is_number and val.has(LambertW):
                    # don't eval this to verify solution since if we got here,
                    # numerical must be False
                    return None

                # add other HACKs here if necessary, otherwise we assume
                # the nz value is correct
                return not nz
            break

        if val == was:
            continue
        elif val.is_Rational:
            return val == 0
        if numerical and val.is_number:
            return (abs(val.n(18).n(12, chop=True)) < 1e-9) is S.true
        was = val

    if flags.get('warn', False):
        warnings.warn("\n\tWarning: could not verify solution %s." % sol)
    # returns None if it can't conclude
    # TODO: improve solution testing


def solve(f, *symbols, **flags):
    r"""
    Algebraically solves equations and systems of equations.

    Explanation
    ===========

    Currently supported:
        - polynomial
        - transcendental
        - piecewise combinations of the above
        - systems of linear and polynomial equations
        - systems containing relational expressions

    Examples
    ========

    The output varies according to the input and can be seen by example:

        >>> from sympy import solve, Poly, Eq, Function, exp
        >>> from sympy.abc import x, y, z, a, b
        >>> f = Function('f')

    Boolean or univariate Relational:

        >>> solve(x < 3)
        (-oo < x) & (x < 3)


    To always get a list of solution mappings, use flag dict=True:

        >>> solve(x - 3, dict=True)
        [{x: 3}]
        >>> sol = solve([x - 3, y - 1], dict=True)
        >>> sol
        [{x: 3, y: 1}]
        >>> sol[0][x]
        3
        >>> sol[0][y]
        1


    To get a list of *symbols* and set of solution(s) use flag set=True:

        >>> solve([x**2 - 3, y - 1], set=True)
        ([x, y], {(-sqrt(3), 1), (sqrt(3), 1)})


    Single expression and single symbol that is in the expression:

        >>> solve(x - y, x)
        [y]
        >>> solve(x - 3, x)
        [3]
        >>> solve(Eq(x, 3), x)
        [3]
        >>> solve(Poly(x - 3), x)
        [3]
        >>> solve(x**2 - y**2, x, set=True)
        ([x], {(-y,), (y,)})
        >>> solve(x**4 - 1, x, set=True)
        ([x], {(-1,), (1,), (-I,), (I,)})

    Single expression with no symbol that is in the expression:

        >>> solve(3, x)
        []
        >>> solve(x - 3, y)
        []

    Single expression with no symbol given. In this case, all free *symbols*
    will be selected as potential *symbols* to solve for. If the equation is
    univariate then a list of solutions is returned; otherwise - as is the case
    when *symbols* are given as an iterable of length greater than 1 - a list of
    mappings will be returned:

        >>> solve(x - 3)
        [3]
        >>> solve(x**2 - y**2)
        [{x: -y}, {x: y}]
        >>> solve(z**2*x**2 - z**2*y**2)
        [{x: -y}, {x: y}, {z: 0}]
        >>> solve(z**2*x - z**2*y**2)
        [{x: y**2}, {z: 0}]

    When an object other than a Symbol is given as a symbol, it is
    isolated algebraically and an implicit solution may be obtained.
    This is mostly provided as a convenience to save you from replacing
    the object with a Symbol and solving for that Symbol. It will only
    work if the specified object can be replaced with a Symbol using the
    subs method:

    >>> solve(f(x) - x, f(x))
    [x]
    >>> solve(f(x).diff(x) - f(x) - x, f(x).diff(x))
    [x + f(x)]
    >>> solve(f(x).diff(x) - f(x) - x, f(x))
    [-x + Derivative(f(x), x)]
    >>> solve(x + exp(x)**2, exp(x), set=True)
    ([exp(x)], {(-sqrt(-x),), (sqrt(-x),)})

    >>> from sympy import Indexed, IndexedBase, Tuple, sqrt
    >>> A = IndexedBase('A')
    >>> eqs = Tuple(A[1] + A[2] - 3, A[1] - A[2] + 1)
    >>> solve(eqs, eqs.atoms(Indexed))
    {A[1]: 1, A[2]: 2}

        * To solve for a symbol implicitly, use implicit=True:

            >>> solve(x + exp(x), x)
            [-LambertW(1)]
            >>> solve(x + exp(x), x, implicit=True)
            [-exp(x)]

        * It is possible to solve for anything that can be targeted with
          subs:

            >>> solve(x + 2 + sqrt(3), x + 2)
            [-sqrt(3)]
            >>> solve((x + 2 + sqrt(3), x + 4 + y), y, x + 2)
            {y: -2 + sqrt(3), x + 2: -sqrt(3)}

        * Nothing heroic is done in this implicit solving so you may end up
          with a symbol still in the solution:

            >>> eqs = (x*y + 3*y + sqrt(3), x + 4 + y)
            >>> solve(eqs, y, x + 2)
            {y: -sqrt(3)/(x + 3), x + 2: -2*x/(x + 3) - 6/(x + 3) + sqrt(3)/(x + 3)}
            >>> solve(eqs, y*x, x)
            {x: -y - 4, x*y: -3*y - sqrt(3)}

        * If you attempt to solve for a number remember that the number
          you have obtained does not necessarily mean that the value is
          equivalent to the expression obtained:

            >>> solve(sqrt(2) - 1, 1)
            [sqrt(2)]
            >>> solve(x - y + 1, 1)  # /!\ -1 is targeted, too
            [x/(y - 1)]
            >>> [_.subs(z, -1) for _ in solve((x - y + 1).subs(-1, z), 1)]
            [-x + y]

        * To solve for a function within a derivative, use ``dsolve``.

    Single expression and more than one symbol:

        * When there is a linear solution:

            >>> solve(x - y**2, x, y)
            [(y**2, y)]
            >>> solve(x**2 - y, x, y)
            [(x, x**2)]
            >>> solve(x**2 - y, x, y, dict=True)
            [{y: x**2}]

        * When undetermined coefficients are identified:

            * That are linear:

                >>> solve((a + b)*x - b + 2, a, b)
                {a: -2, b: 2}

            * That are nonlinear:

                >>> solve((a + b)*x - b**2 + 2, a, b, set=True)
                ([a, b], {(-sqrt(2), sqrt(2)), (sqrt(2), -sqrt(2))})

        * If there is no linear solution, then the first successful
          attempt for a nonlinear solution will be returned:

            >>> solve(x**2 - y**2, x, y, dict=True)
            [{x: -y}, {x: y}]
            >>> solve(x**2 - y**2/exp(x), x, y, dict=True)
            [{x: 2*LambertW(-y/2)}, {x: 2*LambertW(y/2)}]
            >>> solve(x**2 - y**2/exp(x), y, x)
            [(-x*sqrt(exp(x)), x), (x*sqrt(exp(x)), x)]

    Iterable of one or more of the above:

        * Involving relationals or bools:

            >>> solve([x < 3, x - 2])
            Eq(x, 2)
            >>> solve([x > 3, x - 2])
            False

        * When the system is linear:

            * With a solution:

                >>> solve([x - 3], x)
                {x: 3}
                >>> solve((x + 5*y - 2, -3*x + 6*y - 15), x, y)
                {x: -3, y: 1}
                >>> solve((x + 5*y - 2, -3*x + 6*y - 15), x, y, z)
                {x: -3, y: 1}
                >>> solve((x + 5*y - 2, -3*x + 6*y - z), z, x, y)
                {x: 2 - 5*y, z: 21*y - 6}

            * Without a solution:

                >>> solve([x + 3, x - 3])
                []

        * When the system is not linear:

            >>> solve([x**2 + y -2, y**2 - 4], x, y, set=True)
            ([x, y], {(-2, -2), (0, 2), (2, -2)})

        * If no *symbols* are given, all free *symbols* will be selected and a
          list of mappings returned:

            >>> solve([x - 2, x**2 + y])
            [{x: 2, y: -4}]
            >>> solve([x - 2, x**2 + f(x)], {f(x), x})
            [{x: 2, f(x): -4}]

        * If any equation does not depend on the symbol(s) given, it will be
          eliminated from the equation set and an answer may be given
          implicitly in terms of variables that were not of interest:

            >>> solve([x - y, y - 3], x)
            {x: y}

    **Additional Examples**

    ``solve()`` with check=True (default) will run through the symbol tags to
    elimate unwanted solutions. If no assumptions are included, all possible
    solutions will be returned:

        >>> from sympy import Symbol, solve
        >>> x = Symbol("x")
        >>> solve(x**2 - 1)
        [-1, 1]

    By using the positive tag, only one solution will be returned:

        >>> pos = Symbol("pos", positive=True)
        >>> solve(pos**2 - 1)
        [1]

    Assumptions are not checked when ``solve()`` input involves
    relationals or bools.

    When the solutions are checked, those that make any denominator zero
    are automatically excluded. If you do not want to exclude such solutions,
    then use the check=False option:

        >>> from sympy import sin, limit
        >>> solve(sin(x)/x)  # 0 is excluded
        [pi]

    If check=False, then a solution to the numerator being zero is found: x = 0.
    In this case, this is a spurious solution since $\sin(x)/x$ has the well
    known limit (without dicontinuity) of 1 at x = 0:

        >>> solve(sin(x)/x, check=False)
        [0, pi]

    In the following case, however, the limit exists and is equal to the
    value of x = 0 that is excluded when check=True:

        >>> eq = x**2*(1/x - z**2/x)
        >>> solve(eq, x)
        []
        >>> solve(eq, x, check=False)
        [0]
        >>> limit(eq, x, 0, '-')
        0
        >>> limit(eq, x, 0, '+')
        0

    **Disabling High-Order Explicit Solutions**

    When solving polynomial expressions, you might not want explicit solutions
    (which can be quite long). If the expression is univariate, ``CRootOf``
    instances will be returned instead:

        >>> solve(x**3 - x + 1)
        [-1/((-1/2 - sqrt(3)*I/2)*(3*sqrt(69)/2 + 27/2)**(1/3)) - (-1/2 -
        sqrt(3)*I/2)*(3*sqrt(69)/2 + 27/2)**(1/3)/3, -(-1/2 +
        sqrt(3)*I/2)*(3*sqrt(69)/2 + 27/2)**(1/3)/3 - 1/((-1/2 +
        sqrt(3)*I/2)*(3*sqrt(69)/2 + 27/2)**(1/3)), -(3*sqrt(69)/2 +
        27/2)**(1/3)/3 - 1/(3*sqrt(69)/2 + 27/2)**(1/3)]
        >>> solve(x**3 - x + 1, cubics=False)
        [CRootOf(x**3 - x + 1, 0),
         CRootOf(x**3 - x + 1, 1),
         CRootOf(x**3 - x + 1, 2)]

    If the expression is multivariate, no solution might be returned:

        >>> solve(x**3 - x + a, x, cubics=False)
        []

    Sometimes solutions will be obtained even when a flag is False because the
    expression could be factored. In the following example, the equation can
    be factored as the product of a linear and a quadratic factor so explicit
    solutions (which did not require solving a cubic expression) are obtained:

        >>> eq = x**3 + 3*x**2 + x - 1
        >>> solve(eq, cubics=False)
        [-1, -1 + sqrt(2), -sqrt(2) - 1]

    **Solving Equations Involving Radicals**

    Because of SymPy's use of the principle root, some solutions
    to radical equations will be missed unless check=False:

        >>> from sympy import root
        >>> eq = root(x**3 - 3*x**2, 3) + 1 - x
        >>> solve(eq)
        []
        >>> solve(eq, check=False)
        [1/3]

    In the above example, there is only a single solution to the
    equation. Other expressions will yield spurious roots which
    must be checked manually; roots which give a negative argument
    to odd-powered radicals will also need special checking:

        >>> from sympy import real_root, S
        >>> eq = root(x, 3) - root(x, 5) + S(1)/7
        >>> solve(eq)  # this gives 2 solutions but misses a 3rd
        [CRootOf(7*x**5 - 7*x**3 + 1, 1)**15,
        CRootOf(7*x**5 - 7*x**3 + 1, 2)**15]
        >>> sol = solve(eq, check=False)
        >>> [abs(eq.subs(x,i).n(2)) for i in sol]
        [0.48, 0.e-110, 0.e-110, 0.052, 0.052]

    The first solution is negative so ``real_root`` must be used to see that it
    satisfies the expression:

        >>> abs(real_root(eq.subs(x, sol[0])).n(2))
        0.e-110

    If the roots of the equation are not real then more care will be
    necessary to find the roots, especially for higher order equations.
    Consider the following expression:

        >>> expr = root(x, 3) - root(x, 5)

    We will construct a known value for this expression at x = 3 by selecting
    the 1-th root for each radical:

        >>> expr1 = root(x, 3, 1) - root(x, 5, 1)
        >>> v = expr1.subs(x, -3)

    The ``solve`` function is unable to find any exact roots to this equation:

        >>> eq = Eq(expr, v); eq1 = Eq(expr1, v)
        >>> solve(eq, check=False), solve(eq1, check=False)
        ([], [])

    The function ``unrad``, however, can be used to get a form of the equation
    for which numerical roots can be found:

        >>> from sympy.solvers.solvers import unrad
        >>> from sympy import nroots
        >>> e, (p, cov) = unrad(eq)
        >>> pvals = nroots(e)
        >>> inversion = solve(cov, x)[0]
        >>> xvals = [inversion.subs(p, i) for i in pvals]

    Although ``eq`` or ``eq1`` could have been used to find ``xvals``, the
    solution can only be verified with ``expr1``:

        >>> z = expr - v
        >>> [xi.n(chop=1e-9) for xi in xvals if abs(z.subs(x, xi).n()) < 1e-9]
        []
        >>> z1 = expr1 - v
        >>> [xi.n(chop=1e-9) for xi in xvals if abs(z1.subs(x, xi).n()) < 1e-9]
        [-3.0]

    Parameters
    ==========

    f :
        - a single Expr or Poly that must be zero
        - an Equality
        - a Relational expression
        - a Boolean
        - iterable of one or more of the above

    symbols : (object(s) to solve for) specified as
        - none given (other non-numeric objects will be used)
        - single symbol
        - denested list of symbols
          (e.g., ``solve(f, x, y)``)
        - ordered iterable of symbols
          (e.g., ``solve(f, [x, y])``)

    flags :
        dict=True (default is False)
            Return list (perhaps empty) of solution mappings.
        set=True (default is False)
            Return list of symbols and set of tuple(s) of solution(s).
        exclude=[] (default)
            Do not try to solve for any of the free symbols in exclude;
            if expressions are given, the free symbols in them will
            be extracted automatically.
        check=True (default)
            If False, do not do any testing of solutions. This can be
            useful if you want to include solutions that make any
            denominator zero.
        numerical=True (default)
            Do a fast numerical check if *f* has only one symbol.
        minimal=True (default is False)
            A very fast, minimal testing.
        warn=True (default is False)
            Show a warning if ``checksol()`` could not conclude.
        simplify=True (default)
            Simplify all but polynomials of order 3 or greater before
            returning them and (if check is not False) use the
            general simplify function on the solutions and the
            expression obtained when they are substituted into the
            function which should be zero.
        force=True (default is False)
            Make positive all symbols without assumptions regarding sign.
        rational=True (default)
            Recast Floats as Rational; if this option is not used, the
            system containing Floats may fail to solve because of issues
            with polys. If rational=None, Floats will be recast as
            rationals but the answer will be recast as Floats. If the
            flag is False then nothing will be done to the Floats.
        manual=True (default is False)
            Do not use the polys/matrix method to solve a system of
            equations, solve them one at a time as you might "manually."
        implicit=True (default is False)
            Allows ``solve`` to return a solution for a pattern in terms of
            other functions that contain that pattern; this is only
            needed if the pattern is inside of some invertible function
            like cos, exp, ect.
        particular=True (default is False)
            Instructs ``solve`` to try to find a particular solution to a linear
            system with as many zeros as possible; this is very expensive.
        quick=True (default is False)
            When using particular=True, use a fast heuristic to find a
            solution with many zeros (instead of using the very slow method
            guaranteed to find the largest number of zeros possible).
        cubics=True (default)
            Return explicit solutions when cubic expressions are encountered.
        quartics=True (default)
            Return explicit solutions when quartic expressions are encountered.
        quintics=True (default)
            Return explicit solutions (if possible) when quintic expressions
            are encountered.

    See Also
    ========

    rsolve: For solving recurrence relationships
    dsolve: For solving differential equations

    """
    # keeping track of how f was passed since if it is a list
    # a dictionary of results will be returned.
    ###########################################################################

    def _sympified_list(w):
        return list(map(sympify, w if iterable(w) else [w]))
    bare_f = not iterable(f)
    ordered_symbols = (symbols and
                       symbols[0] and
                       (isinstance(symbols[0], Symbol) or
                        is_sequence(symbols[0],
                        include=GeneratorType)
                       )
                      )
    f, symbols = (_sympified_list(w) for w in [f, symbols])
    if isinstance(f, list):
        f = [s for s in f if s is not S.true and s is not True]
    implicit = flags.get('implicit', False)

    # preprocess symbol(s)
    ###########################################################################
    if not symbols:
        # get symbols from equations
        symbols = set().union(*[fi.free_symbols for fi in f])
        if len(symbols) < len(f):
            for fi in f:
                pot = preorder_traversal(fi)
                for p in pot:
                    if isinstance(p, AppliedUndef):
                        flags['dict'] = True  # better show symbols
                        symbols.add(p)
                        pot.skip()  # don't go any deeper
        symbols = list(symbols)

        ordered_symbols = False
    elif len(symbols) == 1 and iterable(symbols[0]):
        symbols = symbols[0]

    # remove symbols the user is not interested in
    exclude = flags.pop('exclude', set())
    if exclude:
        if isinstance(exclude, Expr):
            exclude = [exclude]
        exclude = set().union(*[e.free_symbols for e in sympify(exclude)])
    symbols = [s for s in symbols if s not in exclude]


    # preprocess equation(s)
    ###########################################################################
    for i, fi in enumerate(f):
        if isinstance(fi, (Equality, Unequality)):
            if 'ImmutableDenseMatrix' in [type(a).__name__ for a in fi.args]:
                fi = fi.lhs - fi.rhs
            else:
                L, R = fi.args
                if isinstance(R, BooleanAtom):
                    L, R = R, L
                if isinstance(L, BooleanAtom):
                    if isinstance(fi, Unequality):
                        L = ~L
                    if R.is_Relational:
                        fi = ~R if L is S.false else R
                    elif R.is_Symbol:
                        return L
                    elif R.is_Boolean and (~R).is_Symbol:
                        return ~L
                    else:
                        raise NotImplementedError(filldedent('''
                            Unanticipated argument of Eq when other arg
                            is True or False.
                        '''))
                else:
                    fi = fi.rewrite(Add, evaluate=False)
            f[i] = fi

        if fi.is_Relational:
            return reduce_inequalities(f, symbols=symbols)

        if isinstance(fi, Poly):
            f[i] = fi.as_expr()

        # rewrite hyperbolics in terms of exp
        f[i] = f[i].replace(lambda w: isinstance(w, HyperbolicFunction),
                lambda w: w.rewrite(exp))

        # if we have a Matrix, we need to iterate over its elements again
        if f[i].is_Matrix:
            bare_f = False
            f.extend(list(f[i]))
            f[i] = S.Zero

        # if we can split it into real and imaginary parts then do so
        freei = f[i].free_symbols
        if freei and all(s.is_extended_real or s.is_imaginary for s in freei):
            fr, fi = f[i].as_real_imag()
            # accept as long as new re, im, arg or atan2 are not introduced
            had = f[i].atoms(re, im, arg, atan2)
            if fr and fi and fr != fi and not any(
                    i.atoms(re, im, arg, atan2) - had for i in (fr, fi)):
                if bare_f:
                    bare_f = False
                f[i: i + 1] = [fr, fi]

    # real/imag handling -----------------------------
    if any(isinstance(fi, (bool, BooleanAtom)) for fi in f):
        if flags.get('set', False):
            return [], set()
        return []

    for i, fi in enumerate(f):
        # Abs
        while True:
            was = fi
            fi = fi.replace(Abs, lambda arg:
                separatevars(Abs(arg)).rewrite(Piecewise) if arg.has(*symbols)
                else Abs(arg))
            if was == fi:
                break

        for e in fi.find(Abs):
            if e.has(*symbols):
                raise NotImplementedError('solving %s when the argument '
                    'is not real or imaginary.' % e)

        # arg
        fi = fi.replace(arg, lambda a: arg(a).rewrite(atan2).rewrite(atan))

        # save changes
        f[i] = fi

    # see if re(s) or im(s) appear
    freim = [fi for fi in f if fi.has(re, im)]
    if freim:
        irf = []
        for s in symbols:
            if s.is_real or s.is_imaginary:
                continue  # neither re(x) nor im(x) will appear
            # if re(s) or im(s) appear, the auxiliary equation must be present
            if any(fi.has(re(s), im(s)) for fi in freim):
                irf.append((s, re(s) + S.ImaginaryUnit*im(s)))
        if irf:
            for s, rhs in irf:
                for i, fi in enumerate(f):
                    f[i] = fi.xreplace({s: rhs})
                f.append(s - rhs)
                symbols.extend([re(s), im(s)])
            if bare_f:
                bare_f = False
            flags['dict'] = True
    # end of real/imag handling  -----------------------------

    symbols = list(uniq(symbols))
    if not ordered_symbols:
        # we do this to make the results returned canonical in case f
        # contains a system of nonlinear equations; all other cases should
        # be unambiguous
        symbols = sorted(symbols, key=default_sort_key)

    # we can solve for non-symbol entities by replacing them with Dummy symbols
    f, symbols, swap_sym = recast_to_symbols(f, symbols)

    # this is needed in the next two events
    symset = set(symbols)

    # get rid of equations that have no symbols of interest; we don't
    # try to solve them because the user didn't ask and they might be
    # hard to solve; this means that solutions may be given in terms
    # of the eliminated equations e.g. solve((x-y, y-3), x) -> {x: y}
    newf = []
    for fi in f:
        # let the solver handle equations that..
        # - have no symbols but are expressions
        # - have symbols of interest
        # - have no symbols of interest but are constant
        # but when an expression is not constant and has no symbols of
        # interest, it can't change what we obtain for a solution from
        # the remaining equations so we don't include it; and if it's
        # zero it can be removed and if it's not zero, there is no
        # solution for the equation set as a whole
        #
        # The reason for doing this filtering is to allow an answer
        # to be obtained to queries like solve((x - y, y), x); without
        # this mod the return value is []
        ok = False
        if fi.free_symbols & symset:
            ok = True
        else:
            if fi.is_number:
                if fi.is_Number:
                    if fi.is_zero:
                        continue
                    return []
                ok = True
            else:
                if fi.is_constant():
                    ok = True
        if ok:
            newf.append(fi)
    if not newf:
        return []
    f = newf
    del newf

    # mask off any Object that we aren't going to invert: Derivative,
    # Integral, etc... so that solving for anything that they contain will
    # give an implicit solution
    seen = set()
    non_inverts = set()
    for fi in f:
        pot = preorder_traversal(fi)
        for p in pot:
            if not isinstance(p, Expr) or isinstance(p, Piecewise):
                pass
            elif (isinstance(p, bool) or
                    not p.args or
                    p in symset or
                    p.is_Add or p.is_Mul or
                    p.is_Pow and not implicit or
                    p.is_Function and not implicit) and p.func not in (re, im):
                continue
            elif not p in seen:
                seen.add(p)
                if p.free_symbols & symset:
                    non_inverts.add(p)
                else:
                    continue
            pot.skip()
    del seen
    non_inverts = dict(list(zip(non_inverts, [Dummy() for _ in non_inverts])))
    f = [fi.subs(non_inverts) for fi in f]

    # Both xreplace and subs are needed below: xreplace to force substitution
    # inside Derivative, subs to handle non-straightforward substitutions
    non_inverts = [(v, k.xreplace(swap_sym).subs(swap_sym)) for k, v in non_inverts.items()]

    # rationalize Floats
    floats = False
    if flags.get('rational', True) is not False:
        for i, fi in enumerate(f):
            if fi.has(Float):
                floats = True
                f[i] = nsimplify(fi, rational=True)

    # capture any denominators before rewriting since
    # they may disappear after the rewrite, e.g. issue 14779
    flags['_denominators'] = _simple_dens(f[0], symbols)
    # Any embedded piecewise functions need to be brought out to the
    # top level so that the appropriate strategy gets selected.
    # However, this is necessary only if one of the piecewise
    # functions depends on one of the symbols we are solving for.
    def _has_piecewise(e):
        if e.is_Piecewise:
            return e.has(*symbols)
        return any([_has_piecewise(a) for a in e.args])
    for i, fi in enumerate(f):
        if _has_piecewise(fi):
            f[i] = piecewise_fold(fi)

    #
    # try to get a solution
    ###########################################################################
    if bare_f:
        solution = _solve(f[0], *symbols, **flags)
    else:
        solution = _solve_system(f, symbols, **flags)

    #
    # postprocessing
    ###########################################################################
    # Restore masked-off objects
    if non_inverts:

        def _do_dict(solution):
            return {k: v.subs(non_inverts) for k, v in
                         solution.items()}
        for i in range(1):
            if isinstance(solution, dict):
                solution = _do_dict(solution)
                break
            elif solution and isinstance(solution, list):
                if isinstance(solution[0], dict):
                    solution = [_do_dict(s) for s in solution]
                    break
                elif isinstance(solution[0], tuple):
                    solution = [tuple([v.subs(non_inverts) for v in s]) for s
                                in solution]
                    break
                else:
                    solution = [v.subs(non_inverts) for v in solution]
                    break
            elif not solution:
                break
        else:
            raise NotImplementedError(filldedent('''
                            no handling of %s was implemented''' % solution))

    # Restore original "symbols" if a dictionary is returned.
    # This is not necessary for
    #   - the single univariate equation case
    #     since the symbol will have been removed from the solution;
    #   - the nonlinear poly_system since that only supports zero-dimensional
    #     systems and those results come back as a list
    #
    # ** unless there were Derivatives with the symbols, but those were handled
    #    above.
    if swap_sym:
        symbols = [swap_sym.get(k, k) for k in symbols]
        if isinstance(solution, dict):
            solution = {swap_sym.get(k, k): v.subs(swap_sym)
                             for k, v in solution.items()}
        elif solution and isinstance(solution, list) and isinstance(solution[0], dict):
            for i, sol in enumerate(solution):
                solution[i] = {swap_sym.get(k, k): v.subs(swap_sym)
                              for k, v in sol.items()}

    # undo the dictionary solutions returned when the system was only partially
    # solved with poly-system if all symbols are present
    if (
            not flags.get('dict', False) and
            solution and
            ordered_symbols and
            not isinstance(solution, dict) and
            all(isinstance(sol, dict) for sol in solution)
    ):
        solution = [tuple([r.get(s, s) for s in symbols]) for r in solution]

    # Get assumptions about symbols, to filter solutions.
    # Note that if assumptions about a solution can't be verified, it is still
    # returned.
    check = flags.get('check', True)

    # restore floats
    if floats and solution and flags.get('rational', None) is None:
        solution = nfloat(solution, exponent=False)

    if check and solution:  # assumption checking

        warn = flags.get('warn', False)
        got_None = []  # solutions for which one or more symbols gave None
        no_False = []  # solutions for which no symbols gave False
        if isinstance(solution, tuple):
            # this has already been checked and is in as_set form
            return solution
        elif isinstance(solution, list):
            if isinstance(solution[0], tuple):
                for sol in solution:
                    for symb, val in zip(symbols, sol):
                        test = check_assumptions(val, **symb.assumptions0)
                        if test is False:
                            break
                        if test is None:
                            got_None.append(sol)
                    else:
                        no_False.append(sol)
            elif isinstance(solution[0], dict):
                for sol in solution:
                    a_None = False
                    for symb, val in sol.items():
                        test = check_assumptions(val, **symb.assumptions0)
                        if test:
                            continue
                        if test is False:
                            break
                        a_None = True
                    else:
                        no_False.append(sol)
                        if a_None:
                            got_None.append(sol)
            else:  # list of expressions
                for sol in solution:
                    test = check_assumptions(sol, **symbols[0].assumptions0)
                    if test is False:
                        continue
                    no_False.append(sol)
                    if test is None:
                        got_None.append(sol)

        elif isinstance(solution, dict):
            a_None = False
            for symb, val in solution.items():
                test = check_assumptions(val, **symb.assumptions0)
                if test:
                    continue
                if test is False:
                    no_False = None
                    break
                a_None = True
            else:
                no_False = solution
                if a_None:
                    got_None.append(solution)

        elif isinstance(solution, (Relational, And, Or)):
            if len(symbols) != 1:
                raise ValueError("Length should be 1")
            if warn and symbols[0].assumptions0:
                warnings.warn(filldedent("""
                    \tWarning: assumptions about variable '%s' are
                    not handled currently.""" % symbols[0]))
            # TODO: check also variable assumptions for inequalities

        else:
            raise TypeError('Unrecognized solution')  # improve the checker

        solution = no_False
        if warn and got_None:
            warnings.warn(filldedent("""
                \tWarning: assumptions concerning following solution(s)
                can't be checked:""" + '\n\t' +
                ', '.join(str(s) for s in got_None)))

    #
    # done
    ###########################################################################

    as_dict = flags.get('dict', False)
    as_set = flags.get('set', False)

    if not as_set and isinstance(solution, list):
        # Make sure that a list of solutions is ordered in a canonical way.
        solution.sort(key=default_sort_key)

    if not as_dict and not as_set:
        return solution or []

    # return a list of mappings or []
    if not solution:
        solution = []
    else:
        if isinstance(solution, dict):
            solution = [solution]
        elif iterable(solution[0]):
            solution = [dict(list(zip(symbols, s))) for s in solution]
        elif isinstance(solution[0], dict):
            pass
        else:
            if len(symbols) != 1:
                raise ValueError("Length should be 1")
            solution = [{symbols[0]: s} for s in solution]
    if as_dict:
        return solution
    assert as_set
    if not solution:
        return [], set()
    k = list(ordered(solution[0].keys()))
    return k, {tuple([s[ki] for ki in k]) for s in solution}


def _solve(f, *symbols, **flags):
    """
    Return a checked solution for *f* in terms of one or more of the
    symbols. A list should be returned except for the case when a linear
    undetermined-coefficients equation is encountered (in which case
    a dictionary is returned).

    If no method is implemented to solve the equation, a NotImplementedError
    will be raised. In the case that conversion of an expression to a Poly
    gives None a ValueError will be raised.

    """

    not_impl_msg = "No algorithms are implemented to solve equation %s"

    if len(symbols) != 1:
        soln = None
        free = f.free_symbols
        ex = free - set(symbols)
        if len(ex) != 1:
            ind, dep = f.as_independent(*symbols)
            ex = ind.free_symbols & dep.free_symbols
        if len(ex) == 1:
            ex = ex.pop()
            try:
                # soln may come back as dict, list of dicts or tuples, or
                # tuple of symbol list and set of solution tuples
                soln = solve_undetermined_coeffs(f, symbols, ex, **flags)
            except NotImplementedError:
                pass
        if soln:
            if flags.get('simplify', True):
                if isinstance(soln, dict):
                    for k in soln:
                        soln[k] = simplify(soln[k])
                elif isinstance(soln, list):
                    if isinstance(soln[0], dict):
                        for d in soln:
                            for k in d:
                                d[k] = simplify(d[k])
                    elif isinstance(soln[0], tuple):
                        soln = [tuple(simplify(i) for i in j) for j in soln]
                    else:
                        raise TypeError('unrecognized args in list')
                elif isinstance(soln, tuple):
                    sym, sols = soln
                    soln = sym, {tuple(simplify(i) for i in j) for j in sols}
                else:
                    raise TypeError('unrecognized solution type')
            return soln
        # find first successful solution
        failed = []
        got_s = set()
        result = []
        for s in symbols:
            xi, v = solve_linear(f, symbols=[s])
            if xi == s:
                # no need to check but we should simplify if desired
                if flags.get('simplify', True):
                    v = simplify(v)
                vfree = v.free_symbols
                if got_s and any([ss in vfree for ss in got_s]):
                    # sol depends on previously solved symbols: discard it
                    continue
                got_s.add(xi)
                result.append({xi: v})
            elif xi:  # there might be a non-linear solution if xi is not 0
                failed.append(s)
        if not failed:
            return result
        for s in failed:
            try:
                soln = _solve(f, s, **flags)
                for sol in soln:
                    if got_s and any([ss in sol.free_symbols for ss in got_s]):
                        # sol depends on previously solved symbols: discard it
                        continue
                    got_s.add(s)
                    result.append({s: sol})
            except NotImplementedError:
                continue
        if got_s:
            return result
        else:
            raise NotImplementedError(not_impl_msg % f)
    symbol = symbols[0]

    #expand binomials only if it has the unknown symbol
    f = f.replace(lambda e: isinstance(e, binomial) and e.has(symbol),
        lambda e: expand_func(e))

    # /!\ capture this flag then set it to False so that no checking in
    # recursive calls will be done; only the final answer is checked
    flags['check'] = checkdens = check = flags.pop('check', True)

    # build up solutions if f is a Mul
    if f.is_Mul:
        result = set()
        for m in f.args:
            if m in {S.NegativeInfinity, S.ComplexInfinity, S.Infinity}:
                result = set()
                break
            soln = _solve(m, symbol, **flags)
            result.update(set(soln))
        result = list(result)
        if check:
            # all solutions have been checked but now we must
            # check that the solutions do not set denominators
            # in any factor to zero
            dens = flags.get('_denominators', _simple_dens(f, symbols))
            result = [s for s in result if
                all(not checksol(den, {symbol: s}, **flags) for den in
                dens)]
        # set flags for quick exit at end; solutions for each
        # factor were already checked and simplified
        check = False
        flags['simplify'] = False

    elif f.is_Piecewise:
        result = set()
        for i, (expr, cond) in enumerate(f.args):
            if expr.is_zero:
                raise NotImplementedError(
                    'solve cannot represent interval solutions')
            candidates = _solve(expr, symbol, **flags)
            # the explicit condition for this expr is the current cond
            # and none of the previous conditions
            args = [~c for _, c in f.args[:i]] + [cond]
            cond = And(*args)
            for candidate in candidates:
                if candidate in result:
                    # an unconditional value was already there
                    continue
                try:
                    v = cond.subs(symbol, candidate)
                    _eval_simplify = getattr(v, '_eval_simplify', None)
                    if _eval_simplify is not None:
                        # unconditionally take the simpification of v
                        v = _eval_simplify(ratio=2, measure=lambda x: 1)
                except TypeError:
                    # incompatible type with condition(s)
                    continue
                if v == False:
                    continue
                if v == True:
                    result.add(candidate)
                else:
                    result.add(Piecewise(
                        (candidate, v),
                        (S.NaN, True)))
        # set flags for quick exit at end; solutions for each
        # piece were already checked and simplified
        check = False
        flags['simplify'] = False
    else:
        # first see if it really depends on symbol and whether there
        # is only a linear solution
        f_num, sol = solve_linear(f, symbols=symbols)
        if f_num.is_zero or sol is S.NaN:
            return []
        elif f_num.is_Symbol:
            # no need to check but simplify if desired
            if flags.get('simplify', True):
                sol = simplify(sol)
            return [sol]

        poly = None
        # check for a single non-symbol generator
        dums = f_num.atoms(Dummy)
        D = f_num.replace(
            lambda i: isinstance(i, Add) and symbol in i.free_symbols,
            lambda i: Dummy())
        if not D.is_Dummy:
            dgen = D.atoms(Dummy) - dums
            if len(dgen) == 1:
                d = dgen.pop()
                w = Wild('g')
                gen = f_num.match(D.xreplace({d: w}))[w]
                spart = gen.as_independent(symbol)[1].as_base_exp()[0]
                if spart == symbol:
                    try:
                        poly = Poly(f_num, spart)
                    except PolynomialError:
                        pass

        result = False  # no solution was obtained
        msg = ''  # there is no failure message

        # Poly is generally robust enough to convert anything to
        # a polynomial and tell us the different generators that it
        # contains, so we will inspect the generators identified by
        # polys to figure out what to do.

        # try to identify a single generator that will allow us to solve this
        # as a polynomial, followed (perhaps) by a change of variables if the
        # generator is not a symbol

        try:
            if poly is None:
                poly = Poly(f_num)
            if poly is None:
                raise ValueError('could not convert %s to Poly' % f_num)
        except GeneratorsNeeded:
            simplified_f = simplify(f_num)
            if simplified_f != f_num:
                return _solve(simplified_f, symbol, **flags)
            raise ValueError('expression appears to be a constant')

        gens = [g for g in poly.gens if g.has(symbol)]

        def _as_base_q(x):
            """Return (b**e, q) for x = b**(p*e/q) where p/q is the leading
            Rational of the exponent of x, e.g. exp(-2*x/3) -> (exp(x), 3)
            """
            b, e = x.as_base_exp()
            if e.is_Rational:
                return b, e.q
            if not e.is_Mul:
                return x, 1
            c, ee = e.as_coeff_Mul()
            if c.is_Rational and c is not S.One:  # c could be a Float
                return b**ee, c.q
            return x, 1

        if len(gens) > 1:
            # If there is more than one generator, it could be that the
            # generators have the same base but different powers, e.g.
            #   >>> Poly(exp(x) + 1/exp(x))
            #   Poly(exp(-x) + exp(x), exp(-x), exp(x), domain='ZZ')
            #
            # If unrad was not disabled then there should be no rational
            # exponents appearing as in
            #   >>> Poly(sqrt(x) + sqrt(sqrt(x)))
            #   Poly(sqrt(x) + x**(1/4), sqrt(x), x**(1/4), domain='ZZ')

            bases, qs = list(zip(*[_as_base_q(g) for g in gens]))
            bases = set(bases)

            if len(bases) > 1 or not all(q == 1 for q in qs):
                funcs = {b for b in bases if b.is_Function}

                trig = {_ for _ in funcs if
                    isinstance(_, TrigonometricFunction)}
                other = funcs - trig
                if not other and len(funcs.intersection(trig)) > 1:
                    newf = None
                    if f_num.is_Add and len(f_num.args) == 2:
                        # check for sin(x)**p = cos(x)**p
                        _args = f_num.args
                        t = a, b = [i.atoms(Function).intersection(
                            trig) for i in _args]
                        if all(len(i) == 1 for i in t):
                            a, b = [i.pop() for i in t]
                            if isinstance(a, cos):
                                a, b = b, a
                                _args = _args[::-1]
                            if isinstance(a, sin) and isinstance(b, cos
                                    ) and a.args[0] == b.args[0]:
                                # sin(x) + cos(x) = 0 -> tan(x) + 1 = 0
                                newf, _d = (TR2i(_args[0]/_args[1]) + 1
                                    ).as_numer_denom()
                                if not _d.is_Number:
                                    newf = None
                    if newf is None:
                        newf = TR1(f_num).rewrite(tan)
                    if newf != f_num:
                        # don't check the rewritten form --check
                        # solutions in the un-rewritten form below
                        flags['check'] = False
                        result = _solve(newf, symbol, **flags)
                        flags['check'] = check

                # just a simple case - see if replacement of single function
                # clears all symbol-dependent functions, e.g.
                # log(x) - log(log(x) - 1) - 3 can be solved even though it has
                # two generators.

                if result is False and funcs:
                    funcs = list(ordered(funcs))  # put shallowest function first
                    f1 = funcs[0]
                    t = Dummy('t')
                    # perform the substitution
                    ftry = f_num.subs(f1, t)

                    # if no Functions left, we can proceed with usual solve
                    if not ftry.has(symbol):
                        cv_sols = _solve(ftry, t, **flags)
                        cv_inv = _solve(t - f1, symbol, **flags)[0]
                        sols = list()
                        for sol in cv_sols:
                            sols.append(cv_inv.subs(t, sol))
                        result = list(ordered(sols))

                if result is False:
                    msg = 'multiple generators %s' % gens

            else:
                # e.g. case where gens are exp(x), exp(-x)
                u = bases.pop()
                t = Dummy('t')
                inv = _solve(u - t, symbol, **flags)
                if isinstance(u, (Pow, exp)):
                    # this will be resolved by factor in _tsolve but we might
                    # as well try a simple expansion here to get things in
                    # order so something like the following will work now without
                    # having to factor:
                    #
                    # >>> eq = (exp(I*(-x-2))+exp(I*(x+2)))
                    # >>> eq.subs(exp(x),y)  # fails
                    # exp(I*(-x - 2)) + exp(I*(x + 2))
                    # >>> eq.expand().subs(exp(x),y)  # works
                    # y**I*exp(2*I) + y**(-I)*exp(-2*I)
                    def _expand(p):
                        b, e = p.as_base_exp()
                        e = expand_mul(e)
                        return expand_power_exp(b**e)
                    ftry = f_num.replace(
                        lambda w: w.is_Pow or isinstance(w, exp),
                        _expand).subs(u, t)
                    if not ftry.has(symbol):
                        soln = _solve(ftry, t, **flags)
                        sols = list()
                        for sol in soln:
                            for i in inv:
                                sols.append(i.subs(t, sol))
                        result = list(ordered(sols))

        elif len(gens) == 1:

            # There is only one generator that we are interested in, but
            # there may have been more than one generator identified by
            # polys (e.g. for symbols other than the one we are interested
            # in) so recast the poly in terms of our generator of interest.
            # Also use composite=True with f_num since Poly won't update
            # poly as documented in issue 8810.

            poly = Poly(f_num, gens[0], composite=True)

            # if we aren't on the tsolve-pass, use roots
            if not flags.pop('tsolve', False):
                soln = None
                deg = poly.degree()
                flags['tsolve'] = True
                solvers = {k: flags.get(k, True) for k in
                    ('cubics', 'quartics', 'quintics')}
                soln = roots(poly, **solvers)
                if sum(soln.values()) < deg:
                    # e.g. roots(32*x**5 + 400*x**4 + 2032*x**3 +
                    #            5000*x**2 + 6250*x + 3189) -> {}
                    # so all_roots is used and RootOf instances are
                    # returned *unless* the system is multivariate
                    # or high-order EX domain.
                    try:
                        soln = poly.all_roots()
                    except NotImplementedError:
                        if not flags.get('incomplete', True):
                                raise NotImplementedError(
                                filldedent('''
    Neither high-order multivariate polynomials
    nor sorting of EX-domain polynomials is supported.
    If you want to see any results, pass keyword incomplete=True to
    solve; to see numerical values of roots
    for univariate expressions, use nroots.
    '''))
                        else:
                            pass
                else:
                    soln = list(soln.keys())

                if soln is not None:
                    u = poly.gen
                    if u != symbol:
                        try:
                            t = Dummy('t')
                            iv = _solve(u - t, symbol, **flags)
                            soln = list(ordered({i.subs(t, s) for i in iv for s in soln}))
                        except NotImplementedError:
                            # perhaps _tsolve can handle f_num
                            soln = None
                    else:
                        check = False  # only dens need to be checked
                    if soln is not None:
                        if len(soln) > 2:
                            # if the flag wasn't set then unset it since high-order
                            # results are quite long. Perhaps one could base this
                            # decision on a certain critical length of the
                            # roots. In addition, wester test M2 has an expression
                            # whose roots can be shown to be real with the
                            # unsimplified form of the solution whereas only one of
                            # the simplified forms appears to be real.
                            flags['simplify'] = flags.get('simplify', False)
                        result = soln

    # fallback if above fails
    # -----------------------
    if result is False:
        # try unrad
        if flags.pop('_unrad', True):
            try:
                u = unrad(f_num, symbol)
            except (ValueError, NotImplementedError):
                u = False
            if u:
                eq, cov = u
                if cov:
                    isym, ieq = cov
                    inv = _solve(ieq, symbol, **flags)[0]
                    rv = {inv.subs(isym, xi) for xi in _solve(eq, isym, **flags)}
                else:
                    try:
                        rv = set(_solve(eq, symbol, **flags))
                    except NotImplementedError:
                        rv = None
                if rv is not None:
                    result = list(ordered(rv))
                    # if the flag wasn't set then unset it since unrad results
                    # can be quite long or of very high order
                    flags['simplify'] = flags.get('simplify', False)
            else:
                pass  # for coverage

    # try _tsolve
    if result is False:
        flags.pop('tsolve', None)  # allow tsolve to be used on next pass
        try:
            soln = _tsolve(f_num, symbol, **flags)
            if soln is not None:
                result = soln
        except PolynomialError:
            pass
    # ----------- end of fallback ----------------------------

    if result is False:
        raise NotImplementedError('\n'.join([msg, not_impl_msg % f]))

    if flags.get('simplify', True):
        result = list(map(simplify, result))
        # we just simplified the solution so we now set the flag to
        # False so the simplification doesn't happen again in checksol()
        flags['simplify'] = False

    if checkdens:
        # reject any result that makes any denom. affirmatively 0;
        # if in doubt, keep it
        dens = _simple_dens(f, symbols)
        result = [s for s in result if
                  all(not checksol(d, {symbol: s}, **flags)
                    for d in dens)]
    if check:
        # keep only results if the check is not False
        result = [r for r in result if
                  checksol(f_num, {symbol: r}, **flags) is not False]
    return result


def _solve_system(exprs, symbols, **flags):
    if not exprs:
        return []

    if flags.pop('_split', True):
        # Split the system into connected components
        V = exprs
        symsset = set(symbols)
        exprsyms = {e: e.free_symbols & symsset for e in exprs}
        E = []
        sym_indices = {sym: i for i, sym in enumerate(symbols)}
        for n, e1 in enumerate(exprs):
            for e2 in exprs[:n]:
                # Equations are connected if they share a symbol
                if exprsyms[e1] & exprsyms[e2]:
                    E.append((e1, e2))
        G = V, E
        subexprs = connected_components(G)
        if len(subexprs) > 1:
            subsols = []
            for subexpr in subexprs:
                subsyms = set()
                for e in subexpr:
                    subsyms |= exprsyms[e]
                subsyms = list(sorted(subsyms, key = lambda x: sym_indices[x]))
                # use canonical subset to solve these equations
                # since there may be redundant equations in the set:
                # take the first equation of several that may have the
                # same sub-maximal free symbols of interest; the
                # other equations that weren't used should be checked
                # to see that they did not fail -- does the solver
                # take care of that?
                choices = sift(subexpr, lambda x: tuple(ordered(exprsyms[x])))
                subexpr = choices.pop(tuple(ordered(subsyms)), [])
                for k in choices:
                    subexpr.append(next(ordered(choices[k])))
                flags['_split'] = False  # skip split step
                subsol = _solve_system(subexpr, subsyms, **flags)
                if not isinstance(subsol, list):
                    subsol = [subsol]
                subsols.append(subsol)
            # Full solution is cartesion product of subsystems
            sols = []
            for soldicts in cartes(*subsols):
                sols.append(dict(item for sd in soldicts
                    for item in sd.items()))
            # Return a list with one dict as just the dict
            if len(sols) == 1:
                return sols[0]
            return sols

    polys = []
    dens = set()
    failed = []
    result = False
    linear = False
    manual = flags.get('manual', False)
    checkdens = check = flags.get('check', True)

    for j, g in enumerate(exprs):
        dens.update(_simple_dens(g, symbols))
        i, d = _invert(g, *symbols)
        g = d - i
        g = g.as_numer_denom()[0]
        if manual:
            failed.append(g)
            continue

        poly = g.as_poly(*symbols, extension=True)

        if poly is not None:
            polys.append(poly)
        else:
            failed.append(g)

    if not polys:
        solved_syms = []
    else:
        if all(p.is_linear for p in polys):
            n, m = len(polys), len(symbols)
            matrix = zeros(n, m + 1)

            for i, poly in enumerate(polys):
                for monom, coeff in poly.terms():
                    try:
                        j = monom.index(1)
                        matrix[i, j] = coeff
                    except ValueError:
                        matrix[i, m] = -coeff

            # returns a dictionary ({symbols: values}) or None
            if flags.pop('particular', False):
                result = minsolve_linear_system(matrix, *symbols, **flags)
            else:
                result = solve_linear_system(matrix, *symbols, **flags)
            if failed:
                if result:
                    solved_syms = list(result.keys())
                else:
                    solved_syms = []
            else:
                linear = True

        else:
            if len(symbols) > len(polys):
                from sympy.utilities.iterables import subsets

                free = set().union(*[p.free_symbols for p in polys])
                free = list(ordered(free.intersection(symbols)))
                got_s = set()
                result = []
                for syms in subsets(free, len(polys)):
                    try:
                        # returns [] or list of tuples of solutions for syms
                        res = solve_poly_system(polys, *syms)
                        if res:
                            for r in res:
                                skip = False
                                for r1 in r:
                                    if got_s and any([ss in r1.free_symbols
                                           for ss in got_s]):
                                        # sol depends on previously
                                        # solved symbols: discard it
                                        skip = True
                                if not skip:
                                    got_s.update(syms)
                                    result.extend([dict(list(zip(syms, r)))])
                    except NotImplementedError:
                        pass
                if got_s:
                    solved_syms = list(got_s)
                else:
                    raise NotImplementedError('no valid subset found')
            else:
                try:
                    result = solve_poly_system(polys, *symbols)
                    if result:
                        solved_syms = symbols
                        # we don't know here if the symbols provided
                        # were given or not, so let solve resolve that.
                        # A list of dictionaries is going to always be
                        # returned from here.
                        result = [dict(list(zip(solved_syms, r))) for r in result]
                except NotImplementedError:
                    failed.extend([g.as_expr() for g in polys])
                    solved_syms = []
                    result = None

    if result:
        if isinstance(result, dict):
            result = [result]
    else:
        result = [{}]

    if failed:
        # For each failed equation, see if we can solve for one of the
        # remaining symbols from that equation. If so, we update the
        # solution set and continue with the next failed equation,
        # repeating until we are done or we get an equation that can't
        # be solved.
        def _ok_syms(e, sort=False):
            rv = (e.free_symbols - solved_syms) & legal
            if sort:
                rv = list(rv)
                rv.sort(key=default_sort_key)
            return rv

        solved_syms = set(solved_syms)  # set of symbols we have solved for
        legal = set(symbols)  # what we are interested in
        # sort so equation with the fewest potential symbols is first
        u = Dummy()  # used in solution checking
        for eq in ordered(failed, lambda _: len(_ok_syms(_))):
            newresult = []
            bad_results = []
            got_s = set()
            hit = False
            for r in result:
                # update eq with everything that is known so far
                eq2 = eq.subs(r)
                # if check is True then we see if it satisfies this
                # equation, otherwise we just accept it
                if check and r:
                    b = checksol(u, u, eq2, minimal=True)
                    if b is not None:
                        # this solution is sufficient to know whether
                        # it is valid or not so we either accept or
                        # reject it, then continue
                        if b:
                            newresult.append(r)
                        else:
                            bad_results.append(r)
                        continue
                # search for a symbol amongst those available that
                # can be solved for
                ok_syms = _ok_syms(eq2, sort=True)
                if not ok_syms:
                    if r:
                        newresult.append(r)
                    break  # skip as it's independent of desired symbols
                for s in ok_syms:
                    try:
                        soln = _solve(eq2, s, **flags)
                    except NotImplementedError:
                        continue
                    # put each solution in r and append the now-expanded
                    # result in the new result list; use copy since the
                    # solution for s in being added in-place
                    for sol in soln:
                        if got_s and any([ss in sol.free_symbols for ss in got_s]):
                            # sol depends on previously solved symbols: discard it
                            continue
                        rnew = r.copy()
                        for k, v in r.items():
                            rnew[k] = v.subs(s, sol)
                        # and add this new solution
                        rnew[s] = sol
                        newresult.append(rnew)
                    hit = True
                    got_s.add(s)
                if not hit:
                    raise NotImplementedError('could not solve %s' % eq2)
            else:
                result = newresult
                for b in bad_results:
                    if b in result:
                        result.remove(b)

    default_simplify = bool(failed)  # rely on system-solvers to simplify
    if  flags.get('simplify', default_simplify):
        for r in result:
            for k in r:
                r[k] = simplify(r[k])
        flags['simplify'] = False  # don't need to do so in checksol now

    if checkdens:
        result = [r for r in result
            if not any(checksol(d, r, **flags) for d in dens)]

    if check and not linear:
        result = [r for r in result
            if not any(checksol(e, r, **flags) is False for e in exprs)]

    result = [r for r in result if r]
    if linear and result:
        result = result[0]
    return result


def solve_linear(lhs, rhs=0, symbols=[], exclude=[]):
    r"""
    Return a tuple derived from ``f = lhs - rhs`` that is one of
    the following: ``(0, 1)``, ``(0, 0)``, ``(symbol, solution)``, ``(n, d)``.

    Explanation
    ===========

    ``(0, 1)`` meaning that ``f`` is independent of the symbols in *symbols*
    that are not in *exclude*.

    ``(0, 0)`` meaning that there is no solution to the equation amongst the
    symbols given. If the first element of the tuple is not zero, then the
    function is guaranteed to be dependent on a symbol in *symbols*.

    ``(symbol, solution)`` where symbol appears linearly in the numerator of
    ``f``, is in *symbols* (if given), and is not in *exclude* (if given). No
    simplification is done to ``f`` other than a ``mul=True`` expansion, so the
    solution will correspond strictly to a unique solution.

    ``(n, d)`` where ``n`` and ``d`` are the numerator and denominator of ``f``
    when the numerator was not linear in any symbol of interest; ``n`` will
    never be a symbol unless a solution for that symbol was found (in which case
    the second element is the solution, not the denominator).

    Examples
    ========

    >>> from sympy.core.power import Pow
    >>> from sympy.polys.polytools import cancel

    ``f`` is independent of the symbols in *symbols* that are not in
    *exclude*:

    >>> from sympy.solvers.solvers import solve_linear
    >>> from sympy.abc import x, y, z
    >>> from sympy import cos, sin
    >>> eq = y*cos(x)**2 + y*sin(x)**2 - y  # = y*(1 - 1) = 0
    >>> solve_linear(eq)
    (0, 1)
    >>> eq = cos(x)**2 + sin(x)**2  # = 1
    >>> solve_linear(eq)
    (0, 1)
    >>> solve_linear(x, exclude=[x])
    (0, 1)

    The variable ``x`` appears as a linear variable in each of the
    following:

    >>> solve_linear(x + y**2)
    (x, -y**2)
    >>> solve_linear(1/x - y**2)
    (x, y**(-2))

    When not linear in ``x`` or ``y`` then the numerator and denominator are
    returned:

    >>> solve_linear(x**2/y**2 - 3)
    (x**2 - 3*y**2, y**2)

    If the numerator of the expression is a symbol, then ``(0, 0)`` is
    returned if the solution for that symbol would have set any
    denominator to 0:

    >>> eq = 1/(1/x - 2)
    >>> eq.as_numer_denom()
    (x, 1 - 2*x)
    >>> solve_linear(eq)
    (0, 0)

    But automatic rewriting may cause a symbol in the denominator to
    appear in the numerator so a solution will be returned:

    >>> (1/x)**-1
    x
    >>> solve_linear((1/x)**-1)
    (x, 0)

    Use an unevaluated expression to avoid this:

    >>> solve_linear(Pow(1/x, -1, evaluate=False))
    (0, 0)

    If ``x`` is allowed to cancel in the following expression, then it
    appears to be linear in ``x``, but this sort of cancellation is not
    done by ``solve_linear`` so the solution will always satisfy the
    original expression without causing a division by zero error.

    >>> eq = x**2*(1/x - z**2/x)
    >>> solve_linear(cancel(eq))
    (x, 0)
    >>> solve_linear(eq)
    (x**2*(1 - z**2), x)

    A list of symbols for which a solution is desired may be given:

    >>> solve_linear(x + y + z, symbols=[y])
    (y, -x - z)

    A list of symbols to ignore may also be given:

    >>> solve_linear(x + y + z, exclude=[x])
    (y, -x - z)

    (A solution for ``y`` is obtained because it is the first variable
    from the canonically sorted list of symbols that had a linear
    solution.)

    """
    if isinstance(lhs, Equality):
        if rhs:
            raise ValueError(filldedent('''
            If lhs is an Equality, rhs must be 0 but was %s''' % rhs))
        rhs = lhs.rhs
        lhs = lhs.lhs
    dens = None
    eq = lhs - rhs
    n, d = eq.as_numer_denom()
    if not n:
        return S.Zero, S.One

    free = n.free_symbols
    if not symbols:
        symbols = free
    else:
        bad = [s for s in symbols if not s.is_Symbol]
        if bad:
            if len(bad) == 1:
                bad = bad[0]
            if len(symbols) == 1:
                eg = 'solve(%s, %s)' % (eq, symbols[0])
            else:
                eg = 'solve(%s, *%s)' % (eq, list(symbols))
            raise ValueError(filldedent('''
                solve_linear only handles symbols, not %s. To isolate
                non-symbols use solve, e.g. >>> %s <<<.
                             ''' % (bad, eg)))
        symbols = free.intersection(symbols)
    symbols = symbols.difference(exclude)
    if not symbols:
        return S.Zero, S.One

    # derivatives are easy to do but tricky to analyze to see if they
    # are going to disallow a linear solution, so for simplicity we
    # just evaluate the ones that have the symbols of interest
    derivs = defaultdict(list)
    for der in n.atoms(Derivative):
        csym = der.free_symbols & symbols
        for c in csym:
            derivs[c].append(der)

    all_zero = True
    for xi in sorted(symbols, key=default_sort_key):  # canonical order
        # if there are derivatives in this var, calculate them now
        if isinstance(derivs[xi], list):
            derivs[xi] = {der: der.doit() for der in derivs[xi]}
        newn = n.subs(derivs[xi])
        dnewn_dxi = newn.diff(xi)
        # dnewn_dxi can be nonzero if it survives differentation by any
        # of its free symbols
        free = dnewn_dxi.free_symbols
        if dnewn_dxi and (not free or any(dnewn_dxi.diff(s) for s in free)):
            all_zero = False
            if dnewn_dxi is S.NaN:
                break
            if xi not in dnewn_dxi.free_symbols:
                vi = -1/dnewn_dxi*(newn.subs(xi, 0))
                if dens is None:
                    dens = _simple_dens(eq, symbols)
                if not any(checksol(di, {xi: vi}, minimal=True) is True
                          for di in dens):
                    # simplify any trivial integral
                    irep = [(i, i.doit()) for i in vi.atoms(Integral) if
                            i.function.is_number]
                    # do a slight bit of simplification
                    vi = expand_mul(vi.subs(irep))
                    return xi, vi
    if all_zero:
        return S.Zero, S.One
    if n.is_Symbol: # no solution for this symbol was found
        return S.Zero, S.Zero
    return n, d


def minsolve_linear_system(system, *symbols, **flags):
    r"""
    Find a particular solution to a linear system.

    Explanation
    ===========

    In particular, try to find a solution with the minimal possible number
    of non-zero variables using a naive algorithm with exponential complexity.
    If ``quick=True``, a heuristic is used.

    """
    quick = flags.get('quick', False)
    # Check if there are any non-zero solutions at all
    s0 = solve_linear_system(system, *symbols, **flags)
    if not s0 or all(v == 0 for v in s0.values()):
        return s0
    if quick:
        # We just solve the system and try to heuristically find a nice
        # solution.
        s = solve_linear_system(system, *symbols)
        def update(determined, solution):
            delete = []
            for k, v in solution.items():
                solution[k] = v.subs(determined)
                if not solution[k].free_symbols:
                    delete.append(k)
                    determined[k] = solution[k]
            for k in delete:
                del solution[k]
        determined = {}
        update(determined, s)
        while s:
            # NOTE sort by default_sort_key to get deterministic result
            k = max((k for k in s.values()),
                    key=lambda x: (len(x.free_symbols), default_sort_key(x)))
            x = max(k.free_symbols, key=default_sort_key)
            if len(k.free_symbols) != 1:
                determined[x] = S.Zero
            else:
                val = solve(k)[0]
                if val == 0 and all(v.subs(x, val) == 0 for v in s.values()):
                    determined[x] = S.One
                else:
                    determined[x] = val
            update(determined, s)
        return determined
    else:
        # We try to select n variables which we want to be non-zero.
        # All others will be assumed zero. We try to solve the modified system.
        # If there is a non-trivial solution, just set the free variables to
        # one. If we do this for increasing n, trying all combinations of
        # variables, we will find an optimal solution.
        # We speed up slightly by starting at one less than the number of
        # variables the quick method manages.
        from itertools import combinations
        from sympy.utilities.misc import debug
        N = len(symbols)
        bestsol = minsolve_linear_system(system, *symbols, quick=True)
        n0 = len([x for x in bestsol.values() if x != 0])
        for n in range(n0 - 1, 1, -1):
            debug('minsolve: %s' % n)
            thissol = None
            for nonzeros in combinations(list(range(N)), n):
                subm = Matrix([system.col(i).T for i in nonzeros] + [system.col(-1).T]).T
                s = solve_linear_system(subm, *[symbols[i] for i in nonzeros])
                if s and not all(v == 0 for v in s.values()):
                    subs = [(symbols[v], S.One) for v in nonzeros]
                    for k, v in s.items():
                        s[k] = v.subs(subs)
                    for sym in symbols:
                        if sym not in s:
                            if symbols.index(sym) in nonzeros:
                                s[sym] = S.One
                            else:
                                s[sym] = S.Zero
                    thissol = s
                    break
            if thissol is None:
                break
            bestsol = thissol
        return bestsol


def solve_linear_system(system, *symbols, **flags):
    r"""
    Solve system of $N$ linear equations with $M$ variables, which means
    both under- and overdetermined systems are supported.

    Explanation
    ===========

    The possible number of solutions is zero, one, or infinite. Respectively,
    this procedure will return None or a dictionary with solutions. In the
    case of underdetermined systems, all arbitrary parameters are skipped.
    This may cause a situation in which an empty dictionary is returned.
    In that case, all symbols can be assigned arbitrary values.

    Input to this function is a $N\times M + 1$ matrix, which means it has
    to be in augmented form. If you prefer to enter $N$ equations and $M$
    unknowns then use ``solve(Neqs, *Msymbols)`` instead. Note: a local
    copy of the matrix is made by this routine so the matrix that is
    passed will not be modified.

    The algorithm used here is fraction-free Gaussian elimination,
    which results, after elimination, in an upper-triangular matrix.
    Then solutions are found using back-substitution. This approach
    is more efficient and compact than the Gauss-Jordan method.

    Examples
    ========

    >>> from sympy import Matrix, solve_linear_system
    >>> from sympy.abc import x, y

    Solve the following system::

           x + 4 y ==  2
        -2 x +   y == 14

    >>> system = Matrix(( (1, 4, 2), (-2, 1, 14)))
    >>> solve_linear_system(system, x, y)
    {x: -6, y: 2}

    A degenerate system returns an empty dictionary:

    >>> system = Matrix(( (0,0,0), (0,0,0) ))
    >>> solve_linear_system(system, x, y)
    {}

    """
    assert system.shape[1] == len(symbols) + 1

    # This is just a wrapper for solve_lin_sys
    eqs = list(system * Matrix(symbols + (-1,)))
    eqs, ring = sympy_eqs_to_ring(eqs, symbols)
    sol = solve_lin_sys(eqs, ring, _raw=False)
    if sol is not None:
        sol = {sym:val for sym, val in sol.items() if sym != val}
    return sol


def solve_undetermined_coeffs(equ, coeffs, sym, **flags):
    r"""
    Solve equation of a type $p(x; a_1, \ldots, a_k) = q(x)$ where both
    $p$ and $q$ are univariate polynomials that depend on $k$ parameters.

    Explanation
    ===========

    The result of this function is a dictionary with symbolic values of those
    parameters with respect to coefficients in $q$.

    This function accepts both equations class instances and ordinary
    SymPy expressions. Specification of parameters and variables is
    obligatory for efficiency and simplicity reasons.

    Examples
    ========

    >>> from sympy import Eq
    >>> from sympy.abc import a, b, c, x
    >>> from sympy.solvers import solve_undetermined_coeffs

    >>> solve_undetermined_coeffs(Eq(2*a*x + a+b, x), [a, b], x)
    {a: 1/2, b: -1/2}

    >>> solve_undetermined_coeffs(Eq(a*c*x + a+b, x), [a, b], x)
    {a: 1/c, b: -1/c}

    """
    if isinstance(equ, Equality):
        # got equation, so move all the
        # terms to the left hand side
        equ = equ.lhs - equ.rhs

    equ = cancel(equ).as_numer_denom()[0]

    system = list(collect(equ.expand(), sym, evaluate=False).values())

    if not any(equ.has(sym) for equ in system):
        # consecutive powers in the input expressions have
        # been successfully collected, so solve remaining
        # system using Gaussian elimination algorithm
        return solve(system, *coeffs, **flags)
    else:
        return None  # no solutions


def solve_linear_system_LU(matrix, syms):
    """
    Solves the augmented matrix system using ``LUsolve`` and returns a
    dictionary in which solutions are keyed to the symbols of *syms* as ordered.

    Explanation
    ===========

    The matrix must be invertible.

    Examples
    ========

    >>> from sympy import Matrix
    >>> from sympy.abc import x, y, z
    >>> from sympy.solvers.solvers import solve_linear_system_LU

    >>> solve_linear_system_LU(Matrix([
    ... [1, 2, 0, 1],
    ... [3, 2, 2, 1],
    ... [2, 0, 0, 1]]), [x, y, z])
    {x: 1/2, y: 1/4, z: -1/2}

    See Also
    ========

    LUsolve

    """
    if matrix.rows != matrix.cols - 1:
        raise ValueError("Rows should be equal to columns - 1")
    A = matrix[:matrix.rows, :matrix.rows]
    b = matrix[:, matrix.cols - 1:]
    soln = A.LUsolve(b)
    solutions = {}
    for i in range(soln.rows):
        solutions[syms[i]] = soln[i, 0]
    return solutions


def det_perm(M):
    """
    Return the determinant of *M* by using permutations to select factors.

    Explanation
    ===========

    For sizes larger than 8 the number of permutations becomes prohibitively
    large, or if there are no symbols in the matrix, it is better to use the
    standard determinant routines (e.g., ``M.det()``.)

    See Also
    ========

    det_minor
    det_quick

    """
    args = []
    s = True
    n = M.rows
    list_ = getattr(M, '_mat', None)
    if list_ is None:
        list_ = flatten(M.tolist())
    for perm in generate_bell(n):
        fac = []
        idx = 0
        for j in perm:
            fac.append(list_[idx + j])
            idx += n
        term = Mul(*fac) # disaster with unevaluated Mul -- takes forever for n=7
        args.append(term if s else -term)
        s = not s
    return Add(*args)


def det_minor(M):
    """
    Return the ``det(M)`` computed from minors without
    introducing new nesting in products.

    See Also
    ========

    det_perm
    det_quick

    """
    n = M.rows
    if n == 2:
        return M[0, 0]*M[1, 1] - M[1, 0]*M[0, 1]
    else:
        return sum([(1, -1)[i % 2]*Add(*[M[0, i]*d for d in
            Add.make_args(det_minor(M.minor_submatrix(0, i)))])
            if M[0, i] else S.Zero for i in range(n)])


def det_quick(M, method=None):
    """
    Return ``det(M)`` assuming that either
    there are lots of zeros or the size of the matrix
    is small. If this assumption is not met, then the normal
    Matrix.det function will be used with method = ``method``.

    See Also
    ========

    det_minor
    det_perm

    """
    if any(i.has(Symbol) for i in M):
        if M.rows < 8 and all(i.has(Symbol) for i in M):
            return det_perm(M)
        return det_minor(M)
    else:
        return M.det(method=method) if method else M.det()


def inv_quick(M):
    """Return the inverse of ``M``, assuming that either
    there are lots of zeros or the size of the matrix
    is small.
    """
    from sympy.matrices import zeros
    if not all(i.is_Number for i in M):
        if not any(i.is_Number for i in M):
            det = lambda _: det_perm(_)
        else:
            det = lambda _: det_minor(_)
    else:
        return M.inv()
    n = M.rows
    d = det(M)
    if d == S.Zero:
        raise NonInvertibleMatrixError("Matrix det == 0; not invertible")
    ret = zeros(n)
    s1 = -1
    for i in range(n):
        s = s1 = -s1
        for j in range(n):
            di = det(M.minor_submatrix(i, j))
            ret[j, i] = s*di/d
            s = -s
    return ret


# these are functions that have multiple inverse values per period
multi_inverses = {
    sin: lambda x: (asin(x), S.Pi - asin(x)),
    cos: lambda x: (acos(x), 2*S.Pi - acos(x)),
}


def _tsolve(eq, sym, **flags):
    """
    Helper for ``_solve`` that solves a transcendental equation with respect
    to the given symbol. Various equations containing powers and logarithms,
    can be solved.

    There is currently no guarantee that all solutions will be returned or
    that a real solution will be favored over a complex one.

    Either a list of potential solutions will be returned or None will be
    returned (in the case that no method was known to get a solution
    for the equation). All other errors (like the inability to cast an
    expression as a Poly) are unhandled.

    Examples
    ========

    >>> from sympy import log
    >>> from sympy.solvers.solvers import _tsolve as tsolve
    >>> from sympy.abc import x

    >>> tsolve(3**(2*x + 5) - 4, x)
    [-5/2 + log(2)/log(3), (-5*log(3)/2 + log(2) + I*pi)/log(3)]

    >>> tsolve(log(x) + 2*x, x)
    [LambertW(2)/2]

    """
    if 'tsolve_saw' not in flags:
        flags['tsolve_saw'] = []
    if eq in flags['tsolve_saw']:
        return None
    else:
        flags['tsolve_saw'].append(eq)

    rhs, lhs = _invert(eq, sym)

    if lhs == sym:
        return [rhs]
    try:
        if lhs.is_Add:
            # it's time to try factoring; powdenest is used
            # to try get powers in standard form for better factoring
            f = factor(powdenest(lhs - rhs))
            if f.is_Mul:
                return _solve(f, sym, **flags)
            if rhs:
                f = logcombine(lhs, force=flags.get('force', True))
                if f.count(log) != lhs.count(log):
                    if isinstance(f, log):
                        return _solve(f.args[0] - exp(rhs), sym, **flags)
                    return _tsolve(f - rhs, sym, **flags)

        elif lhs.is_Pow:
            if lhs.exp.is_Integer:
                if lhs - rhs != eq:
                    return _solve(lhs - rhs, sym, **flags)

            if sym not in lhs.exp.free_symbols:
                return _solve(lhs.base - rhs**(1/lhs.exp), sym, **flags)

            # _tsolve calls this with Dummy before passing the actual number in.
            if any(t.is_Dummy for t in rhs.free_symbols):
                raise NotImplementedError # _tsolve will call here again...

            # a ** g(x) == 0
            if not rhs:
                # f(x)**g(x) only has solutions where f(x) == 0 and g(x) != 0 at
                # the same place
                sol_base = _solve(lhs.base, sym, **flags)
                return [s for s in sol_base if lhs.exp.subs(sym, s) != 0]

            # a ** g(x) == b
            if not lhs.base.has(sym):
                if lhs.base == 0:
                    return _solve(lhs.exp, sym, **flags) if rhs != 0 else []

                # Gets most solutions...
                if lhs.base == rhs.as_base_exp()[0]:
                    # handles case when bases are equal
                    sol = _solve(lhs.exp - rhs.as_base_exp()[1], sym, **flags)
                else:
                    # handles cases when bases are not equal and exp
                    # may or may not be equal
                    sol = _solve(exp(log(lhs.base)*lhs.exp)-exp(log(rhs)), sym, **flags)

                # Check for duplicate solutions
                def equal(expr1, expr2):
                    _ = Dummy()
                    eq = checksol(expr1 - _, _, expr2)
                    if eq is None:
                        if nsimplify(expr1) != nsimplify(expr2):
                            return False
                        # they might be coincidentally the same
                        # so check more rigorously
                        eq = expr1.equals(expr2)
                    return eq

                # Guess a rational exponent
                e_rat = nsimplify(log(abs(rhs))/log(abs(lhs.base)))
                e_rat = simplify(posify(e_rat)[0])
                n, d = fraction(e_rat)
                if expand(lhs.base**n - rhs**d) == 0:
                    sol = [s for s in sol if not equal(lhs.exp.subs(sym, s), e_rat)]
                    sol.extend(_solve(lhs.exp - e_rat, sym, **flags))

                return list(ordered(set(sol)))

            # f(x) ** g(x) == c
            else:
                sol = []
                logform = lhs.exp*log(lhs.base) - log(rhs)
                if logform != lhs - rhs:
                    try:
                        sol.extend(_solve(logform, sym, **flags))
                    except NotImplementedError:
                        pass

                # Collect possible solutions and check with substitution later.
                check = []
                if rhs == 1:
                    # f(x) ** g(x) = 1 -- g(x)=0 or f(x)=+-1
                    check.extend(_solve(lhs.exp, sym, **flags))
                    check.extend(_solve(lhs.base - 1, sym, **flags))
                    check.extend(_solve(lhs.base + 1, sym, **flags))
                elif rhs.is_Rational:
                    for d in (i for i in divisors(abs(rhs.p)) if i != 1):
                        e, t = integer_log(rhs.p, d)
                        if not t:
                            continue  # rhs.p != d**b
                        for s in divisors(abs(rhs.q)):
                            if s**e== rhs.q:
                                r = Rational(d, s)
                                check.extend(_solve(lhs.base - r, sym, **flags))
                                check.extend(_solve(lhs.base + r, sym, **flags))
                                check.extend(_solve(lhs.exp - e, sym, **flags))
                elif rhs.is_irrational:
                    b_l, e_l = lhs.base.as_base_exp()
                    n, d = (e_l*lhs.exp).as_numer_denom()
                    b, e = sqrtdenest(rhs).as_base_exp()
                    check = [sqrtdenest(i) for i in (_solve(lhs.base - b, sym, **flags))]
                    check.extend([sqrtdenest(i) for i in (_solve(lhs.exp - e, sym, **flags))])
                    if e_l*d != 1:
                        check.extend(_solve(b_l**n - rhs**(e_l*d), sym, **flags))
                for s in check:
                    ok = checksol(eq, sym, s)
                    if ok is None:
                        ok = eq.subs(sym, s).equals(0)
                    if ok:
                        sol.append(s)
                return list(ordered(set(sol)))

        elif lhs.is_Function and len(lhs.args) == 1:
            if lhs.func in multi_inverses:
                # sin(x) = 1/3 -> x - asin(1/3) & x - (pi - asin(1/3))
                soln = []
                for i in multi_inverses[lhs.func](rhs):
                    soln.extend(_solve(lhs.args[0] - i, sym, **flags))
                return list(ordered(soln))
            elif lhs.func == LambertW:
                return _solve(lhs.args[0] - rhs*exp(rhs), sym, **flags)

        rewrite = lhs.rewrite(exp)
        if rewrite != lhs:
            return _solve(rewrite - rhs, sym, **flags)
    except NotImplementedError:
        pass

    # maybe it is a lambert pattern
    if flags.pop('bivariate', True):
        # lambert forms may need some help being recognized, e.g. changing
        # 2**(3*x) + x**3*log(2)**3 + 3*x**2*log(2)**2 + 3*x*log(2) + 1
        # to 2**(3*x) + (x*log(2) + 1)**3
        g = _filtered_gens(eq.as_poly(), sym)
        up_or_log = set()
        for gi in g:
            if isinstance(gi, exp) or (gi.is_Pow and gi.base == S.Exp1) or isinstance(gi, log):
                up_or_log.add(gi)
            elif gi.is_Pow:
                gisimp = powdenest(expand_power_exp(gi))
                if gisimp.is_Pow and sym in gisimp.exp.free_symbols:
                    up_or_log.add(gi)
        eq_down = expand_log(expand_power_exp(eq)).subs(
            dict(list(zip(up_or_log, [0]*len(up_or_log)))))
        eq = expand_power_exp(factor(eq_down, deep=True) + (eq - eq_down))
        rhs, lhs = _invert(eq, sym)
        if lhs.has(sym):
            try:
                poly = lhs.as_poly()
                g = _filtered_gens(poly, sym)
                _eq = lhs - rhs
                sols = _solve_lambert(_eq, sym, g)
                # use a simplified form if it satisfies eq
                # and has fewer operations
                for n, s in enumerate(sols):
                    ns = nsimplify(s)
                    if ns != s and ns.count_ops() <= s.count_ops():
                        ok = checksol(_eq, sym, ns)
                        if ok is None:
                            ok = _eq.subs(sym, ns).equals(0)
                        if ok:
                            sols[n] = ns
                return sols
            except NotImplementedError:
                # maybe it's a convoluted function
                if len(g) == 2:
                    try:
                        gpu = bivariate_type(lhs - rhs, *g)
                        if gpu is None:
                            raise NotImplementedError
                        g, p, u = gpu
                        flags['bivariate'] = False
                        inversion = _tsolve(g - u, sym, **flags)
                        if inversion:
                            sol = _solve(p, u, **flags)
                            return list(ordered({i.subs(u, s)
                                for i in inversion for s in sol}))
                    except NotImplementedError:
                        pass
                else:
                    pass

    if flags.pop('force', True):
        flags['force'] = False
        pos, reps = posify(lhs - rhs)
        if rhs == S.ComplexInfinity:
            return []
        for u, s in reps.items():
            if s == sym:
                break
        else:
            u = sym
        if pos.has(u):
            try:
                soln = _solve(pos, u, **flags)
                return list(ordered([s.subs(reps) for s in soln]))
            except NotImplementedError:
                pass
        else:
            pass  # here for coverage

    return  # here for coverage


# TODO: option for calculating J numerically

@conserve_mpmath_dps
def nsolve(*args, dict=False, **kwargs):
    r"""
    Solve a nonlinear equation system numerically: ``nsolve(f, [args,] x0,
    modules=['mpmath'], **kwargs)``.

    Explanation
    ===========

    ``f`` is a vector function of symbolic expressions representing the system.
    *args* are the variables. If there is only one variable, this argument can
    be omitted. ``x0`` is a starting vector close to a solution.

    Use the modules keyword to specify which modules should be used to
    evaluate the function and the Jacobian matrix. Make sure to use a module
    that supports matrices. For more information on the syntax, please see the
    docstring of ``lambdify``.

    If the keyword arguments contain ``dict=True`` (default is False) ``nsolve``
    will return a list (perhaps empty) of solution mappings. This might be
    especially useful if you want to use ``nsolve`` as a fallback to solve since
    using the dict argument for both methods produces return values of
    consistent type structure. Please note: to keep this consistent with
    ``solve``, the solution will be returned in a list even though ``nsolve``
    (currently at least) only finds one solution at a time.

    Overdetermined systems are supported.

    Examples
    ========

    >>> from sympy import Symbol, nsolve
    >>> import mpmath
    >>> mpmath.mp.dps = 15
    >>> x1 = Symbol('x1')
    >>> x2 = Symbol('x2')
    >>> f1 = 3 * x1**2 - 2 * x2**2 - 1
    >>> f2 = x1**2 - 2 * x1 + x2**2 + 2 * x2 - 8
    >>> print(nsolve((f1, f2), (x1, x2), (-1, 1)))
    Matrix([[-1.19287309935246], [1.27844411169911]])

    For one-dimensional functions the syntax is simplified:

    >>> from sympy import sin, nsolve
    >>> from sympy.abc import x
    >>> nsolve(sin(x), x, 2)
    3.14159265358979
    >>> nsolve(sin(x), 2)
    3.14159265358979

    To solve with higher precision than the default, use the prec argument:

    >>> from sympy import cos
    >>> nsolve(cos(x) - x, 1)
    0.739085133215161
    >>> nsolve(cos(x) - x, 1, prec=50)
    0.73908513321516064165531208767387340401341175890076
    >>> cos(_)
    0.73908513321516064165531208767387340401341175890076

    To solve for complex roots of real functions, a nonreal initial point
    must be specified:

    >>> from sympy import I
    >>> nsolve(x**2 + 2, I)
    1.4142135623731*I

    ``mpmath.findroot`` is used and you can find their more extensive
    documentation, especially concerning keyword parameters and
    available solvers. Note, however, that functions which are very
    steep near the root, the verification of the solution may fail. In
    this case you should use the flag ``verify=False`` and
    independently verify the solution.

    >>> from sympy import cos, cosh
    >>> f = cos(x)*cosh(x) - 1
    >>> nsolve(f, 3.14*100)
    Traceback (most recent call last):
    ...
    ValueError: Could not find root within given tolerance. (1.39267e+230 > 2.1684e-19)
    >>> ans = nsolve(f, 3.14*100, verify=False); ans
    312.588469032184
    >>> f.subs(x, ans).n(2)
    2.1e+121
    >>> (f/f.diff(x)).subs(x, ans).n(2)
    7.4e-15

    One might safely skip the verification if bounds of the root are known
    and a bisection method is used:

    >>> bounds = lambda i: (3.14*i, 3.14*(i + 1))
    >>> nsolve(f, bounds(100), solver='bisect', verify=False)
    315.730061685774

    Alternatively, a function may be better behaved when the
    denominator is ignored. Since this is not always the case, however,
    the decision of what function to use is left to the discretion of
    the user.

    >>> eq = x**2/(1 - x)/(1 - 2*x)**2 - 100
    >>> nsolve(eq, 0.46)
    Traceback (most recent call last):
    ...
    ValueError: Could not find root within given tolerance. (10000 > 2.1684e-19)
    Try another starting point or tweak arguments.
    >>> nsolve(eq.as_numer_denom()[0], 0.46)
    0.46792545969349058

    """
    # there are several other SymPy functions that use method= so
    # guard against that here
    if 'method' in kwargs:
        raise ValueError(filldedent('''
            Keyword "method" should not be used in this context.  When using
            some mpmath solvers directly, the keyword "method" is
            used, but when using nsolve (and findroot) the keyword to use is
            "solver".'''))

    if 'prec' in kwargs:
        prec = kwargs.pop('prec')
        import mpmath
        mpmath.mp.dps = prec
    else:
        prec = None

    # keyword argument to return result as a dictionary
    as_dict = dict
    from builtins import dict  # to unhide the builtin

    # interpret arguments
    if len(args) == 3:
        f = args[0]
        fargs = args[1]
        x0 = args[2]
        if iterable(fargs) and iterable(x0):
            if len(x0) != len(fargs):
                raise TypeError('nsolve expected exactly %i guess vectors, got %i'
                                % (len(fargs), len(x0)))
    elif len(args) == 2:
        f = args[0]
        fargs = None
        x0 = args[1]
        if iterable(f):
            raise TypeError('nsolve expected 3 arguments, got 2')
    elif len(args) < 2:
        raise TypeError('nsolve expected at least 2 arguments, got %i'
                        % len(args))
    else:
        raise TypeError('nsolve expected at most 3 arguments, got %i'
                        % len(args))
    modules = kwargs.get('modules', ['mpmath'])
    if iterable(f):
        f = list(f)
        for i, fi in enumerate(f):
            if isinstance(fi, Equality):
                f[i] = fi.lhs - fi.rhs
        f = Matrix(f).T
    if iterable(x0):
        x0 = list(x0)
    if not isinstance(f, Matrix):
        # assume it's a sympy expression
        if isinstance(f, Equality):
            f = f.lhs - f.rhs
        syms = f.free_symbols
        if fargs is None:
            fargs = syms.copy().pop()
        if not (len(syms) == 1 and (fargs in syms or fargs[0] in syms)):
            raise ValueError(filldedent('''
                expected a one-dimensional and numerical function'''))

        # the function is much better behaved if there is no denominator
        # but sending the numerator is left to the user since sometimes
        # the function is better behaved when the denominator is present
        # e.g., issue 11768

        f = lambdify(fargs, f, modules)
        x = sympify(findroot(f, x0, **kwargs))
        if as_dict:
            return [{fargs: x}]
        return x

    if len(fargs) > f.cols:
        raise NotImplementedError(filldedent('''
            need at least as many equations as variables'''))
    verbose = kwargs.get('verbose', False)
    if verbose:
        print('f(x):')
        print(f)
    # derive Jacobian
    J = f.jacobian(fargs)
    if verbose:
        print('J(x):')
        print(J)
    # create functions
    f = lambdify(fargs, f.T, modules)
    J = lambdify(fargs, J, modules)
    # solve the system numerically
    x = findroot(f, x0, J=J, **kwargs)
    if as_dict:
        return [dict(zip(fargs, [sympify(xi) for xi in x]))]
    return Matrix(x)


def _invert(eq, *symbols, **kwargs):
    """
    Return tuple (i, d) where ``i`` is independent of *symbols* and ``d``
    contains symbols.

    Explanation
    ===========

    ``i`` and ``d`` are obtained after recursively using algebraic inversion
    until an uninvertible ``d`` remains. If there are no free symbols then
    ``d`` will be zero. Some (but not necessarily all) solutions to the
    expression ``i - d`` will be related to the solutions of the original
    expression.

    Examples
    ========

    >>> from sympy.solvers.solvers import _invert as invert
    >>> from sympy import sqrt, cos
    >>> from sympy.abc import x, y
    >>> invert(x - 3)
    (3, x)
    >>> invert(3)
    (3, 0)
    >>> invert(2*cos(x) - 1)
    (1/2, cos(x))
    >>> invert(sqrt(x) - 3)
    (3, sqrt(x))
    >>> invert(sqrt(x) + y, x)
    (-y, sqrt(x))
    >>> invert(sqrt(x) + y, y)
    (-sqrt(x), y)
    >>> invert(sqrt(x) + y, x, y)
    (0, sqrt(x) + y)

    If there is more than one symbol in a power's base and the exponent
    is not an Integer, then the principal root will be used for the
    inversion:

    >>> invert(sqrt(x + y) - 2)
    (4, x + y)
    >>> invert(sqrt(x + y) - 2)
    (4, x + y)

    If the exponent is an Integer, setting ``integer_power`` to True
    will force the principal root to be selected:

    >>> invert(x**2 - 4, integer_power=True)
    (2, x)

    """
    eq = sympify(eq)
    if eq.args:
        # make sure we are working with flat eq
        eq = eq.func(*eq.args)
    free = eq.free_symbols
    if not symbols:
        symbols = free
    if not free & set(symbols):
        return eq, S.Zero

    dointpow = bool(kwargs.get('integer_power', False))

    lhs = eq
    rhs = S.Zero
    while True:
        was = lhs
        while True:
            indep, dep = lhs.as_independent(*symbols)

            # dep + indep == rhs
            if lhs.is_Add:
                # this indicates we have done it all
                if indep.is_zero:
                    break

                lhs = dep
                rhs -= indep

            # dep * indep == rhs
            else:
                # this indicates we have done it all
                if indep is S.One:
                    break

                lhs = dep
                rhs /= indep

        # collect like-terms in symbols
        if lhs.is_Add:
            terms = {}
            for a in lhs.args:
                i, d = a.as_independent(*symbols)
                terms.setdefault(d, []).append(i)
            if any(len(v) > 1 for v in terms.values()):
                args = []
                for d, i in terms.items():
                    if len(i) > 1:
                        args.append(Add(*i)*d)
                    else:
                        args.append(i[0]*d)
                lhs = Add(*args)

        # if it's a two-term Add with rhs = 0 and two powers we can get the
        # dependent terms together, e.g. 3*f(x) + 2*g(x) -> f(x)/g(x) = -2/3
        if lhs.is_Add and not rhs and len(lhs.args) == 2 and \
                not lhs.is_polynomial(*symbols):
            a, b = ordered(lhs.args)
            ai, ad = a.as_independent(*symbols)
            bi, bd = b.as_independent(*symbols)
            if any(_ispow(i) for i in (ad, bd)):
                a_base, a_exp = ad.as_base_exp()
                b_base, b_exp = bd.as_base_exp()
                if a_base == b_base:
                    # a = -b
                    lhs = powsimp(powdenest(ad/bd))
                    rhs = -bi/ai
                else:
                    rat = ad/bd
                    _lhs = powsimp(ad/bd)
                    if _lhs != rat:
                        lhs = _lhs
                        rhs = -bi/ai
            elif ai == -bi:
                if isinstance(ad, Function) and ad.func == bd.func:
                    if len(ad.args) == len(bd.args) == 1:
                        lhs = ad.args[0] - bd.args[0]
                    elif len(ad.args) == len(bd.args):
                        # should be able to solve
                        # f(x, y) - f(2 - x, 0) == 0 -> x == 1
                        raise NotImplementedError(
                            'equal function with more than 1 argument')
                    else:
                        raise ValueError(
                            'function with different numbers of args')

        elif lhs.is_Mul and any(_ispow(a) for a in lhs.args):
            lhs = powsimp(powdenest(lhs))

        if lhs.is_Function:
            if hasattr(lhs, 'inverse') and lhs.inverse() is not None and len(lhs.args) == 1:
                #                    -1
                # f(x) = g  ->  x = f  (g)
                #
                # /!\ inverse should not be defined if there are multiple values
                # for the function -- these are handled in _tsolve
                #
                rhs = lhs.inverse()(rhs)
                lhs = lhs.args[0]
            elif isinstance(lhs, atan2):
                y, x = lhs.args
                lhs = 2*atan(y/(sqrt(x**2 + y**2) + x))
            elif lhs.func == rhs.func:
                if len(lhs.args) == len(rhs.args) == 1:
                    lhs = lhs.args[0]
                    rhs = rhs.args[0]
                elif len(lhs.args) == len(rhs.args):
                    # should be able to solve
                    # f(x, y) == f(2, 3) -> x == 2
                    # f(x, x + y) == f(2, 3) -> x == 2
                    raise NotImplementedError(
                        'equal function with more than 1 argument')
                else:
                    raise ValueError(
                        'function with different numbers of args')


        if rhs and lhs.is_Pow and lhs.exp.is_Integer and lhs.exp < 0:
            lhs = 1/lhs
            rhs = 1/rhs

        # base**a = b -> base = b**(1/a) if
        #    a is an Integer and dointpow=True (this gives real branch of root)
        #    a is not an Integer and the equation is multivariate and the
        #      base has more than 1 symbol in it
        # The rationale for this is that right now the multi-system solvers
        # doesn't try to resolve generators to see, for example, if the whole
        # system is written in terms of sqrt(x + y) so it will just fail, so we
        # do that step here.
        if lhs.is_Pow and (
            lhs.exp.is_Integer and dointpow or not lhs.exp.is_Integer and
                len(symbols) > 1 and len(lhs.base.free_symbols & set(symbols)) > 1):
            rhs = rhs**(1/lhs.exp)
            lhs = lhs.base

        if lhs == was:
            break
    return rhs, lhs


def unrad(eq, *syms, **flags):
    """
    Remove radicals with symbolic arguments and return (eq, cov),
    None, or raise an error.

    Explanation
    ===========

    None is returned if there are no radicals to remove.

    NotImplementedError is raised if there are radicals and they cannot be
    removed or if the relationship between the original symbols and the
    change of variable needed to rewrite the system as a polynomial cannot
    be solved.

    Otherwise the tuple, ``(eq, cov)``, is returned where:

    *eq*, ``cov``
        *eq* is an equation without radicals (in the symbol(s) of
        interest) whose solutions are a superset of the solutions to the
        original expression. *eq* might be rewritten in terms of a new
        variable; the relationship to the original variables is given by
        ``cov`` which is a list containing ``v`` and ``v**p - b`` where
        ``p`` is the power needed to clear the radical and ``b`` is the
        radical now expressed as a polynomial in the symbols of interest.
        For example, for sqrt(2 - x) the tuple would be
        ``(c, c**2 - 2 + x)``. The solutions of *eq* will contain
        solutions to the original equation (if there are any).

    *syms*
        An iterable of symbols which, if provided, will limit the focus of
        radical removal: only radicals with one or more of the symbols of
        interest will be cleared. All free symbols are used if *syms* is not
        set.

    *flags* are used internally for communication during recursive calls.
    Two options are also recognized:

        ``take``, when defined, is interpreted as a single-argument function
        that returns True if a given Pow should be handled.

    Radicals can be removed from an expression if:

        *   All bases of the radicals are the same; a change of variables is
            done in this case.
        *   If all radicals appear in one term of the expression.
        *   There are only four terms with sqrt() factors or there are less than
            four terms having sqrt() factors.
        *   There are only two terms with radicals.

    Examples
    ========

    >>> from sympy.solvers.solvers import unrad
    >>> from sympy.abc import x
    >>> from sympy import sqrt, Rational, root

    >>> unrad(sqrt(x)*x**Rational(1, 3) + 2)
    (x**5 - 64, [])
    >>> unrad(sqrt(x) + root(x + 1, 3))
    (-x**3 + x**2 + 2*x + 1, [])
    >>> eq = sqrt(x) + root(x, 3) - 2
    >>> unrad(eq)
    (_p**3 + _p**2 - 2, [_p, _p**6 - x])

    """
    from sympy import Equality as Eq

    uflags = dict(check=False, simplify=False)

    def _cov(p, e):
        if cov:
            # XXX - uncovered
            oldp, olde = cov
            if Poly(e, p).degree(p) in (1, 2):
                cov[:] = [p, olde.subs(oldp, _solve(e, p, **uflags)[0])]
            else:
                raise NotImplementedError
        else:
            cov[:] = [p, e]

    def _canonical(eq, cov):
        if cov:
            # change symbol to vanilla so no solutions are eliminated
            p, e = cov
            rep = {p: Dummy(p.name)}
            eq = eq.xreplace(rep)
            cov = [p.xreplace(rep), e.xreplace(rep)]

        # remove constants and powers of factors since these don't change
        # the location of the root; XXX should factor or factor_terms be used?
        eq = factor_terms(_mexpand(eq.as_numer_denom()[0], recursive=True), clear=True)
        if eq.is_Mul:
            args = []
            for f in eq.args:
                if f.is_number:
                    continue
                if f.is_Pow:
                    args.append(f.base)
                else:
                    args.append(f)
            eq = Mul(*args)  # leave as Mul for more efficient solving

        # make the sign canonical
        margs = list(Mul.make_args(eq))
        changed = False
        for i, m in enumerate(margs):
            if m.could_extract_minus_sign():
                margs[i] = -m
                changed = True
        if changed:
            eq = Mul(*margs, evaluate=False)

        return eq, cov

    def _Q(pow):
        # return leading Rational of denominator of Pow's exponent
        c = pow.as_base_exp()[1].as_coeff_Mul()[0]
        if not c.is_Rational:
            return S.One
        return c.q

    # define the _take method that will determine whether a term is of interest
    def _take(d):
        # return True if coefficient of any factor's exponent's den is not 1
        for pow in Mul.make_args(d):
            if not pow.is_Pow:
                continue
            if _Q(pow) == 1:
                continue
            if pow.free_symbols & syms:
                return True
        return False
    _take = flags.setdefault('_take', _take)

    if isinstance(eq, Eq):
        eq = eq.rewrite(Add)
        assert isinstance(eq, Expr)
    elif not isinstance(eq, Expr):
        return

    cov, nwas, rpt = [flags.setdefault(k, v) for k, v in
        sorted(dict(cov=[], n=None, rpt=0).items())]

    # preconditioning
    eq = powdenest(factor_terms(eq, radical=True, clear=True))
    eq = eq.as_numer_denom()[0]
    eq = _mexpand(eq, recursive=True)
    if eq.is_number:
        return

    # see if there are radicals in symbols of interest
    syms = set(syms) or eq.free_symbols  # _take uses this
    poly = eq.as_poly()
    gens = [g for g in poly.gens if _take(g)]
    if not gens:
        return

    # recast poly in terms of eigen-gens
    poly = eq.as_poly(*gens)

    # - an exponent has a symbol of interest (don't handle)
    if any(g.exp.has(*syms) for g in gens):
        return

    def _rads_bases_lcm(poly):
        # if all the bases are the same or all the radicals are in one
        # term, `lcm` will be the lcm of the denominators of the
        # exponents of the radicals
        lcm = 1
        rads = set()
        bases = set()
        for g in poly.gens:
            q = _Q(g)
            if q != 1:
                rads.add(g)
                lcm = ilcm(lcm, q)
                bases.add(g.base)
        return rads, bases, lcm
    rads, bases, lcm = _rads_bases_lcm(poly)

    covsym = Dummy('p', nonnegative=True)

    # only keep in syms symbols that actually appear in radicals;
    # and update gens
    newsyms = set()
    for r in rads:
        newsyms.update(syms & r.free_symbols)
    if newsyms != syms:
        syms = newsyms
        gens = [g for g in gens if g.free_symbols & syms]

    # get terms together that have common generators
    drad = dict(list(zip(rads, list(range(len(rads))))))
    rterms = {(): []}
    args = Add.make_args(poly.as_expr())
    for t in args:
        if _take(t):
            common = set(t.as_poly().gens).intersection(rads)
            key = tuple(sorted([drad[i] for i in common]))
        else:
            key = ()
        rterms.setdefault(key, []).append(t)
    others = Add(*rterms.pop(()))
    rterms = [Add(*rterms[k]) for k in rterms.keys()]

    # the output will depend on the order terms are processed, so
    # make it canonical quickly
    rterms = list(reversed(list(ordered(rterms))))

    ok = False  # we don't have a solution yet
    depth = sqrt_depth(eq)

    if len(rterms) == 1 and not (rterms[0].is_Add and lcm > 2):
        eq = rterms[0]**lcm - ((-others)**lcm)
        ok = True
    else:
        if len(rterms) == 1 and rterms[0].is_Add:
            rterms = list(rterms[0].args)
        if len(bases) == 1:
            b = bases.pop()
            if len(syms) > 1:
                x = b.free_symbols
            else:
                x = syms
            x = list(ordered(x))[0]
            try:
                inv = _solve(covsym**lcm - b, x, **uflags)
                if not inv:
                    raise NotImplementedError
                eq = poly.as_expr().subs(b, covsym**lcm).subs(x, inv[0])
                _cov(covsym, covsym**lcm - b)
                return _canonical(eq, cov)
            except NotImplementedError:
                pass

        if len(rterms) == 2:
            if not others:
                eq = rterms[0]**lcm - (-rterms[1])**lcm
                ok = True
            elif not log(lcm, 2).is_Integer:
                # the lcm-is-power-of-two case is handled below
                r0, r1 = rterms
                if flags.get('_reverse', False):
                    r1, r0 = r0, r1
                i0 = _rads0, _bases0, lcm0 = _rads_bases_lcm(r0.as_poly())
                i1 = _rads1, _bases1, lcm1 = _rads_bases_lcm(r1.as_poly())
                for reverse in range(2):
                    if reverse:
                        i0, i1 = i1, i0
                        r0, r1 = r1, r0
                    _rads1, _, lcm1 = i1
                    _rads1 = Mul(*_rads1)
                    t1 = _rads1**lcm1
                    c = covsym**lcm1 - t1
                    for x in syms:
                        try:
                            sol = _solve(c, x, **uflags)
                            if not sol:
                                raise NotImplementedError
                            neweq = r0.subs(x, sol[0]) + covsym*r1/_rads1 + \
                                others
                            tmp = unrad(neweq, covsym)
                            if tmp:
                                eq, newcov = tmp
                                if newcov:
                                    newp, newc = newcov
                                    _cov(newp, c.subs(covsym,
                                        _solve(newc, covsym, **uflags)[0]))
                                else:
                                    _cov(covsym, c)
                            else:
                                eq = neweq
                                _cov(covsym, c)
                            ok = True
                            break
                        except NotImplementedError:
                            if reverse:
                                raise NotImplementedError(
                                    'no successful change of variable found')
                            else:
                                pass
                    if ok:
                        break
        elif len(rterms) == 3:
            # two cube roots and another with order less than 5
            # (so an analytical solution can be found) or a base
            # that matches one of the cube root bases
            info = [_rads_bases_lcm(i.as_poly()) for i in rterms]
            RAD = 0
            BASES = 1
            LCM = 2
            if info[0][LCM] != 3:
                info.append(info.pop(0))
                rterms.append(rterms.pop(0))
            elif info[1][LCM] != 3:
                info.append(info.pop(1))
                rterms.append(rterms.pop(1))
            if info[0][LCM] == info[1][LCM] == 3:
                if info[1][BASES] != info[2][BASES]:
                    info[0], info[1] = info[1], info[0]
                    rterms[0], rterms[1] = rterms[1], rterms[0]
                if info[1][BASES] == info[2][BASES]:
                    eq = rterms[0]**3 + (rterms[1] + rterms[2] + others)**3
                    ok = True
                elif info[2][LCM] < 5:
                    # a*root(A, 3) + b*root(B, 3) + others = c
                    a, b, c, d, A, B = [Dummy(i) for i in 'abcdAB']
                    # zz represents the unraded expression into which the
                    # specifics for this case are substituted
                    zz = (c - d)*(A**3*a**9 + 3*A**2*B*a**6*b**3 -
                        3*A**2*a**6*c**3 + 9*A**2*a**6*c**2*d - 9*A**2*a**6*c*d**2 +
                        3*A**2*a**6*d**3 + 3*A*B**2*a**3*b**6 + 21*A*B*a**3*b**3*c**3 -
                        63*A*B*a**3*b**3*c**2*d + 63*A*B*a**3*b**3*c*d**2 -
                        21*A*B*a**3*b**3*d**3 + 3*A*a**3*c**6 - 18*A*a**3*c**5*d +
                        45*A*a**3*c**4*d**2 - 60*A*a**3*c**3*d**3 + 45*A*a**3*c**2*d**4 -
                        18*A*a**3*c*d**5 + 3*A*a**3*d**6 + B**3*b**9 - 3*B**2*b**6*c**3 +
                        9*B**2*b**6*c**2*d - 9*B**2*b**6*c*d**2 + 3*B**2*b**6*d**3 +
                        3*B*b**3*c**6 - 18*B*b**3*c**5*d + 45*B*b**3*c**4*d**2 -
                        60*B*b**3*c**3*d**3 + 45*B*b**3*c**2*d**4 - 18*B*b**3*c*d**5 +
                        3*B*b**3*d**6 - c**9 + 9*c**8*d - 36*c**7*d**2 + 84*c**6*d**3 -
                        126*c**5*d**4 + 126*c**4*d**5 - 84*c**3*d**6 + 36*c**2*d**7 -
                        9*c*d**8 + d**9)
                    def _t(i):
                        b = Mul(*info[i][RAD])
                        return cancel(rterms[i]/b), Mul(*info[i][BASES])
                    aa, AA = _t(0)
                    bb, BB = _t(1)
                    cc = -rterms[2]
                    dd = others
                    eq = zz.xreplace(dict(zip(
                        (a, A, b, B, c, d),
                        (aa, AA, bb, BB, cc, dd))))
                    ok = True
        # handle power-of-2 cases
        if not ok:
            if log(lcm, 2).is_Integer and (not others and
                    len(rterms) == 4 or len(rterms) < 4):
                def _norm2(a, b):
                    return a**2 + b**2 + 2*a*b

                if len(rterms) == 4:
                    # (r0+r1)**2 - (r2+r3)**2
                    r0, r1, r2, r3 = rterms
                    eq = _norm2(r0, r1) - _norm2(r2, r3)
                    ok = True
                elif len(rterms) == 3:
                    # (r1+r2)**2 - (r0+others)**2
                    r0, r1, r2 = rterms
                    eq = _norm2(r1, r2) - _norm2(r0, others)
                    ok = True
                elif len(rterms) == 2:
                    # r0**2 - (r1+others)**2
                    r0, r1 = rterms
                    eq = r0**2 - _norm2(r1, others)
                    ok = True

    new_depth = sqrt_depth(eq) if ok else depth
    rpt += 1  # XXX how many repeats with others unchanging is enough?
    if not ok or (
                nwas is not None and len(rterms) == nwas and
                new_depth is not None and new_depth == depth and
                rpt > 3):
        raise NotImplementedError('Cannot remove all radicals')

    flags.update(dict(cov=cov, n=len(rterms), rpt=rpt))
    neq = unrad(eq, *syms, **flags)
    if neq:
        eq, cov = neq
    eq, cov = _canonical(eq, cov)
    return eq, cov

from sympy.solvers.bivariate import (
    bivariate_type, _solve_lambert, _filtered_gens)