"""Implementation of RootOf class and related tools. """
from __future__ import print_function, division
from sympy.core import (S, Expr, Integer, Float, I, Add, Lambda, symbols,
sympify, Rational, Dummy)
from sympy.core.cache import cacheit
from sympy.core.function import AppliedUndef
from sympy.functions.elementary.miscellaneous import root as _root
from sympy.polys.polytools import Poly, PurePoly, factor
from sympy.polys.rationaltools import together
from sympy.polys.polyfuncs import symmetrize, viete
from sympy.polys.rootisolation import (
dup_isolate_complex_roots_sqf,
dup_isolate_real_roots_sqf)
from sympy.polys.polyroots import (
roots_linear, roots_quadratic, roots_binomial,
preprocess_roots, roots)
from sympy.polys.polyerrors import (
MultivariatePolynomialError,
GeneratorsNeeded,
PolynomialError,
DomainError)
from sympy.polys.domains import QQ
from mpmath import mpf, mpc, findroot, workprec
from mpmath.libmp.libmpf import prec_to_dps
from sympy.utilities import lambdify, public
from sympy.core.compatibility import range
from math import log as mathlog
__all__ = ['CRootOf']
def _ispow2(i):
v = mathlog(i, 2)
return v == int(v)
_reals_cache = {}
_complexes_cache = {}
@public
[docs]def rootof(f, x, index=None, radicals=True, expand=True):
"""An indexed root of a univariate polynomial.
Returns either a ``ComplexRootOf`` object or an explicit
expression involving radicals.
Parameters
----------
f : Expr
Univariate polynomial.
x : Symbol, optional
Generator for ``f``.
index : int or Integer
radicals : bool
Return a radical expression if possible.
expand : bool
Expand ``f``.
"""
return CRootOf(f, x, index=index, radicals=radicals, expand=expand)
@public
[docs]class RootOf(Expr):
"""Represents a root of a univariate polynomial.
Base class for roots of different kinds of polynomials.
Only complex roots are currently supported.
"""
__slots__ = ['poly']
def __new__(cls, f, x, index=None, radicals=True, expand=True):
"""Construct a new ``CRootOf`` object for ``k``-th root of ``f``."""
return rootof(f, x, index=index, radicals=radicals, expand=expand)
@public
[docs]class ComplexRootOf(RootOf):
"""Represents an indexed complex root of a polynomial.
Roots of a univariate polynomial separated into disjoint
real or complex intervals and indexed in a fixed order.
Currently only rational coefficients are allowed.
Can be imported as ``CRootOf``.
"""
__slots__ = ['index']
is_complex = True
is_number = True
def __new__(cls, f, x, index=None, radicals=False, expand=True):
""" Construct an indexed complex root of a polynomial.
See ``rootof`` for the parameters.
The default value of ``radicals`` is ``False`` to satisfy
``eval(srepr(expr) == expr``.
"""
x = sympify(x)
if index is None and x.is_Integer:
x, index = None, x
else:
index = sympify(index)
if index is not None and index.is_Integer:
index = int(index)
else:
raise ValueError("expected an integer root index, got %s" % index)
poly = PurePoly(f, x, greedy=False, expand=expand)
if not poly.is_univariate:
raise PolynomialError("only univariate polynomials are allowed")
degree = poly.degree()
if degree <= 0:
raise PolynomialError("can't construct CRootOf object for %s" % f)
if index < -degree or index >= degree:
raise IndexError("root index out of [%d, %d] range, got %d" %
(-degree, degree - 1, index))
elif index < 0:
index += degree
dom = poly.get_domain()
if not dom.is_Exact:
poly = poly.to_exact()
roots = cls._roots_trivial(poly, radicals)
if roots is not None:
return roots[index]
coeff, poly = preprocess_roots(poly)
dom = poly.get_domain()
if not dom.is_ZZ:
raise NotImplementedError("CRootOf is not supported over %s" % dom)
root = cls._indexed_root(poly, index)
return coeff * cls._postprocess_root(root, radicals)
@classmethod
def _new(cls, poly, index):
"""Construct new ``CRootOf`` object from raw data. """
obj = Expr.__new__(cls)
obj.poly = PurePoly(poly)
obj.index = index
try:
_reals_cache[obj.poly] = _reals_cache[poly]
_complexes_cache[obj.poly] = _complexes_cache[poly]
except KeyError:
pass
return obj
def _hashable_content(self):
return (self.poly, self.index)
@property
def expr(self):
return self.poly.as_expr()
@property
def args(self):
return (self.expr, Integer(self.index))
@property
def free_symbols(self):
# CRootOf currently only works with univariate expressions and although
# the poly attribute is often a PurePoly, sometimes it is a Poly. In
# either case no free symbols should be reported.
return set()
def _eval_is_real(self):
"""Return ``True`` if the root is real. """
return self.index < len(_reals_cache[self.poly])
@classmethod
def real_roots(cls, poly, radicals=True):
"""Get real roots of a polynomial. """
return cls._get_roots("_real_roots", poly, radicals)
@classmethod
def all_roots(cls, poly, radicals=True):
"""Get real and complex roots of a polynomial. """
return cls._get_roots("_all_roots", poly, radicals)
@classmethod
def _get_reals_sqf(cls, factor):
"""Get real root isolating intervals for a square-free factor."""
if factor in _reals_cache:
real_part = _reals_cache[factor]
else:
_reals_cache[factor] = real_part = \
dup_isolate_real_roots_sqf(
factor.rep.rep, factor.rep.dom, blackbox=True)
return real_part
@classmethod
def _get_complexes_sqf(cls, factor):
"""Get complex root isolating intervals for a square-free factor."""
if factor in _complexes_cache:
complex_part = _complexes_cache[factor]
else:
_complexes_cache[factor] = complex_part = \
dup_isolate_complex_roots_sqf(
factor.rep.rep, factor.rep.dom, blackbox=True)
return complex_part
@classmethod
def _get_reals(cls, factors):
"""Compute real root isolating intervals for a list of factors. """
reals = []
for factor, k in factors:
real_part = cls._get_reals_sqf(factor)
reals.extend([(root, factor, k) for root in real_part])
return reals
@classmethod
def _get_complexes(cls, factors):
"""Compute complex root isolating intervals for a list of factors. """
complexes = []
for factor, k in factors:
complex_part = cls._get_complexes_sqf(factor)
complexes.extend([(root, factor, k) for root in complex_part])
return complexes
@classmethod
def _reals_sorted(cls, reals):
"""Make real isolating intervals disjoint and sort roots. """
cache = {}
for i, (u, f, k) in enumerate(reals):
for j, (v, g, m) in enumerate(reals[i + 1:]):
u, v = u.refine_disjoint(v)
reals[i + j + 1] = (v, g, m)
reals[i] = (u, f, k)
reals = sorted(reals, key=lambda r: r[0].a)
for root, factor, _ in reals:
if factor in cache:
cache[factor].append(root)
else:
cache[factor] = [root]
for factor, roots in cache.items():
_reals_cache[factor] = roots
return reals
@classmethod
def _separate_imaginary_from_complex(cls, complexes):
from sympy.utilities.iterables import sift
def is_imag(c):
'''
return True if all roots are imaginary (ax**2 + b)
return False if no roots are imaginary
return None if 2 roots are imaginary (ax**N'''
u, f, k = c
deg = f.degree()
if f.length() == 2:
if deg == 2:
return True # both imag
elif _ispow2(deg):
if f.LC()*f.TC() < 0:
return None # 2 are imag
return False # none are imag
# separate according to the function
sifted = sift(complexes, lambda c: c[1])
del complexes
imag = []
complexes = []
for f in sifted:
isift = sift(sifted[f], lambda c: is_imag(c))
imag.extend(isift.pop(True, []))
complexes.extend(isift.pop(False, []))
mixed = isift.pop(None, [])
assert not isift
if not mixed:
continue
while True:
# the non-imaginary ones will be on one side or the other
# of the y-axis
i = 0
while i < len(mixed):
u, f, k = mixed[i]
if u.ax*u.bx > 0:
complexes.append(mixed.pop(i))
else:
i += 1
if len(mixed) == 2:
imag.extend(mixed)
break
# refine
for i, (u, f, k) in enumerate(mixed):
u = u._inner_refine()
mixed[i] = u, f, k
return imag, complexes
@classmethod
def _refine_complexes(cls, complexes):
"""return complexes such that no bounding rectangles of non-conjugate
roots would intersect if slid horizontally or vertically/
"""
while complexes: # break when all are distinct
# get the intervals pairwise-disjoint.
# If rectangles were drawn around the coordinates of the bounding
# rectangles, no rectangles would intersect after this procedure.
for i, (u, f, k) in enumerate(complexes):
for j, (v, g, m) in enumerate(complexes[i + 1:]):
u, v = u.refine_disjoint(v)
complexes[i + j + 1] = (v, g, m)
complexes[i] = (u, f, k)
# Although there are no intersecting rectangles, a given rectangle
# might intersect another when slid horizontally. We have to refine
# intervals until this is not true so we can sort the roots
# unambiguously. Since complex roots come in conjugate pairs, we
# will always have 2 rectangles above each other but we should not
# have more than that.
N = len(complexes)//2 - 1
# check x (real) parts: there must be N + 1 disjoint x ranges, i.e.
# the first one must be different from N others
uu = set([(u.ax, u.bx) for u, _, _ in complexes])
u = uu.pop()
if sum([u[1] <= v[0] or v[1] <= u[0] for v in uu]) < N:
# refine
for i, (u, f, k) in enumerate(complexes):
u = u._inner_refine()
complexes[i] = u, f, k
else:
# intervals with identical x-values have disjoint y-values or
# else they would not be disjoint so there is no need for
# further checks
break
return complexes
@classmethod
def _complexes_sorted(cls, complexes):
"""Make complex isolating intervals disjoint and sort roots. """
if not complexes:
return []
cache = {}
# imaginary roots can cause a problem in terms of sorting since
# their x-intervals will never refine as distinct from others
# so we handle them separately
imag, complexes = cls._separate_imaginary_from_complex(complexes)
complexes = cls._refine_complexes(complexes)
# sort imaginary roots
def key(c):
'''return, for ax**n+b, +/-root(abs(b/a), b) according to the
apparent sign of the imaginary interval, e.g. if the interval
were (0, 3) the positive root would be returned.
'''
u, f, k = c
r = _root(abs(f.TC()/f.LC()), f.degree())
if u.ay < 0 or u.by < 0:
return -r
return r
imag = sorted(imag, key=lambda c: key(c))
# sort complexes and combine with imag
if complexes:
# key is (x1, y1) e.g. (1, 2)x(3, 4) -> (1,3)
complexes = sorted(complexes, key=lambda c: c[0].a)
# find insertion point for imaginary
for i, c in enumerate(reversed(complexes)):
if c[0].bx <= 0:
break
i = len(complexes) - i - 1
if i:
i += 1
complexes = complexes[:i] + imag + complexes[i:]
else:
complexes = imag
# update cache
for root, factor, _ in complexes:
if factor in cache:
cache[factor].append(root)
else:
cache[factor] = [root]
for factor, roots in cache.items():
_complexes_cache[factor] = roots
return complexes
@classmethod
def _reals_index(cls, reals, index):
"""
Map initial real root index to an index in a factor where
the root belongs.
"""
i = 0
for j, (_, factor, k) in enumerate(reals):
if index < i + k:
poly, index = factor, 0
for _, factor, _ in reals[:j]:
if factor == poly:
index += 1
return poly, index
else:
i += k
@classmethod
def _complexes_index(cls, complexes, index):
"""
Map initial complex root index to an index in a factor where
the root belongs.
"""
index, i = index, 0
for j, (_, factor, k) in enumerate(complexes):
if index < i + k:
poly, index = factor, 0
for _, factor, _ in complexes[:j]:
if factor == poly:
index += 1
index += len(_reals_cache[poly])
return poly, index
else:
i += k
@classmethod
def _count_roots(cls, roots):
"""Count the number of real or complex roots with multiplicities."""
return sum([k for _, _, k in roots])
@classmethod
def _indexed_root(cls, poly, index):
"""Get a root of a composite polynomial by index. """
(_, factors) = poly.factor_list()
reals = cls._get_reals(factors)
reals_count = cls._count_roots(reals)
if index < reals_count:
reals = cls._reals_sorted(reals)
return cls._reals_index(reals, index)
else:
complexes = cls._get_complexes(factors)
complexes = cls._complexes_sorted(complexes)
return cls._complexes_index(complexes, index - reals_count)
@classmethod
def _real_roots(cls, poly):
"""Get real roots of a composite polynomial. """
(_, factors) = poly.factor_list()
reals = cls._get_reals(factors)
reals = cls._reals_sorted(reals)
reals_count = cls._count_roots(reals)
roots = []
for index in range(0, reals_count):
roots.append(cls._reals_index(reals, index))
return roots
@classmethod
def _all_roots(cls, poly):
"""Get real and complex roots of a composite polynomial. """
(_, factors) = poly.factor_list()
reals = cls._get_reals(factors)
reals = cls._reals_sorted(reals)
reals_count = cls._count_roots(reals)
roots = []
for index in range(0, reals_count):
roots.append(cls._reals_index(reals, index))
complexes = cls._get_complexes(factors)
complexes = cls._complexes_sorted(complexes)
complexes_count = cls._count_roots(complexes)
for index in range(0, complexes_count):
roots.append(cls._complexes_index(complexes, index))
return roots
@classmethod
@cacheit
def _roots_trivial(cls, poly, radicals):
"""Compute roots in linear, quadratic and binomial cases. """
if poly.degree() == 1:
return roots_linear(poly)
if not radicals:
return None
if poly.degree() == 2:
return roots_quadratic(poly)
elif poly.length() == 2 and poly.TC():
return roots_binomial(poly)
else:
return None
@classmethod
def _preprocess_roots(cls, poly):
"""Take heroic measures to make ``poly`` compatible with ``CRootOf``."""
dom = poly.get_domain()
if not dom.is_Exact:
poly = poly.to_exact()
coeff, poly = preprocess_roots(poly)
dom = poly.get_domain()
if not dom.is_ZZ:
raise NotImplementedError(
"sorted roots not supported over %s" % dom)
return coeff, poly
@classmethod
def _postprocess_root(cls, root, radicals):
"""Return the root if it is trivial or a ``CRootOf`` object. """
poly, index = root
roots = cls._roots_trivial(poly, radicals)
if roots is not None:
return roots[index]
else:
return cls._new(poly, index)
@classmethod
def _get_roots(cls, method, poly, radicals):
"""Return postprocessed roots of specified kind. """
if not poly.is_univariate:
raise PolynomialError("only univariate polynomials are allowed")
coeff, poly = cls._preprocess_roots(poly)
roots = []
for root in getattr(cls, method)(poly):
roots.append(coeff*cls._postprocess_root(root, radicals))
return roots
def _get_interval(self):
"""Internal function for retrieving isolation interval from cache. """
if self.is_real:
return _reals_cache[self.poly][self.index]
else:
reals_count = len(_reals_cache[self.poly])
return _complexes_cache[self.poly][self.index - reals_count]
def _set_interval(self, interval):
"""Internal function for updating isolation interval in cache. """
if self.is_real:
_reals_cache[self.poly][self.index] = interval
else:
reals_count = len(_reals_cache[self.poly])
_complexes_cache[self.poly][self.index - reals_count] = interval
def _eval_subs(self, old, new):
# don't allow subs to change anything
return self
def _eval_evalf(self, prec):
"""Evaluate this complex root to the given precision. """
with workprec(prec):
g = self.poly.gen
if not g.is_Symbol:
d = Dummy('x')
func = lambdify(d, self.expr.subs(g, d))
else:
func = lambdify(g, self.expr)
interval = self._get_interval()
if not self.is_real:
# For complex intervals, we need to keep refining until the
# imaginary interval is disjunct with other roots, that is,
# until both ends get refined.
ay = interval.ay
by = interval.by
while interval.ay == ay or interval.by == by:
interval = interval.refine()
while True:
if self.is_real:
a = mpf(str(interval.a))
b = mpf(str(interval.b))
if a == b:
root = a
break
x0 = mpf(str(interval.center))
else:
ax = mpf(str(interval.ax))
bx = mpf(str(interval.bx))
ay = mpf(str(interval.ay))
by = mpf(str(interval.by))
if ax == bx and ay == by:
# the sign of the imaginary part will be assigned
# according to the desired index using the fact that
# roots are sorted with negative imag parts coming
# before positive (and all imag roots coming after real
# roots)
deg = self.poly.degree()
i = self.index # a positive attribute after creation
if (deg - i) % 2:
if ay < 0:
ay = -ay
else:
if ay > 0:
ay = -ay
root = mpc(ax, ay)
break
x0 = mpc(*map(str, interval.center))
try:
root = findroot(func, x0)
# If the (real or complex) root is not in the 'interval',
# then keep refining the interval. This happens if findroot
# accidentally finds a different root outside of this
# interval because our initial estimate 'x0' was not close
# enough. It is also possible that the secant method will
# get trapped by a max/min in the interval; the root
# verification by findroot will raise a ValueError in this
# case and the interval will then be tightened -- and
# eventually the root will be found.
#
# It is also possible that findroot will not have any
# successful iterations to process (in which case it
# will fail to initialize a variable that is tested
# after the iterations and raise an UnboundLocalError).
if self.is_real:
if (a <= root <= b):
break
elif (ax <= root.real <= bx and ay <= root.imag <= by):
break
except (UnboundLocalError, ValueError):
pass
interval = interval.refine()
return (Float._new(root.real._mpf_, prec)
+ I*Float._new(root.imag._mpf_, prec))
def eval_rational(self, tol):
"""
Return a Rational approximation to ``self`` with the tolerance ``tol``.
This method uses bisection, which is very robust and it will always
converge. The returned Rational instance will be at most 'tol' from the
exact root.
The following example first obtains Rational approximation to 1e-7
accuracy for all roots of the 4-th order Legendre polynomial, and then
evaluates it to 5 decimal digits (so all digits will be correct
including rounding):
>>> from sympy import S, legendre_poly, Symbol
>>> x = Symbol("x")
>>> p = legendre_poly(4, x, polys=True)
>>> roots = [r.eval_rational(S(1)/10**7) for r in p.real_roots()]
>>> roots = [str(r.n(5)) for r in roots]
>>> roots
['-0.86114', '-0.33998', '0.33998', '0.86114']
"""
if not self.is_real:
raise NotImplementedError(
"eval_rational() only works for real polynomials so far")
func = lambdify(self.poly.gen, self.expr)
interval = self._get_interval()
a = Rational(str(interval.a))
b = Rational(str(interval.b))
return bisect(func, a, b, tol)
def _eval_Eq(self, other):
# CRootOf represents a Root, so if other is that root, it should set
# the expression to zero *and* it should be in the interval of the
# CRootOf instance. It must also be a number that agrees with the
# is_real value of the CRootOf instance.
if type(self) == type(other):
return sympify(self.__eq__(other))
if not (other.is_number and not other.has(AppliedUndef)):
return S.false
if not other.is_finite:
return S.false
z = self.expr.subs(self.expr.free_symbols.pop(), other).is_zero
if z is False: # all roots will make z True but we don't know
# whether this is the right root if z is True
return S.false
o = other.is_real, other.is_imaginary
s = self.is_real, self.is_imaginary
if o != s and None not in o and None not in s:
return S.false
i = self._get_interval()
was = i.a, i.b
need = [True]*2
# make sure it would be distinct from others
while any(need):
i = i.refine()
a, b = i.a, i.b
if need[0] and a != was[0]:
need[0] = False
if need[1] and b != was[1]:
need[1] = False
re, im = other.as_real_imag()
if not im:
if self.is_real:
a, b = [Rational(str(i)) for i in (a, b)]
return sympify(a < other and other < b)
return S.false
if self.is_real:
return S.false
z = r1, r2, i1, i2 = [Rational(str(j)) for j in (
i.ax, i.bx, i.ay, i.by)]
return sympify((
r1 < re and re < r2) and (
i1 < im and im < i2))
CRootOf = ComplexRootOf
@public
[docs]class RootSum(Expr):
"""Represents a sum of all roots of a univariate polynomial. """
__slots__ = ['poly', 'fun', 'auto']
def __new__(cls, expr, func=None, x=None, auto=True, quadratic=False):
"""Construct a new ``RootSum`` instance of roots of a polynomial."""
coeff, poly = cls._transform(expr, x)
if not poly.is_univariate:
raise MultivariatePolynomialError(
"only univariate polynomials are allowed")
if func is None:
func = Lambda(poly.gen, poly.gen)
else:
try:
is_func = func.is_Function
except AttributeError:
is_func = False
if is_func and 1 in func.nargs:
if not isinstance(func, Lambda):
func = Lambda(poly.gen, func(poly.gen))
else:
raise ValueError(
"expected a univariate function, got %s" % func)
var, expr = func.variables[0], func.expr
if coeff is not S.One:
expr = expr.subs(var, coeff*var)
deg = poly.degree()
if not expr.has(var):
return deg*expr
if expr.is_Add:
add_const, expr = expr.as_independent(var)
else:
add_const = S.Zero
if expr.is_Mul:
mul_const, expr = expr.as_independent(var)
else:
mul_const = S.One
func = Lambda(var, expr)
rational = cls._is_func_rational(poly, func)
(_, factors), terms = poly.factor_list(), []
for poly, k in factors:
if poly.is_linear:
term = func(roots_linear(poly)[0])
elif quadratic and poly.is_quadratic:
term = sum(map(func, roots_quadratic(poly)))
else:
if not rational or not auto:
term = cls._new(poly, func, auto)
else:
term = cls._rational_case(poly, func)
terms.append(k*term)
return mul_const*Add(*terms) + deg*add_const
@classmethod
def _new(cls, poly, func, auto=True):
"""Construct new raw ``RootSum`` instance. """
obj = Expr.__new__(cls)
obj.poly = poly
obj.fun = func
obj.auto = auto
return obj
@classmethod
def new(cls, poly, func, auto=True):
"""Construct new ``RootSum`` instance. """
if not func.expr.has(*func.variables):
return func.expr
rational = cls._is_func_rational(poly, func)
if not rational or not auto:
return cls._new(poly, func, auto)
else:
return cls._rational_case(poly, func)
@classmethod
def _transform(cls, expr, x):
"""Transform an expression to a polynomial. """
poly = PurePoly(expr, x, greedy=False)
return preprocess_roots(poly)
@classmethod
def _is_func_rational(cls, poly, func):
"""Check if a lambda is areational function. """
var, expr = func.variables[0], func.expr
return expr.is_rational_function(var)
@classmethod
def _rational_case(cls, poly, func):
"""Handle the rational function case. """
roots = symbols('r:%d' % poly.degree())
var, expr = func.variables[0], func.expr
f = sum(expr.subs(var, r) for r in roots)
p, q = together(f).as_numer_denom()
domain = QQ[roots]
p = p.expand()
q = q.expand()
try:
p = Poly(p, domain=domain, expand=False)
except GeneratorsNeeded:
p, p_coeff = None, (p,)
else:
p_monom, p_coeff = zip(*p.terms())
try:
q = Poly(q, domain=domain, expand=False)
except GeneratorsNeeded:
q, q_coeff = None, (q,)
else:
q_monom, q_coeff = zip(*q.terms())
coeffs, mapping = symmetrize(p_coeff + q_coeff, formal=True)
formulas, values = viete(poly, roots), []
for (sym, _), (_, val) in zip(mapping, formulas):
values.append((sym, val))
for i, (coeff, _) in enumerate(coeffs):
coeffs[i] = coeff.subs(values)
n = len(p_coeff)
p_coeff = coeffs[:n]
q_coeff = coeffs[n:]
if p is not None:
p = Poly(dict(zip(p_monom, p_coeff)), *p.gens).as_expr()
else:
(p,) = p_coeff
if q is not None:
q = Poly(dict(zip(q_monom, q_coeff)), *q.gens).as_expr()
else:
(q,) = q_coeff
return factor(p/q)
def _hashable_content(self):
return (self.poly, self.fun)
@property
def expr(self):
return self.poly.as_expr()
@property
def args(self):
return (self.expr, self.fun, self.poly.gen)
@property
def free_symbols(self):
return self.poly.free_symbols | self.fun.free_symbols
@property
def is_commutative(self):
return True
def doit(self, **hints):
if not hints.get('roots', True):
return self
_roots = roots(self.poly, multiple=True)
if len(_roots) < self.poly.degree():
return self
else:
return Add(*[self.fun(r) for r in _roots])
def _eval_evalf(self, prec):
try:
_roots = self.poly.nroots(n=prec_to_dps(prec))
except (DomainError, PolynomialError):
return self
else:
return Add(*[self.fun(r) for r in _roots])
def _eval_derivative(self, x):
var, expr = self.fun.args
func = Lambda(var, expr.diff(x))
return self.new(self.poly, func, self.auto)
def bisect(f, a, b, tol):
"""
Implements bisection. This function is used in RootOf.eval_rational() and
it needs to be robust.
Examples
========
>>> from sympy import S
>>> from sympy.polys.rootoftools import bisect
>>> bisect(lambda x: x**2-1, -10, 0, S(1)/10**2)
-1025/1024
>>> bisect(lambda x: x**2-1, -10, 0, S(1)/10**4)
-131075/131072
"""
a = sympify(a)
b = sympify(b)
fa = f(a)
fb = f(b)
if fa * fb >= 0:
raise ValueError("bisect: f(a) and f(b) must have opposite signs")
while (b - a > tol):
c = (a + b)/2
fc = f(c)
if (fc == 0):
return c # We need to make sure f(c) is not zero below
if (fa * fc < 0):
b = c
fb = fc
else:
a = c
fa = fc
return (a + b)/2
