739 lines
24 KiB
Python
739 lines
24 KiB
Python
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"""
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Limits
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======
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Implemented according to the PhD thesis
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https://www.cybertester.com/data/gruntz.pdf, which contains very thorough
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descriptions of the algorithm including many examples. We summarize here
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the gist of it.
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All functions are sorted according to how rapidly varying they are at
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infinity using the following rules. Any two functions f and g can be
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compared using the properties of L:
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L=lim log|f(x)| / log|g(x)| (for x -> oo)
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We define >, < ~ according to::
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1. f > g .... L=+-oo
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we say that:
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- f is greater than any power of g
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- f is more rapidly varying than g
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- f goes to infinity/zero faster than g
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2. f < g .... L=0
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we say that:
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- f is lower than any power of g
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3. f ~ g .... L!=0, +-oo
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we say that:
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- both f and g are bounded from above and below by suitable integral
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powers of the other
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Examples
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========
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::
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2 < x < exp(x) < exp(x**2) < exp(exp(x))
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2 ~ 3 ~ -5
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x ~ x**2 ~ x**3 ~ 1/x ~ x**m ~ -x
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exp(x) ~ exp(-x) ~ exp(2x) ~ exp(x)**2 ~ exp(x+exp(-x))
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f ~ 1/f
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So we can divide all the functions into comparability classes (x and x^2
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belong to one class, exp(x) and exp(-x) belong to some other class). In
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principle, we could compare any two functions, but in our algorithm, we
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do not compare anything below the class 2~3~-5 (for example log(x) is
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below this), so we set 2~3~-5 as the lowest comparability class.
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Given the function f, we find the list of most rapidly varying (mrv set)
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subexpressions of it. This list belongs to the same comparability class.
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Let's say it is {exp(x), exp(2x)}. Using the rule f ~ 1/f we find an
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element "w" (either from the list or a new one) from the same
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comparability class which goes to zero at infinity. In our example we
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set w=exp(-x) (but we could also set w=exp(-2x) or w=exp(-3x) ...). We
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rewrite the mrv set using w, in our case {1/w, 1/w^2}, and substitute it
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into f. Then we expand f into a series in w::
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f = c0*w^e0 + c1*w^e1 + ... + O(w^en), where e0<e1<...<en, c0!=0
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but for x->oo, lim f = lim c0*w^e0, because all the other terms go to zero,
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because w goes to zero faster than the ci and ei. So::
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for e0>0, lim f = 0
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for e0<0, lim f = +-oo (the sign depends on the sign of c0)
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for e0=0, lim f = lim c0
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We need to recursively compute limits at several places of the algorithm, but
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as is shown in the PhD thesis, it always finishes.
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Important functions from the implementation:
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compare(a, b, x) compares "a" and "b" by computing the limit L.
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mrv(e, x) returns list of most rapidly varying (mrv) subexpressions of "e"
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rewrite(e, Omega, x, wsym) rewrites "e" in terms of w
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leadterm(f, x) returns the lowest power term in the series of f
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mrv_leadterm(e, x) returns the lead term (c0, e0) for e
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limitinf(e, x) computes lim e (for x->oo)
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limit(e, z, z0) computes any limit by converting it to the case x->oo
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All the functions are really simple and straightforward except
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rewrite(), which is the most difficult/complex part of the algorithm.
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When the algorithm fails, the bugs are usually in the series expansion
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(i.e. in SymPy) or in rewrite.
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This code is almost exact rewrite of the Maple code inside the Gruntz
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thesis.
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Debugging
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---------
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Because the gruntz algorithm is highly recursive, it's difficult to
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figure out what went wrong inside a debugger. Instead, turn on nice
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debug prints by defining the environment variable SYMPY_DEBUG. For
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example:
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[user@localhost]: SYMPY_DEBUG=True ./bin/isympy
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In [1]: limit(sin(x)/x, x, 0)
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limitinf(_x*sin(1/_x), _x) = 1
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+-mrv_leadterm(_x*sin(1/_x), _x) = (1, 0)
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| +-mrv(_x*sin(1/_x), _x) = set([_x])
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| | +-mrv(_x, _x) = set([_x])
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| | +-mrv(sin(1/_x), _x) = set([_x])
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| | +-mrv(1/_x, _x) = set([_x])
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| | +-mrv(_x, _x) = set([_x])
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| +-mrv_leadterm(exp(_x)*sin(exp(-_x)), _x, set([exp(_x)])) = (1, 0)
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| +-rewrite(exp(_x)*sin(exp(-_x)), set([exp(_x)]), _x, _w) = (1/_w*sin(_w), -_x)
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| +-sign(_x, _x) = 1
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| +-mrv_leadterm(1, _x) = (1, 0)
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+-sign(0, _x) = 0
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+-limitinf(1, _x) = 1
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And check manually which line is wrong. Then go to the source code and
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debug this function to figure out the exact problem.
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"""
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from functools import reduce
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from sympy.core import Basic, S, Mul, PoleError, expand_mul
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from sympy.core.cache import cacheit
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from sympy.core.numbers import ilcm, I, oo
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from sympy.core.symbol import Dummy, Wild
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from sympy.core.traversal import bottom_up
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from sympy.functions import log, exp, sign as _sign
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from sympy.series.order import Order
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from sympy.utilities.exceptions import SymPyDeprecationWarning
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from sympy.utilities.misc import debug_decorator as debug
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from sympy.utilities.timeutils import timethis
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timeit = timethis('gruntz')
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def compare(a, b, x):
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"""Returns "<" if a<b, "=" for a == b, ">" for a>b"""
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# log(exp(...)) must always be simplified here for termination
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la, lb = log(a), log(b)
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if isinstance(a, Basic) and (isinstance(a, exp) or (a.is_Pow and a.base == S.Exp1)):
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la = a.exp
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if isinstance(b, Basic) and (isinstance(b, exp) or (b.is_Pow and b.base == S.Exp1)):
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lb = b.exp
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c = limitinf(la/lb, x)
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if c == 0:
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return "<"
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elif c.is_infinite:
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return ">"
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else:
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return "="
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class SubsSet(dict):
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"""
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Stores (expr, dummy) pairs, and how to rewrite expr-s.
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Explanation
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===========
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The gruntz algorithm needs to rewrite certain expressions in term of a new
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variable w. We cannot use subs, because it is just too smart for us. For
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example::
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> Omega=[exp(exp(_p - exp(-_p))/(1 - 1/_p)), exp(exp(_p))]
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> O2=[exp(-exp(_p) + exp(-exp(-_p))*exp(_p)/(1 - 1/_p))/_w, 1/_w]
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> e = exp(exp(_p - exp(-_p))/(1 - 1/_p)) - exp(exp(_p))
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> e.subs(Omega[0],O2[0]).subs(Omega[1],O2[1])
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-1/w + exp(exp(p)*exp(-exp(-p))/(1 - 1/p))
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is really not what we want!
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So we do it the hard way and keep track of all the things we potentially
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want to substitute by dummy variables. Consider the expression::
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exp(x - exp(-x)) + exp(x) + x.
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The mrv set is {exp(x), exp(-x), exp(x - exp(-x))}.
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We introduce corresponding dummy variables d1, d2, d3 and rewrite::
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d3 + d1 + x.
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This class first of all keeps track of the mapping expr->variable, i.e.
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will at this stage be a dictionary::
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{exp(x): d1, exp(-x): d2, exp(x - exp(-x)): d3}.
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[It turns out to be more convenient this way round.]
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But sometimes expressions in the mrv set have other expressions from the
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mrv set as subexpressions, and we need to keep track of that as well. In
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this case, d3 is really exp(x - d2), so rewrites at this stage is::
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{d3: exp(x-d2)}.
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The function rewrite uses all this information to correctly rewrite our
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expression in terms of w. In this case w can be chosen to be exp(-x),
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i.e. d2. The correct rewriting then is::
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exp(-w)/w + 1/w + x.
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"""
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def __init__(self):
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self.rewrites = {}
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def __repr__(self):
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return super().__repr__() + ', ' + self.rewrites.__repr__()
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def __getitem__(self, key):
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if key not in self:
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self[key] = Dummy()
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return dict.__getitem__(self, key)
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def do_subs(self, e):
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"""Substitute the variables with expressions"""
|
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for expr, var in self.items():
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e = e.xreplace({var: expr})
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return e
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def meets(self, s2):
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"""Tell whether or not self and s2 have non-empty intersection"""
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return set(self.keys()).intersection(list(s2.keys())) != set()
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def union(self, s2, exps=None):
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"""Compute the union of self and s2, adjusting exps"""
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res = self.copy()
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tr = {}
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for expr, var in s2.items():
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if expr in self:
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if exps:
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exps = exps.xreplace({var: res[expr]})
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tr[var] = res[expr]
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else:
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res[expr] = var
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for var, rewr in s2.rewrites.items():
|
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res.rewrites[var] = rewr.xreplace(tr)
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return res, exps
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def copy(self):
|
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|
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"""Create a shallow copy of SubsSet"""
|
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r = SubsSet()
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r.rewrites = self.rewrites.copy()
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for expr, var in self.items():
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r[expr] = var
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return r
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@debug
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def mrv(e, x):
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"""Returns a SubsSet of most rapidly varying (mrv) subexpressions of 'e',
|
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and e rewritten in terms of these"""
|
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|
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from sympy.simplify.powsimp import powsimp
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e = powsimp(e, deep=True, combine='exp')
|
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if not isinstance(e, Basic):
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raise TypeError("e should be an instance of Basic")
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if not e.has(x):
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return SubsSet(), e
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elif e == x:
|
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s = SubsSet()
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return s, s[x]
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elif e.is_Mul or e.is_Add:
|
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i, d = e.as_independent(x) # throw away x-independent terms
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if d.func != e.func:
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s, expr = mrv(d, x)
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return s, e.func(i, expr)
|
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|
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a, b = d.as_two_terms()
|
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s1, e1 = mrv(a, x)
|
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s2, e2 = mrv(b, x)
|
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return mrv_max1(s1, s2, e.func(i, e1, e2), x)
|
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elif e.is_Pow and e.base != S.Exp1:
|
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e1 = S.One
|
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|
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while e.is_Pow:
|
||
|
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b1 = e.base
|
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|
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e1 *= e.exp
|
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e = b1
|
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|
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if b1 == 1:
|
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return SubsSet(), b1
|
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|
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if e1.has(x):
|
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|
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base_lim = limitinf(b1, x)
|
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|
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if base_lim is S.One:
|
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|
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return mrv(exp(e1 * (b1 - 1)), x)
|
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|
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return mrv(exp(e1 * log(b1)), x)
|
||
|
|
else:
|
||
|
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s, expr = mrv(b1, x)
|
||
|
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return s, expr**e1
|
||
|
|
elif isinstance(e, log):
|
||
|
|
s, expr = mrv(e.args[0], x)
|
||
|
|
return s, log(expr)
|
||
|
|
elif isinstance(e, exp) or (e.is_Pow and e.base == S.Exp1):
|
||
|
|
# We know from the theory of this algorithm that exp(log(...)) may always
|
||
|
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# be simplified here, and doing so is vital for termination.
|
||
|
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if isinstance(e.exp, log):
|
||
|
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return mrv(e.exp.args[0], x)
|
||
|
|
# if a product has an infinite factor the result will be
|
||
|
|
# infinite if there is no zero, otherwise NaN; here, we
|
||
|
|
# consider the result infinite if any factor is infinite
|
||
|
|
li = limitinf(e.exp, x)
|
||
|
|
if any(_.is_infinite for _ in Mul.make_args(li)):
|
||
|
|
s1 = SubsSet()
|
||
|
|
e1 = s1[e]
|
||
|
|
s2, e2 = mrv(e.exp, x)
|
||
|
|
su = s1.union(s2)[0]
|
||
|
|
su.rewrites[e1] = exp(e2)
|
||
|
|
return mrv_max3(s1, e1, s2, exp(e2), su, e1, x)
|
||
|
|
else:
|
||
|
|
s, expr = mrv(e.exp, x)
|
||
|
|
return s, exp(expr)
|
||
|
|
elif e.is_Function:
|
||
|
|
l = [mrv(a, x) for a in e.args]
|
||
|
|
l2 = [s for (s, _) in l if s != SubsSet()]
|
||
|
|
if len(l2) != 1:
|
||
|
|
# e.g. something like BesselJ(x, x)
|
||
|
|
raise NotImplementedError("MRV set computation for functions in"
|
||
|
|
" several variables not implemented.")
|
||
|
|
s, ss = l2[0], SubsSet()
|
||
|
|
args = [ss.do_subs(x[1]) for x in l]
|
||
|
|
return s, e.func(*args)
|
||
|
|
elif e.is_Derivative:
|
||
|
|
raise NotImplementedError("MRV set computation for derivatives"
|
||
|
|
" not implemented yet.")
|
||
|
|
raise NotImplementedError(
|
||
|
|
"Don't know how to calculate the mrv of '%s'" % e)
|
||
|
|
|
||
|
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|
||
|
|
def mrv_max3(f, expsf, g, expsg, union, expsboth, x):
|
||
|
|
"""
|
||
|
|
Computes the maximum of two sets of expressions f and g, which
|
||
|
|
are in the same comparability class, i.e. max() compares (two elements of)
|
||
|
|
f and g and returns either (f, expsf) [if f is larger], (g, expsg)
|
||
|
|
[if g is larger] or (union, expsboth) [if f, g are of the same class].
|
||
|
|
"""
|
||
|
|
if not isinstance(f, SubsSet):
|
||
|
|
raise TypeError("f should be an instance of SubsSet")
|
||
|
|
if not isinstance(g, SubsSet):
|
||
|
|
raise TypeError("g should be an instance of SubsSet")
|
||
|
|
if f == SubsSet():
|
||
|
|
return g, expsg
|
||
|
|
elif g == SubsSet():
|
||
|
|
return f, expsf
|
||
|
|
elif f.meets(g):
|
||
|
|
return union, expsboth
|
||
|
|
|
||
|
|
c = compare(list(f.keys())[0], list(g.keys())[0], x)
|
||
|
|
if c == ">":
|
||
|
|
return f, expsf
|
||
|
|
elif c == "<":
|
||
|
|
return g, expsg
|
||
|
|
else:
|
||
|
|
if c != "=":
|
||
|
|
raise ValueError("c should be =")
|
||
|
|
return union, expsboth
|
||
|
|
|
||
|
|
|
||
|
|
def mrv_max1(f, g, exps, x):
|
||
|
|
"""Computes the maximum of two sets of expressions f and g, which
|
||
|
|
are in the same comparability class, i.e. mrv_max1() compares (two elements of)
|
||
|
|
f and g and returns the set, which is in the higher comparability class
|
||
|
|
of the union of both, if they have the same order of variation.
|
||
|
|
Also returns exps, with the appropriate substitutions made.
|
||
|
|
"""
|
||
|
|
u, b = f.union(g, exps)
|
||
|
|
return mrv_max3(f, g.do_subs(exps), g, f.do_subs(exps),
|
||
|
|
u, b, x)
|
||
|
|
|
||
|
|
|
||
|
|
@debug
|
||
|
|
@cacheit
|
||
|
|
@timeit
|
||
|
|
def sign(e, x):
|
||
|
|
"""
|
||
|
|
Returns a sign of an expression e(x) for x->oo.
|
||
|
|
|
||
|
|
::
|
||
|
|
|
||
|
|
e > 0 for x sufficiently large ... 1
|
||
|
|
e == 0 for x sufficiently large ... 0
|
||
|
|
e < 0 for x sufficiently large ... -1
|
||
|
|
|
||
|
|
The result of this function is currently undefined if e changes sign
|
||
|
|
arbitrarily often for arbitrarily large x (e.g. sin(x)).
|
||
|
|
|
||
|
|
Note that this returns zero only if e is *constantly* zero
|
||
|
|
for x sufficiently large. [If e is constant, of course, this is just
|
||
|
|
the same thing as the sign of e.]
|
||
|
|
"""
|
||
|
|
if not isinstance(e, Basic):
|
||
|
|
raise TypeError("e should be an instance of Basic")
|
||
|
|
|
||
|
|
if e.is_positive:
|
||
|
|
return 1
|
||
|
|
elif e.is_negative:
|
||
|
|
return -1
|
||
|
|
elif e.is_zero:
|
||
|
|
return 0
|
||
|
|
|
||
|
|
elif not e.has(x):
|
||
|
|
from sympy.simplify import logcombine
|
||
|
|
e = logcombine(e)
|
||
|
|
return _sign(e)
|
||
|
|
elif e == x:
|
||
|
|
return 1
|
||
|
|
elif e.is_Mul:
|
||
|
|
a, b = e.as_two_terms()
|
||
|
|
sa = sign(a, x)
|
||
|
|
if not sa:
|
||
|
|
return 0
|
||
|
|
return sa * sign(b, x)
|
||
|
|
elif isinstance(e, exp):
|
||
|
|
return 1
|
||
|
|
elif e.is_Pow:
|
||
|
|
if e.base == S.Exp1:
|
||
|
|
return 1
|
||
|
|
s = sign(e.base, x)
|
||
|
|
if s == 1:
|
||
|
|
return 1
|
||
|
|
if e.exp.is_Integer:
|
||
|
|
return s**e.exp
|
||
|
|
elif isinstance(e, log):
|
||
|
|
return sign(e.args[0] - 1, x)
|
||
|
|
|
||
|
|
# if all else fails, do it the hard way
|
||
|
|
c0, e0 = mrv_leadterm(e, x)
|
||
|
|
return sign(c0, x)
|
||
|
|
|
||
|
|
|
||
|
|
@debug
|
||
|
|
@timeit
|
||
|
|
@cacheit
|
||
|
|
def limitinf(e, x):
|
||
|
|
"""Limit e(x) for x-> oo."""
|
||
|
|
# rewrite e in terms of tractable functions only
|
||
|
|
|
||
|
|
old = e
|
||
|
|
if not e.has(x):
|
||
|
|
return e # e is a constant
|
||
|
|
from sympy.simplify.powsimp import powdenest
|
||
|
|
from sympy.calculus.util import AccumBounds
|
||
|
|
if e.has(Order):
|
||
|
|
e = e.expand().removeO()
|
||
|
|
if not x.is_positive or x.is_integer:
|
||
|
|
# We make sure that x.is_positive is True and x.is_integer is None
|
||
|
|
# so we get all the correct mathematical behavior from the expression.
|
||
|
|
# We need a fresh variable.
|
||
|
|
p = Dummy('p', positive=True)
|
||
|
|
e = e.subs(x, p)
|
||
|
|
x = p
|
||
|
|
e = e.rewrite('tractable', deep=True, limitvar=x)
|
||
|
|
e = powdenest(e)
|
||
|
|
if isinstance(e, AccumBounds):
|
||
|
|
if mrv_leadterm(e.min, x) != mrv_leadterm(e.max, x):
|
||
|
|
raise NotImplementedError
|
||
|
|
c0, e0 = mrv_leadterm(e.min, x)
|
||
|
|
else:
|
||
|
|
c0, e0 = mrv_leadterm(e, x)
|
||
|
|
sig = sign(e0, x)
|
||
|
|
if sig == 1:
|
||
|
|
return S.Zero # e0>0: lim f = 0
|
||
|
|
elif sig == -1: # e0<0: lim f = +-oo (the sign depends on the sign of c0)
|
||
|
|
if c0.match(I*Wild("a", exclude=[I])):
|
||
|
|
return c0*oo
|
||
|
|
s = sign(c0, x)
|
||
|
|
# the leading term shouldn't be 0:
|
||
|
|
if s == 0:
|
||
|
|
raise ValueError("Leading term should not be 0")
|
||
|
|
return s*oo
|
||
|
|
elif sig == 0:
|
||
|
|
if c0 == old:
|
||
|
|
c0 = c0.cancel()
|
||
|
|
return limitinf(c0, x) # e0=0: lim f = lim c0
|
||
|
|
else:
|
||
|
|
raise ValueError("{} could not be evaluated".format(sig))
|
||
|
|
|
||
|
|
|
||
|
|
def moveup2(s, x):
|
||
|
|
r = SubsSet()
|
||
|
|
for expr, var in s.items():
|
||
|
|
r[expr.xreplace({x: exp(x)})] = var
|
||
|
|
for var, expr in s.rewrites.items():
|
||
|
|
r.rewrites[var] = s.rewrites[var].xreplace({x: exp(x)})
|
||
|
|
return r
|
||
|
|
|
||
|
|
|
||
|
|
def moveup(l, x):
|
||
|
|
return [e.xreplace({x: exp(x)}) for e in l]
|
||
|
|
|
||
|
|
|
||
|
|
@debug
|
||
|
|
@timeit
|
||
|
|
def calculate_series(e, x, logx=None):
|
||
|
|
""" Calculates at least one term of the series of ``e`` in ``x``.
|
||
|
|
|
||
|
|
This is a place that fails most often, so it is in its own function.
|
||
|
|
"""
|
||
|
|
|
||
|
|
SymPyDeprecationWarning(
|
||
|
|
feature="calculate_series",
|
||
|
|
useinstead="series() with suitable n, or as_leading_term",
|
||
|
|
issue=21838,
|
||
|
|
deprecated_since_version="1.12"
|
||
|
|
).warn()
|
||
|
|
|
||
|
|
from sympy.simplify.powsimp import powdenest
|
||
|
|
|
||
|
|
for t in e.lseries(x, logx=logx):
|
||
|
|
# bottom_up function is required for a specific case - when e is
|
||
|
|
# -exp(p/(p + 1)) + exp(-p**2/(p + 1) + p)
|
||
|
|
t = bottom_up(t, lambda w:
|
||
|
|
getattr(w, 'normal', lambda: w)())
|
||
|
|
# And the expression
|
||
|
|
# `(-sin(1/x) + sin((x + exp(x))*exp(-x)/x))*exp(x)`
|
||
|
|
# from the first test of test_gruntz_eval_special needs to
|
||
|
|
# be expanded. But other forms need to be have at least
|
||
|
|
# factor_terms applied. `factor` accomplishes both and is
|
||
|
|
# faster than using `factor_terms` for the gruntz suite. It
|
||
|
|
# does not appear that use of `cancel` is necessary.
|
||
|
|
# t = cancel(t, expand=False)
|
||
|
|
t = t.factor()
|
||
|
|
|
||
|
|
if t.has(exp) and t.has(log):
|
||
|
|
t = powdenest(t)
|
||
|
|
|
||
|
|
if not t.is_zero:
|
||
|
|
break
|
||
|
|
|
||
|
|
return t
|
||
|
|
|
||
|
|
|
||
|
|
@debug
|
||
|
|
@timeit
|
||
|
|
@cacheit
|
||
|
|
def mrv_leadterm(e, x):
|
||
|
|
"""Returns (c0, e0) for e."""
|
||
|
|
Omega = SubsSet()
|
||
|
|
if not e.has(x):
|
||
|
|
return (e, S.Zero)
|
||
|
|
if Omega == SubsSet():
|
||
|
|
Omega, exps = mrv(e, x)
|
||
|
|
if not Omega:
|
||
|
|
# e really does not depend on x after simplification
|
||
|
|
return exps, S.Zero
|
||
|
|
if x in Omega:
|
||
|
|
# move the whole omega up (exponentiate each term):
|
||
|
|
Omega_up = moveup2(Omega, x)
|
||
|
|
exps_up = moveup([exps], x)[0]
|
||
|
|
# NOTE: there is no need to move this down!
|
||
|
|
Omega = Omega_up
|
||
|
|
exps = exps_up
|
||
|
|
#
|
||
|
|
# The positive dummy, w, is used here so log(w*2) etc. will expand;
|
||
|
|
# a unique dummy is needed in this algorithm
|
||
|
|
#
|
||
|
|
# For limits of complex functions, the algorithm would have to be
|
||
|
|
# improved, or just find limits of Re and Im components separately.
|
||
|
|
#
|
||
|
|
w = Dummy("w", positive=True)
|
||
|
|
f, logw = rewrite(exps, Omega, x, w)
|
||
|
|
try:
|
||
|
|
lt = f.leadterm(w, logx=logw)
|
||
|
|
except (NotImplementedError, PoleError, ValueError):
|
||
|
|
n0 = 1
|
||
|
|
_series = Order(1)
|
||
|
|
incr = S.One
|
||
|
|
while _series.is_Order:
|
||
|
|
_series = f._eval_nseries(w, n=n0+incr, logx=logw)
|
||
|
|
incr *= 2
|
||
|
|
series = _series.expand().removeO()
|
||
|
|
try:
|
||
|
|
lt = series.leadterm(w, logx=logw)
|
||
|
|
except (NotImplementedError, PoleError, ValueError):
|
||
|
|
lt = f.as_coeff_exponent(w)
|
||
|
|
if lt[0].has(w):
|
||
|
|
base = f.as_base_exp()[0].as_coeff_exponent(w)
|
||
|
|
ex = f.as_base_exp()[1]
|
||
|
|
lt = (base[0]**ex, base[1]*ex)
|
||
|
|
return (lt[0].subs(log(w), logw), lt[1])
|
||
|
|
|
||
|
|
|
||
|
|
def build_expression_tree(Omega, rewrites):
|
||
|
|
r""" Helper function for rewrite.
|
||
|
|
|
||
|
|
We need to sort Omega (mrv set) so that we replace an expression before
|
||
|
|
we replace any expression in terms of which it has to be rewritten::
|
||
|
|
|
||
|
|
e1 ---> e2 ---> e3
|
||
|
|
\
|
||
|
|
-> e4
|
||
|
|
|
||
|
|
Here we can do e1, e2, e3, e4 or e1, e2, e4, e3.
|
||
|
|
To do this we assemble the nodes into a tree, and sort them by height.
|
||
|
|
|
||
|
|
This function builds the tree, rewrites then sorts the nodes.
|
||
|
|
"""
|
||
|
|
class Node:
|
||
|
|
def __init__(self):
|
||
|
|
self.before = []
|
||
|
|
self.expr = None
|
||
|
|
self.var = None
|
||
|
|
def ht(self):
|
||
|
|
return reduce(lambda x, y: x + y,
|
||
|
|
[x.ht() for x in self.before], 1)
|
||
|
|
nodes = {}
|
||
|
|
for expr, v in Omega:
|
||
|
|
n = Node()
|
||
|
|
n.var = v
|
||
|
|
n.expr = expr
|
||
|
|
nodes[v] = n
|
||
|
|
for _, v in Omega:
|
||
|
|
if v in rewrites:
|
||
|
|
n = nodes[v]
|
||
|
|
r = rewrites[v]
|
||
|
|
for _, v2 in Omega:
|
||
|
|
if r.has(v2):
|
||
|
|
n.before.append(nodes[v2])
|
||
|
|
|
||
|
|
return nodes
|
||
|
|
|
||
|
|
|
||
|
|
@debug
|
||
|
|
@timeit
|
||
|
|
def rewrite(e, Omega, x, wsym):
|
||
|
|
"""e(x) ... the function
|
||
|
|
Omega ... the mrv set
|
||
|
|
wsym ... the symbol which is going to be used for w
|
||
|
|
|
||
|
|
Returns the rewritten e in terms of w and log(w). See test_rewrite1()
|
||
|
|
for examples and correct results.
|
||
|
|
"""
|
||
|
|
|
||
|
|
from sympy import AccumBounds
|
||
|
|
if not isinstance(Omega, SubsSet):
|
||
|
|
raise TypeError("Omega should be an instance of SubsSet")
|
||
|
|
if len(Omega) == 0:
|
||
|
|
raise ValueError("Length cannot be 0")
|
||
|
|
# all items in Omega must be exponentials
|
||
|
|
for t in Omega.keys():
|
||
|
|
if not isinstance(t, exp):
|
||
|
|
raise ValueError("Value should be exp")
|
||
|
|
rewrites = Omega.rewrites
|
||
|
|
Omega = list(Omega.items())
|
||
|
|
|
||
|
|
nodes = build_expression_tree(Omega, rewrites)
|
||
|
|
Omega.sort(key=lambda x: nodes[x[1]].ht(), reverse=True)
|
||
|
|
|
||
|
|
# make sure we know the sign of each exp() term; after the loop,
|
||
|
|
# g is going to be the "w" - the simplest one in the mrv set
|
||
|
|
for g, _ in Omega:
|
||
|
|
sig = sign(g.exp, x)
|
||
|
|
if sig != 1 and sig != -1 and not sig.has(AccumBounds):
|
||
|
|
raise NotImplementedError('Result depends on the sign of %s' % sig)
|
||
|
|
if sig == 1:
|
||
|
|
wsym = 1/wsym # if g goes to oo, substitute 1/w
|
||
|
|
# O2 is a list, which results by rewriting each item in Omega using "w"
|
||
|
|
O2 = []
|
||
|
|
denominators = []
|
||
|
|
for f, var in Omega:
|
||
|
|
c = limitinf(f.exp/g.exp, x)
|
||
|
|
if c.is_Rational:
|
||
|
|
denominators.append(c.q)
|
||
|
|
arg = f.exp
|
||
|
|
if var in rewrites:
|
||
|
|
if not isinstance(rewrites[var], exp):
|
||
|
|
raise ValueError("Value should be exp")
|
||
|
|
arg = rewrites[var].args[0]
|
||
|
|
O2.append((var, exp((arg - c*g.exp).expand())*wsym**c))
|
||
|
|
|
||
|
|
# Remember that Omega contains subexpressions of "e". So now we find
|
||
|
|
# them in "e" and substitute them for our rewriting, stored in O2
|
||
|
|
|
||
|
|
# the following powsimp is necessary to automatically combine exponentials,
|
||
|
|
# so that the .xreplace() below succeeds:
|
||
|
|
# TODO this should not be necessary
|
||
|
|
from sympy.simplify.powsimp import powsimp
|
||
|
|
f = powsimp(e, deep=True, combine='exp')
|
||
|
|
for a, b in O2:
|
||
|
|
f = f.xreplace({a: b})
|
||
|
|
|
||
|
|
for _, var in Omega:
|
||
|
|
assert not f.has(var)
|
||
|
|
|
||
|
|
# finally compute the logarithm of w (logw).
|
||
|
|
logw = g.exp
|
||
|
|
if sig == 1:
|
||
|
|
logw = -logw # log(w)->log(1/w)=-log(w)
|
||
|
|
|
||
|
|
# Some parts of SymPy have difficulty computing series expansions with
|
||
|
|
# non-integral exponents. The following heuristic improves the situation:
|
||
|
|
exponent = reduce(ilcm, denominators, 1)
|
||
|
|
f = f.subs({wsym: wsym**exponent})
|
||
|
|
logw /= exponent
|
||
|
|
|
||
|
|
# bottom_up function is required for a specific case - when f is
|
||
|
|
# -exp(p/(p + 1)) + exp(-p**2/(p + 1) + p). No current simplification
|
||
|
|
# methods reduce this to 0 while not expanding polynomials.
|
||
|
|
f = bottom_up(f, lambda w: getattr(w, 'normal', lambda: w)())
|
||
|
|
f = expand_mul(f)
|
||
|
|
|
||
|
|
return f, logw
|
||
|
|
|
||
|
|
|
||
|
|
def gruntz(e, z, z0, dir="+"):
|
||
|
|
"""
|
||
|
|
Compute the limit of e(z) at the point z0 using the Gruntz algorithm.
|
||
|
|
|
||
|
|
Explanation
|
||
|
|
===========
|
||
|
|
|
||
|
|
``z0`` can be any expression, including oo and -oo.
|
||
|
|
|
||
|
|
For ``dir="+"`` (default) it calculates the limit from the right
|
||
|
|
(z->z0+) and for ``dir="-"`` the limit from the left (z->z0-). For infinite z0
|
||
|
|
(oo or -oo), the dir argument does not matter.
|
||
|
|
|
||
|
|
This algorithm is fully described in the module docstring in the gruntz.py
|
||
|
|
file. It relies heavily on the series expansion. Most frequently, gruntz()
|
||
|
|
is only used if the faster limit() function (which uses heuristics) fails.
|
||
|
|
"""
|
||
|
|
if not z.is_symbol:
|
||
|
|
raise NotImplementedError("Second argument must be a Symbol")
|
||
|
|
|
||
|
|
# convert all limits to the limit z->oo; sign of z is handled in limitinf
|
||
|
|
r = None
|
||
|
|
if z0 in (oo, I*oo):
|
||
|
|
e0 = e
|
||
|
|
elif z0 in (-oo, -I*oo):
|
||
|
|
e0 = e.subs(z, -z)
|
||
|
|
else:
|
||
|
|
if str(dir) == "-":
|
||
|
|
e0 = e.subs(z, z0 - 1/z)
|
||
|
|
elif str(dir) == "+":
|
||
|
|
e0 = e.subs(z, z0 + 1/z)
|
||
|
|
else:
|
||
|
|
raise NotImplementedError("dir must be '+' or '-'")
|
||
|
|
|
||
|
|
r = limitinf(e0, z)
|
||
|
|
|
||
|
|
# This is a bit of a heuristic for nice results... we always rewrite
|
||
|
|
# tractable functions in terms of familiar intractable ones.
|
||
|
|
# It might be nicer to rewrite the exactly to what they were initially,
|
||
|
|
# but that would take some work to implement.
|
||
|
|
return r.rewrite('intractable', deep=True)
|