Source code for diofant.integrals.heurisch

from functools import reduce
from itertools import permutations

from ..core import Add, Basic, Dummy, E, Eq, Mul, S, Wild, pi, sympify
from ..core.compatibility import ordered
from ..functions import (Ei, LambertW, Piecewise, acosh, asin, asinh, atan,
                         cos, cosh, cot, coth, erf, erfi, exp, li, log, root,
                         sin, sinh, sqrt, tan, tanh)
from ..logic import And
from ..polys import PolynomialError, cancel, factor, gcd, lcm, quo
from ..polys.constructor import construct_domain
from ..polys.monomials import itermonomials
from ..polys.polyroots import root_factors
from ..polys.solvers import solve_lin_sys
from ..utilities.iterables import uniq

[docs]def components(f, x): """ Returns a set of all functional components of the given expression which includes symbols, function applications and compositions and non-integer powers. Fractional powers are collected with with minimal, positive exponents. >>> components(sin(x)*cos(x)**2, x) {x, sin(x), cos(x)} See Also ======== diofant.integrals.heurisch.heurisch """ result = set() if x in f.free_symbols: if f.is_Symbol: result.add(f) elif f.is_Function or f.is_Derivative: for g in f.args: result |= components(g, x) result.add(f) elif f.is_Pow: result |= components(f.base, x) if not f.exp.is_Integer: if f.exp.is_Rational: result.add(root(f.base, f.exp.denominator)) else: result |= components(f.exp, x) | {f} else: for g in f.args: result |= components(g, x) return result
# name -> [] of symbols _symbols_cache = {} # NB @cacheit is not convenient here def _symbols(name, n): """get vector of symbols local to this module""" try: lsyms = _symbols_cache[name] except KeyError: lsyms = [] _symbols_cache[name] = lsyms while len(lsyms) < n: lsyms.append( Dummy('%s%i' % (name, len(lsyms))) ) return lsyms[:n]
[docs]def heurisch_wrapper(f, x, rewrite=False, hints=None, mappings=None, retries=3, degree_offset=0, unnecessary_permutations=None): """ A wrapper around the heurisch integration algorithm. This method takes the result from heurisch and checks for poles in the denominator. For each of these poles, the integral is reevaluated, and the final integration result is given in terms of a Piecewise. Examples ======== >>> heurisch(cos(n*x), x) sin(n*x)/n >>> heurisch_wrapper(cos(n*x), x) Piecewise((x, Eq(n, 0)), (sin(n*x)/n, true)) See Also ======== diofant.integrals.heurisch.heurisch """ from ..solvers.solvers import solve, denoms f = sympify(f) if x not in f.free_symbols: return f*x res = heurisch(f, x, rewrite, hints, mappings, retries, degree_offset, unnecessary_permutations) if not isinstance(res, Basic): return res # We consider each denominator in the expression, and try to find # cases where one or more symbolic denominator might be zero. The # conditions for these cases are stored in the list slns. slns = [] for d in denoms(res): try: ds = list(ordered(d.free_symbols - {x})) if ds: slns += solve(d, *ds) except NotImplementedError: pass if not slns: return res slns = list(uniq(slns)) # Remove the solutions corresponding to poles in the original expression. slns0 = [] for d in denoms(f): try: ds = list(ordered(d.free_symbols - {x})) if ds: slns0 += solve(d, *ds) except NotImplementedError: pass slns = [s for s in slns if s not in slns0] if not slns: return res if len(slns) > 1: eqs = [] for sub_dict in slns: eqs.extend([Eq(key, value) for key, value in sub_dict.items()]) slns = solve(eqs, *ordered(set().union(*[e.free_symbols for e in eqs]) - {x})) + slns # For each case listed in the list slns, we reevaluate the integral. pairs = [] for sub_dict in slns: expr = heurisch(f.subs(sub_dict), x, rewrite, hints, mappings, retries, degree_offset, unnecessary_permutations) cond = And(*[Eq(key, value) for key, value in sub_dict.items()]) pairs.append((expr, cond)) pairs.append((heurisch(f, x, rewrite, hints, mappings, retries, degree_offset, unnecessary_permutations), True)) return Piecewise(*pairs)
[docs]def heurisch(f, x, rewrite=False, hints=None, mappings=None, retries=3, degree_offset=0, unnecessary_permutations=None): """ Compute indefinite integral using heuristic Risch algorithm. This is a heuristic approach to indefinite integration in finite terms using the extended heuristic (parallel) Risch algorithm, based on Manuel Bronstein's "Poor Man's Integrator". The algorithm supports various classes of functions including transcendental elementary or special functions like Airy, Bessel, Whittaker and Lambert. Note that this algorithm is not a decision procedure. If it isn't able to compute the antiderivative for a given function, then this is not a proof that such a functions does not exist. One should use recursive Risch algorithm in such case. It's an open question if this algorithm can be made a full decision procedure. This is an internal integrator procedure. You should use toplevel 'integrate' function in most cases, as this procedure needs some preprocessing steps and otherwise may fail. Parameters ========== f : Expr expression x : Symbol variable rewrite : Boolean, optional force rewrite 'f' in terms of 'tan' and 'tanh', default False. hints : None or list a list of functions that may appear in anti-derivate. If None (default) - no suggestions at all, if empty list - try to figure out. Examples ======== >>> heurisch(y*tan(x), x) y*log(tan(x)**2 + 1)/2 References ========== * Manuel Bronstein's "Poor Man's Integrator", See Also ======== diofant.integrals.integrals.Integral.doit diofant.integrals.integrals.Integral diofant.integrals.heurisch.components """ f = sympify(f) if x not in f.free_symbols: return f*x if not f.is_Add: indep, f = f.as_independent(x) else: indep = S.One rewritables = { (sin, cos, cot): tan, (sinh, cosh, coth): tanh, } if rewrite: for candidates, rule in rewritables.items(): f = f.rewrite(candidates, rule) else: for candidates in rewritables: if f.has(*candidates): break else: rewrite = True terms = components(f, x) if hints is not None: if not hints: a = Wild('a', exclude=[x]) b = Wild('b', exclude=[x]) c = Wild('c', exclude=[x]) for g in set(terms): # using copy of terms if g.is_Function: if isinstance(g, li): M = g.args[0].match(a*x**b) if M is not None: terms.add( x*(li(M[a]*x**M[b]) - (M[a]*x**M[b])**(-1/M[b])*Ei((M[b]+1)*log(M[a]*x**M[b])/M[b])) ) elif g.is_Pow: if g.base is E: M = g.exp.match(a*x**2) if M is not None: if M[a].is_positive: terms.add(erfi(sqrt(M[a])*x)) else: # M[a].is_negative or unknown terms.add(erf(sqrt(-M[a])*x)) M = g.exp.match(a*x**2 + b*x + c) if M is not None: if M[a].is_positive: terms.add(sqrt(pi/4*(-M[a]))*exp(M[c] - M[b]**2/(4*M[a])) * erfi(sqrt(M[a])*x + M[b]/(2*sqrt(M[a])))) elif M[a].is_negative: terms.add(sqrt(pi/4*(-M[a]))*exp(M[c] - M[b]**2/(4*M[a])) * erf(sqrt(-M[a])*x - M[b]/(2*sqrt(-M[a])))) M = g.exp.match(a*log(x)**2) if M is not None: if M[a].is_positive: terms.add(erfi(sqrt(M[a])*log(x) + 1/(2*sqrt(M[a])))) if M[a].is_negative: terms.add(erf(sqrt(-M[a])*log(x) - 1/(2*sqrt(-M[a])))) elif g.exp.is_Rational and g.exp.denominator == 2: M = g.base.match(a*x**2 + b) if M is not None and M[b].is_positive: if M[a].is_positive: terms.add(asinh(sqrt(M[a]/M[b])*x)) elif M[a].is_negative: terms.add(asin(sqrt(-M[a]/M[b])*x)) M = g.base.match(a*x**2 - b) if M is not None and M[b].is_positive: if M[a].is_positive: terms.add(acosh(sqrt(M[a]/M[b])*x)) elif M[a].is_negative: terms.add((-M[b]/2*sqrt(-M[a]) * atan(sqrt(-M[a])*x/sqrt(M[a]*x**2 - M[b])))) else: terms |= set(hints) for g in set(terms): # using copy of terms terms |= components(cancel(g.diff(x)), x) # TODO: caching is significant factor for why permutations work at all. Change this. V = _symbols('x', len(terms)) # sort mapping expressions from largest to smallest (last is always x). mapping = list(reversed(list(zip(*ordered( # [(a[0].as_independent(x)[1], a) for a in zip(terms, V)])))[1])) # rev_mapping = {v: k for k, v in mapping} # if mappings is None: # # optimizing the number of permutations of mapping # assert mapping[-1][0] == x # if not, find it and correct this comment unnecessary_permutations = [mapping.pop(-1)] mappings = permutations(mapping) else: unnecessary_permutations = unnecessary_permutations or [] def _substitute(expr): return expr.subs(mapping) for mapping in mappings: mapping = list(mapping) mapping = mapping + unnecessary_permutations diffs = [ _substitute(cancel(g.diff(x))) for g in terms ] denoms = [ g.as_numer_denom()[1] for g in diffs ] if all(h.is_polynomial(*V) for h in denoms) and _substitute(f).is_rational_function(*V): denom = reduce(lambda p, q: lcm(p, q, *V), denoms) break else: if not rewrite: result = heurisch(f, x, rewrite=True, hints=hints, unnecessary_permutations=unnecessary_permutations) if result is not None: return indep*result return numers = [ cancel(denom*g) for g in diffs ] def _derivation(h): return Add(*[ d * h.diff(v) for d, v in zip(numers, V) ]) def _deflation(p): for y in V: if not p.has(y): continue if _derivation(p) is not S.Zero: c, q = p.as_poly(y).primitive() return _deflation(c)*gcd(q, q.diff(y)).as_expr() else: return p def _splitter(p): for y in V: if not p.has(y): continue if _derivation(y) is not S.Zero: c, q = p.as_poly(y).primitive() q = q.as_expr() h = gcd(q, _derivation(q), y) s = quo(h, gcd(q, q.diff(y), y), y) c_split = _splitter(c) if s.as_poly(y).degree() == 0: return c_split[0], q*c_split[1] q_split = _splitter(cancel(q / s)) return c_split[0]*q_split[0]*s, c_split[1]*q_split[1] else: return S.One, p special = {} for term in terms: if term.is_Function: if isinstance(term, tan): special[1 + _substitute(term)**2] = False elif isinstance(term, tanh): special[1 + _substitute(term)] = False special[1 - _substitute(term)] = False elif isinstance(term, LambertW): special[_substitute(term)] = True F = _substitute(f) P, Q = F.as_numer_denom() u_split = _splitter(denom) v_split = _splitter(Q) polys = set(list(v_split) + [u_split[0]] + list(special)) s = u_split[0] * Mul(*[ k for k, v in special.items() if v ]) polified = [ p.as_poly(*V) for p in [s, P, Q] ] if None in polified: return # --- definitions for _integrate --- a, b, c = [ p.total_degree() for p in polified ] poly_denom = (s * v_split[0] * _deflation(v_split[1])).as_expr() def _exponent(g): if g.is_Pow: if g.exp.is_Rational and g.exp.denominator != 1: if g.exp.numerator > 0: return g.exp.numerator + g.exp.denominator - 1 else: return abs(g.exp.numerator + g.exp.denominator) else: return 1 elif not g.is_Atom and g.args: return max(_exponent(h) for h in g.args) else: return 1 A, B = _exponent(f), a + max(b, c) if A > 1 and B > 1: monoms = itermonomials(V, A + B - 1 + degree_offset) else: monoms = itermonomials(V, A + B + degree_offset) poly_coeffs = _symbols('A', len(monoms)) poly_part = Add(*[ poly_coeffs[i]*monomial for i, monomial in enumerate(ordered(monoms)) ]) reducibles = set() for poly in polys: if poly.has(*V): try: factorization = factor(poly, greedy=True) except PolynomialError: factorization = poly factorization = poly if factorization.is_Mul: reducibles |= set(factorization.args) else: reducibles.add(factorization) def _integrate(field=None): irreducibles = set() for poly in reducibles: for z in poly.free_symbols: if z in V: break # should this be: `irreducibles |= \ else: # set(root_factors(poly, z, filter=field))` continue # and the line below deleted? # | # V irreducibles |= set(root_factors(poly, z, filter=field)) log_part = [] B = _symbols('B', len(irreducibles)) # Note: the ordering matters here for poly, b in reversed(list(ordered(zip(irreducibles, B)))): if poly.has(*V): poly_coeffs.append(b) log_part.append(b * log(poly)) # TODO: Currently it's better to use symbolic expressions here instead # of rational functions, because it's simpler and FracElement doesn't # give big speed improvement yet. This is because cancellation is slow # due to slow polynomial GCD algorithms. If this gets improved then # revise this code. candidate = poly_part/poly_denom + Add(*log_part) h = F - _derivation(candidate) / denom raw_numer = h.as_numer_denom()[0] # Rewrite raw_numer as a polynomial in K[coeffs][V] where K is a field # that we have to determine. We can't use simply atoms() because log(3), # sqrt(y) and similar expressions can appear, leading to non-trivial # domains. syms = set(poly_coeffs) | set(V) non_syms = set() def find_non_syms(expr): if expr.is_Integer or expr.is_Rational: pass # ignore trivial numbers elif expr in syms: pass # ignore variables elif not expr.has(*syms): non_syms.add(expr) elif expr.is_Add or expr.is_Mul or expr.is_Pow: list(map(find_non_syms, expr.args)) else: # TODO: Non-polynomial expression. This should have been # filtered out at an earlier stage. raise PolynomialError try: find_non_syms(raw_numer) except PolynomialError: return else: ground, _ = construct_domain(non_syms, field=True) coeff_ring = ground.poly_ring(*poly_coeffs) ring = coeff_ring.poly_ring(*V) try: numer = ring.from_expr(raw_numer) except ValueError: raise PolynomialError solution = solve_lin_sys(numer.coeffs(), coeff_ring) if solution is None: return else: solution = [(coeff_ring.symbols[coeff_ring.index(k)], v.as_expr()) for k, v in solution.items()] return candidate.subs(solution).subs( list(zip(poly_coeffs, [S.Zero]*len(poly_coeffs)))) if not (F.free_symbols - set(V)): solution = _integrate('Q') if solution is None: solution = _integrate() else: solution = _integrate() if solution is not None: antideriv = solution.subs(rev_mapping) antideriv = cancel(antideriv).expand(force=True) if antideriv.is_Add: antideriv = antideriv.as_independent(x)[1] return indep*antideriv else: if retries >= 0: result = heurisch(f, x, mappings=mappings, rewrite=rewrite, hints=hints, retries=retries - 1, unnecessary_permutations=unnecessary_permutations) if result is not None: return indep*result return