Source code for sympy.polys.polyfuncs

"""High-level polynomials manipulation functions. """

from __future__ import print_function, division

from sympy.polys.polytools import (
    poly_from_expr, parallel_poly_from_expr, Poly)
from sympy.polys.polyoptions import allowed_flags

from sympy.polys.specialpolys import (
    symmetric_poly, interpolating_poly)

from sympy.polys.polyerrors import (
    PolificationFailed, ComputationFailed,
    MultivariatePolynomialError)

from sympy.utilities import numbered_symbols, take, public

from sympy.core import S, Basic, Add, Mul

from sympy.core.compatibility import xrange


@public
[docs]def symmetrize(F, *gens, **args): """ Rewrite a polynomial in terms of elementary symmetric polynomials. A symmetric polynomial is a multivariate polynomial that remains invariant under any variable permutation, i.e., if ``f = f(x_1, x_2, ..., x_n)``, then ``f = f(x_{i_1}, x_{i_2}, ..., x_{i_n})``, where ``(i_1, i_2, ..., i_n)`` is a permutation of ``(1, 2, ..., n)`` (an element of the group ``S_n``). Returns a tuple of symmetric polynomials ``(f1, f2, ..., fn)`` such that ``f = f1 + f2 + ... + fn``. Examples ======== >>> from sympy.polys.polyfuncs import symmetrize >>> from sympy.abc import x, y >>> symmetrize(x**2 + y**2) (-2*x*y + (x + y)**2, 0) >>> symmetrize(x**2 + y**2, formal=True) (s1**2 - 2*s2, 0, [(s1, x + y), (s2, x*y)]) >>> symmetrize(x**2 - y**2) (-2*x*y + (x + y)**2, -2*y**2) >>> symmetrize(x**2 - y**2, formal=True) (s1**2 - 2*s2, -2*y**2, [(s1, x + y), (s2, x*y)]) """ allowed_flags(args, ['formal', 'symbols']) iterable = True if not hasattr(F, '__iter__'): iterable = False F = [F] try: F, opt = parallel_poly_from_expr(F, *gens, **args) except PolificationFailed as exc: result = [] for expr in exc.exprs: if expr.is_Number: result.append((expr, S.Zero)) else: raise ComputationFailed('symmetrize', len(F), exc) else: if not iterable: result, = result if not exc.opt.formal: return result else: if iterable: return result, [] else: return result + ([],) polys, symbols = [], opt.symbols gens, dom = opt.gens, opt.domain for i in xrange(0, len(gens)): poly = symmetric_poly(i + 1, gens, polys=True) polys.append((next(symbols), poly.set_domain(dom))) indices = list(range(0, len(gens) - 1)) weights = list(range(len(gens), 0, -1)) result = [] for f in F: symmetric = [] if not f.is_homogeneous: symmetric.append(f.TC()) f -= f.TC() while f: _height, _monom, _coeff = -1, None, None for i, (monom, coeff) in enumerate(f.terms()): if all(monom[i] >= monom[i + 1] for i in indices): height = max([ n*m for n, m in zip(weights, monom) ]) if height > _height: _height, _monom, _coeff = height, monom, coeff if _height != -1: monom, coeff = _monom, _coeff else: break exponents = [] for m1, m2 in zip(monom, monom[1:] + (0,)): exponents.append(m1 - m2) term = [ s**n for (s, _), n in zip(polys, exponents) ] poly = [ p**n for (_, p), n in zip(polys, exponents) ] symmetric.append(Mul(coeff, *term)) product = poly[0].mul(coeff) for p in poly[1:]: product = product.mul(p) f -= product result.append((Add(*symmetric), f.as_expr())) polys = [ (s, p.as_expr()) for s, p in polys ] if not opt.formal: for i, (sym, non_sym) in enumerate(result): result[i] = (sym.subs(polys), non_sym) if not iterable: result, = result if not opt.formal: return result else: if iterable: return result, polys else: return result + (polys,)
@public
[docs]def horner(f, *gens, **args): """ Rewrite a polynomial in Horner form. Among other applications, evaluation of a polynomial at a point is optimal when it is applied using the Horner scheme ([1]). Examples ======== >>> from sympy.polys.polyfuncs import horner >>> from sympy.abc import x, y, a, b, c, d, e >>> horner(9*x**4 + 8*x**3 + 7*x**2 + 6*x + 5) x*(x*(x*(9*x + 8) + 7) + 6) + 5 >>> horner(a*x**4 + b*x**3 + c*x**2 + d*x + e) e + x*(d + x*(c + x*(a*x + b))) >>> f = 4*x**2*y**2 + 2*x**2*y + 2*x*y**2 + x*y >>> horner(f, wrt=x) x*(x*y*(4*y + 2) + y*(2*y + 1)) >>> horner(f, wrt=y) y*(x*y*(4*x + 2) + x*(2*x + 1)) References ========== [1] - http://en.wikipedia.org/wiki/Horner_scheme """ allowed_flags(args, []) try: F, opt = poly_from_expr(f, *gens, **args) except PolificationFailed as exc: return exc.expr form, gen = S.Zero, F.gen if F.is_univariate: for coeff in F.all_coeffs(): form = form*gen + coeff else: F, gens = Poly(F, gen), gens[1:] for coeff in F.all_coeffs(): form = form*gen + horner(coeff, *gens, **args) return form
@public
[docs]def interpolate(data, x): """ Construct an interpolating polynomial for the data points. Examples ======== >>> from sympy.polys.polyfuncs import interpolate >>> from sympy.abc import x A list is interpreted as though it were paired with a range starting from 1: >>> interpolate([1, 4, 9, 16], x) x**2 This can be made explicit by giving a list of coordinates: >>> interpolate([(1, 1), (2, 4), (3, 9)], x) x**2 The (x, y) coordinates can also be given as keys and values of a dictionary (and the points need not be equispaced): >>> interpolate([(-1, 2), (1, 2), (2, 5)], x) x**2 + 1 >>> interpolate({-1: 2, 1: 2, 2: 5}, x) x**2 + 1 """ n = len(data) if isinstance(data, dict): X, Y = list(zip(*data.items())) else: if isinstance(data[0], tuple): X, Y = list(zip(*data)) else: X = list(range(1, n + 1)) Y = list(data) poly = interpolating_poly(n, x, X, Y) return poly.expand()
@public
[docs]def viete(f, roots=None, *gens, **args): """ Generate Viete's formulas for ``f``. Examples ======== >>> from sympy.polys.polyfuncs import viete >>> from sympy import symbols >>> x, a, b, c, r1, r2 = symbols('x,a:c,r1:3') >>> viete(a*x**2 + b*x + c, [r1, r2], x) [(r1 + r2, -b/a), (r1*r2, c/a)] """ allowed_flags(args, []) if isinstance(roots, Basic): gens, roots = (roots,) + gens, None try: f, opt = poly_from_expr(f, *gens, **args) except PolificationFailed as exc: raise ComputationFailed('viete', 1, exc) if f.is_multivariate: raise MultivariatePolynomialError( "multivariate polynomials are not allowed") n = f.degree() if n < 1: raise ValueError( "can't derive Viete's formulas for a constant polynomial") if roots is None: roots = numbered_symbols('r', start=1) roots = take(roots, n) if n != len(roots): raise ValueError("required %s roots, got %s" % (n, len(roots))) lc, coeffs = f.LC(), f.all_coeffs() result, sign = [], -1 for i, coeff in enumerate(coeffs[1:]): poly = symmetric_poly(i + 1, roots) coeff = sign*(coeff/lc) result.append((poly, coeff)) sign = -sign return result