Source code for sympy.concrete.products

from __future__ import print_function, division

from sympy.tensor.indexed import Idx
from sympy.core.mul import Mul
from sympy.core.singleton import S
from sympy.core.symbol import symbols
from sympy.concrete.expr_with_intlimits import ExprWithIntLimits
from sympy.functions.elementary.exponential import exp, log
from sympy.polys import quo, roots
from sympy.simplify import powsimp
from sympy.core.compatibility import range


[docs]class Product(ExprWithIntLimits): r"""Represents unevaluated products. ``Product`` represents a finite or infinite product, with the first argument being the general form of terms in the series, and the second argument being ``(dummy_variable, start, end)``, with ``dummy_variable`` taking all integer values from ``start`` through ``end``. In accordance with long-standing mathematical convention, the end term is included in the product. Finite products =============== For finite products (and products with symbolic limits assumed to be finite) we follow the analogue of the summation convention described by Karr [1], especially definition 3 of section 1.4. The product: .. math:: \prod_{m \leq i < n} f(i) has *the obvious meaning* for `m < n`, namely: .. math:: \prod_{m \leq i < n} f(i) = f(m) f(m+1) \cdot \ldots \cdot f(n-2) f(n-1) with the upper limit value `f(n)` excluded. The product over an empty set is one if and only if `m = n`: .. math:: \prod_{m \leq i < n} f(i) = 1 \quad \mathrm{for} \quad m = n Finally, for all other products over empty sets we assume the following definition: .. math:: \prod_{m \leq i < n} f(i) = \frac{1}{\prod_{n \leq i < m} f(i)} \quad \mathrm{for} \quad m > n It is important to note that above we define all products with the upper limit being exclusive. This is in contrast to the usual mathematical notation, but does not affect the product convention. Indeed we have: .. math:: \prod_{m \leq i < n} f(i) = \prod_{i = m}^{n - 1} f(i) where the difference in notation is intentional to emphasize the meaning, with limits typeset on the top being inclusive. Examples ======== >>> from sympy.abc import a, b, i, k, m, n, x >>> from sympy import Product, factorial, oo >>> Product(k, (k, 1, m)) Product(k, (k, 1, m)) >>> Product(k, (k, 1, m)).doit() factorial(m) >>> Product(k**2,(k, 1, m)) Product(k**2, (k, 1, m)) >>> Product(k**2,(k, 1, m)).doit() factorial(m)**2 Wallis' product for pi: >>> W = Product(2*i/(2*i-1) * 2*i/(2*i+1), (i, 1, oo)) >>> W Product(4*i**2/((2*i - 1)*(2*i + 1)), (i, 1, oo)) Direct computation currently fails: >>> W.doit() Product(4*i**2/((2*i - 1)*(2*i + 1)), (i, 1, oo)) But we can approach the infinite product by a limit of finite products: >>> from sympy import limit >>> W2 = Product(2*i/(2*i-1)*2*i/(2*i+1), (i, 1, n)) >>> W2 Product(4*i**2/((2*i - 1)*(2*i + 1)), (i, 1, n)) >>> W2e = W2.doit() >>> W2e 2**(-2*n)*4**n*factorial(n)**2/(RisingFactorial(1/2, n)*RisingFactorial(3/2, n)) >>> limit(W2e, n, oo) pi/2 By the same formula we can compute sin(pi/2): >>> from sympy import pi, gamma, simplify >>> P = pi * x * Product(1 - x**2/k**2, (k, 1, n)) >>> P = P.subs(x, pi/2) >>> P pi**2*Product(1 - pi**2/(4*k**2), (k, 1, n))/2 >>> Pe = P.doit() >>> Pe pi**2*RisingFactorial(1 + pi/2, n)*RisingFactorial(-pi/2 + 1, n)/(2*factorial(n)**2) >>> Pe = Pe.rewrite(gamma) >>> Pe pi**2*gamma(n + 1 + pi/2)*gamma(n - pi/2 + 1)/(2*gamma(1 + pi/2)*gamma(-pi/2 + 1)*gamma(n + 1)**2) >>> Pe = simplify(Pe) >>> Pe sin(pi**2/2)*gamma(n + 1 + pi/2)*gamma(n - pi/2 + 1)/gamma(n + 1)**2 >>> limit(Pe, n, oo) sin(pi**2/2) Products with the lower limit being larger than the upper one: >>> Product(1/i, (i, 6, 1)).doit() 120 >>> Product(i, (i, 2, 5)).doit() 120 The empty product: >>> Product(i, (i, n, n-1)).doit() 1 An example showing that the symbolic result of a product is still valid for seemingly nonsensical values of the limits. Then the Karr convention allows us to give a perfectly valid interpretation to those products by interchanging the limits according to the above rules: >>> P = Product(2, (i, 10, n)).doit() >>> P 2**(n - 9) >>> P.subs(n, 5) 1/16 >>> Product(2, (i, 10, 5)).doit() 1/16 >>> 1/Product(2, (i, 6, 9)).doit() 1/16 An explicit example of the Karr summation convention applied to products: >>> P1 = Product(x, (i, a, b)).doit() >>> P1 x**(-a + b + 1) >>> P2 = Product(x, (i, b+1, a-1)).doit() >>> P2 x**(a - b - 1) >>> simplify(P1 * P2) 1 And another one: >>> P1 = Product(i, (i, b, a)).doit() >>> P1 RisingFactorial(b, a - b + 1) >>> P2 = Product(i, (i, a+1, b-1)).doit() >>> P2 RisingFactorial(a + 1, -a + b - 1) >>> P1 * P2 RisingFactorial(b, a - b + 1)*RisingFactorial(a + 1, -a + b - 1) >>> simplify(P1 * P2) 1 See Also ======== Sum, summation product References ========== .. [1] Michael Karr, "Summation in Finite Terms", Journal of the ACM, Volume 28 Issue 2, April 1981, Pages 305-350 http://dl.acm.org/citation.cfm?doid=322248.322255 .. [2] http://en.wikipedia.org/wiki/Multiplication#Capital_Pi_notation .. [3] http://en.wikipedia.org/wiki/Empty_product """ __slots__ = ['is_commutative'] def __new__(cls, function, *symbols, **assumptions): obj = ExprWithIntLimits.__new__(cls, function, *symbols, **assumptions) return obj def _eval_rewrite_as_Sum(self, *args): from sympy.concrete.summations import Sum return exp(Sum(log(self.function), *self.limits)) @property def term(self): return self._args[0] function = term def _eval_is_zero(self): # a Product is zero only if its term is zero. return self.term.is_zero def doit(self, **hints): f = self.function for index, limit in enumerate(self.limits): i, a, b = limit dif = b - a if dif.is_Integer and dif < 0: a, b = b + 1, a - 1 f = 1 / f g = self._eval_product(f, (i, a, b)) if g in (None, S.NaN): return self.func(powsimp(f), *self.limits[index:]) else: f = g if hints.get('deep', True): return f.doit(**hints) else: return powsimp(f) def _eval_adjoint(self): if self.is_commutative: return self.func(self.function.adjoint(), *self.limits) return None def _eval_conjugate(self): return self.func(self.function.conjugate(), *self.limits) def _eval_product(self, term, limits): from sympy.concrete.delta import deltaproduct, _has_simple_delta from sympy.concrete.summations import summation from sympy.functions import KroneckerDelta, RisingFactorial (k, a, n) = limits if k not in term.free_symbols: if (term - 1).is_zero: return S.One return term**(n - a + 1) if a == n: return term.subs(k, a) if term.has(KroneckerDelta) and _has_simple_delta(term, limits[0]): return deltaproduct(term, limits) dif = n - a if dif.is_Integer: return Mul(*[term.subs(k, a + i) for i in range(dif + 1)]) elif term.is_polynomial(k): poly = term.as_poly(k) A = B = Q = S.One all_roots = roots(poly) M = 0 for r, m in all_roots.items(): M += m A *= RisingFactorial(a - r, n - a + 1)**m Q *= (n - r)**m if M < poly.degree(): arg = quo(poly, Q.as_poly(k)) B = self.func(arg, (k, a, n)).doit() return poly.LC()**(n - a + 1) * A * B elif term.is_Add: p, q = term.as_numer_denom() q = self._eval_product(q, (k, a, n)) if q.is_Number: # There is expression, which couldn't change by # as_numer_denom(). E.g. n**(2/3) + 1 --> (n**(2/3) + 1, 1). # We have to catch this case. p = sum([self._eval_product(i, (k, a, n)) for i in p.as_coeff_Add()]) else: p = self._eval_product(p, (k, a, n)) return p / q elif term.is_Mul: exclude, include = [], [] for t in term.args: p = self._eval_product(t, (k, a, n)) if p is not None: exclude.append(p) else: include.append(t) if not exclude: return None else: arg = term._new_rawargs(*include) A = Mul(*exclude) B = self.func(arg, (k, a, n)).doit() return A * B elif term.is_Pow: if not term.base.has(k): s = summation(term.exp, (k, a, n)) return term.base**s elif not term.exp.has(k): p = self._eval_product(term.base, (k, a, n)) if p is not None: return p**term.exp elif isinstance(term, Product): evaluated = term.doit() f = self._eval_product(evaluated, limits) if f is None: return self.func(evaluated, limits) else: return f def _eval_simplify(self, ratio, measure): from sympy.simplify.simplify import product_simplify return product_simplify(self) def _eval_transpose(self): if self.is_commutative: return self.func(self.function.transpose(), *self.limits) return None
[docs] def is_convergent(self): r""" See docs of Sum.is_convergent() for explanation of convergence in SymPy. The infinite product: .. math:: \prod_{1 \leq i < \infty} f(i) is defined by the sequence of partial products: .. math:: \prod_{i=1}^{n} f(i) = f(1) f(2) \cdots f(n) as n increases without bound. The product converges to a non-zero value if and only if the sum: .. math:: \sum_{1 \leq i < \infty} \log{f(n)} converges. References ========== .. [1] https://en.wikipedia.org/wiki/Infinite_product Examples ======== >>> from sympy import Interval, S, Product, Symbol, cos, pi, exp, oo >>> n = Symbol('n', integer=True) >>> Product(n/(n + 1), (n, 1, oo)).is_convergent() False >>> Product(1/n**2, (n, 1, oo)).is_convergent() False >>> Product(cos(pi/n), (n, 1, oo)).is_convergent() True >>> Product(exp(-n**2), (n, 1, oo)).is_convergent() False """ from sympy.concrete.summations import Sum sequence_term = self.function log_sum = log(sequence_term) lim = self.limits try: is_conv = Sum(log_sum, *lim).is_convergent() except NotImplementedError: if Sum(sequence_term - 1, *lim).is_absolutely_convergent() is S.true: return S.true raise NotImplementedError("The algorithm to find the product convergence of %s " "is not yet implemented" % (sequence_term)) return is_conv
[docs] def reverse_order(expr, *indices): """ Reverse the order of a limit in a Product. Usage ===== ``reverse_order(expr, *indices)`` reverses some limits in the expression ``expr`` which can be either a ``Sum`` or a ``Product``. The selectors in the argument ``indices`` specify some indices whose limits get reversed. These selectors are either variable names or numerical indices counted starting from the inner-most limit tuple. Examples ======== >>> from sympy import Product, simplify, RisingFactorial, gamma, Sum >>> from sympy.abc import x, y, a, b, c, d >>> P = Product(x, (x, a, b)) >>> Pr = P.reverse_order(x) >>> Pr Product(1/x, (x, b + 1, a - 1)) >>> Pr = Pr.doit() >>> Pr 1/RisingFactorial(b + 1, a - b - 1) >>> simplify(Pr) gamma(b + 1)/gamma(a) >>> P = P.doit() >>> P RisingFactorial(a, -a + b + 1) >>> simplify(P) gamma(b + 1)/gamma(a) While one should prefer variable names when specifying which limits to reverse, the index counting notation comes in handy in case there are several symbols with the same name. >>> S = Sum(x*y, (x, a, b), (y, c, d)) >>> S Sum(x*y, (x, a, b), (y, c, d)) >>> S0 = S.reverse_order(0) >>> S0 Sum(-x*y, (x, b + 1, a - 1), (y, c, d)) >>> S1 = S0.reverse_order(1) >>> S1 Sum(x*y, (x, b + 1, a - 1), (y, d + 1, c - 1)) Of course we can mix both notations: >>> Sum(x*y, (x, a, b), (y, 2, 5)).reverse_order(x, 1) Sum(x*y, (x, b + 1, a - 1), (y, 6, 1)) >>> Sum(x*y, (x, a, b), (y, 2, 5)).reverse_order(y, x) Sum(x*y, (x, b + 1, a - 1), (y, 6, 1)) See Also ======== index, reorder_limit, reorder References ========== .. [1] Michael Karr, "Summation in Finite Terms", Journal of the ACM, Volume 28 Issue 2, April 1981, Pages 305-350 http://dl.acm.org/citation.cfm?doid=322248.322255 """ l_indices = list(indices) for i, indx in enumerate(l_indices): if not isinstance(indx, int): l_indices[i] = expr.index(indx) e = 1 limits = [] for i, limit in enumerate(expr.limits): l = limit if i in l_indices: e = -e l = (limit[0], limit[2] + 1, limit[1] - 1) limits.append(l) return Product(expr.function ** e, *limits)
[docs]def product(*args, **kwargs): r""" Compute the product. The notation for symbols is similar to the notation used in Sum or Integral. product(f, (i, a, b)) computes the product of f with respect to i from a to b, i.e., :: b _____ product(f(n), (i, a, b)) = | | f(n) | | i = a If it cannot compute the product, it returns an unevaluated Product object. Repeated products can be computed by introducing additional symbols tuples:: >>> from sympy import product, symbols >>> i, n, m, k = symbols('i n m k', integer=True) >>> product(i, (i, 1, k)) factorial(k) >>> product(m, (i, 1, k)) m**k >>> product(i, (i, 1, k), (k, 1, n)) Product(factorial(k), (k, 1, n)) """ prod = Product(*args, **kwargs) if isinstance(prod, Product): return prod.doit(deep=False) else: return prod