1/* Compute a product of 1 + (T/X), 1 + (T/(X+1)), ....
2 Copyright (C) 2015-2023 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19#include <math.h>
20#include <math_private.h>
21#include <float.h>
22
23/* Compute the product of 1 + (T / (X + X_EPS)), 1 + (T / (X + X_EPS +
24 1)), ..., 1 + (T / (X + X_EPS + N - 1)), minus 1. X is such that
25 all the values X + 1, ..., X + N - 1 are exactly representable, and
26 X_EPS / X is small enough that factors quadratic in it can be
27 neglected. */
28
29double
30__lgamma_product (double t, double x, double x_eps, int n)
31{
32 long double x_full = (long double) x + (long double) x_eps;
33 long double ret = 0;
34 for (int i = 0; i < n; i++)
35 ret += (t / (x_full + i)) * (1 + ret);
36 return ret;
37}
38