small_gamma2_series.js (1745B)
1 /** 2 * @license Apache-2.0 3 * 4 * Copyright (c) 2018 The Stdlib Authors. 5 * 6 * Licensed under the Apache License, Version 2.0 (the "License"); 7 * you may not use this file except in compliance with the License. 8 * You may obtain a copy of the License at 9 * 10 * http://www.apache.org/licenses/LICENSE-2.0 11 * 12 * Unless required by applicable law or agreed to in writing, software 13 * distributed under the License is distributed on an "AS IS" BASIS, 14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15 * See the License for the specific language governing permissions and 16 * limitations under the License. 17 * 18 * 19 * ## Notice 20 * 21 * The original C++ code and copyright notice are from the [Boost library]{@link http://www.boost.org/doc/libs/1_37_0/boost/math/special_functions/gamma.hpp}. The implementation has been modified for JavaScript. 22 * 23 * ```text 24 * (C) Copyright John Maddock 2006. 25 * (C) Copyright Paul A. Bristow 2007. 26 * 27 * Use, modification and distribution are subject to the 28 * Boost Software License, Version 1.0. (See accompanying file 29 * LICENSE or copy at http://www.boost.org/LICENSE_1_0.txt) 30 * ``` 31 */ 32 33 'use strict'; 34 35 /** 36 * Series representation for upper fraction when `z` is small. 37 * 38 * @private 39 * @param {number} a - function parameter 40 * @param {number} x - function parameter 41 * @returns {Function} series function 42 */ 43 function smallGamma2Series( a, x ) { 44 var result; 45 var apn; 46 var n; 47 var r; 48 49 result = -x; 50 x = -x; 51 apn = a + 1.0; 52 n = 1; 53 return next; 54 55 /** 56 * Calculate the next term of the series. 57 * 58 * @private 59 * @returns {number} series expansion term 60 */ 61 function next() { 62 r = result / apn; 63 result *= x; 64 n += 1; 65 result /= n; 66 apn += 1.0; 67 return r; 68 } 69 } 70 71 72 // EXPORTS // 73 74 module.exports = smallGamma2Series;