upper_gamma_fraction.js (1761B)
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 // MODULES // 36 37 var continuedFraction = require( './../../../../base/tools/continued-fraction' ); 38 var upperIncompleteGammaFract = require( './upper_incomplete_gamma_fract.js' ); 39 40 41 // MAIN // 42 43 /** 44 * Evaluate the lower incomplete gamma integral via a series expansion and divide by gamma(z) to normalize. 45 * 46 * @private 47 * @param {number} a - function parameter 48 * @param {number} z - function parameter 49 * @returns {number} function value 50 */ 51 function upperGammaFraction( a, z ) { 52 var f = upperIncompleteGammaFract( a, z ); 53 return 1.0 / ( z - a + 1.0 + continuedFraction( f ) ); 54 } 55 56 57 // EXPORTS // 58 59 module.exports = upperGammaFraction;