File: upper_incomplete_gamma_fract.js

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/**
* @license Apache-2.0
*
* Copyright (c) 2018 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*    http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
*
* ## Notice
*
* The original C++ code and copyright notice are from the [Boost library]{@link http://www.boost.org/doc/libs/1_61_0/boost/math/special_functions/gamma.hpp}. The implementation has been modified for JavaScript.
*
* ```text
* Copyright John Maddock 2006-7, 2013-14.
* Copyright Paul A. Bristow 2007, 2013-14.
* Copyright Nikhar Agrawal 2013-14.
* Copyright Christopher Kormanyos 2013-14.
*
* Use, modification and distribution are subject to the
* Boost Software License, Version 1.0. (See accompanying file
* LICENSE or copy at http://www.boost.org/LICENSE_1_0.txt)
* ```
*/

'use strict';

// MAIN //

/**
* Creates a function to evaluate a series expansion of the upper incomplete gamma fraction.
*
* @private
* @param {number} a - function parameter
* @param {number} z - function parameter
* @returns {Function} series function
*/
function upperIncompleteGammaFract( a, z ) {
	var k = 0;
	z = z - a + 1.0;
	return next;

	/**
	* Calculate the next term of the series.
	*
	* @private
	* @returns {Array} series expansion terms
	*/
	function next() {
		k += 1;
		z += 2.0;
		return [ k * ( a - k ), z ];
	}
}


// EXPORTS //

module.exports = upperIncompleteGammaFract;