1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.numbers.gamma;
18
19 /**
20 * Ratio of <a href="https://mathworld.wolfram.com/GammaFunction.html">Gamma
21 * functions</a>.
22 *
23 * <p>\[ \frac{\Gamma(a)}{\Gamma(b)} \]
24 *
25 * <p>This code has been adapted from:
26 * <ul>
27 * <li>The <a href="https://www.boost.org/">Boost</a>
28 * {@code c++} implementation {@code <boost/math/special_functions/gamma.hpp>}.
29 * </ul>
30 *
31 * @see
32 * <a href="https://www.boost.org/doc/libs/1_77_0/libs/math/doc/html/math_toolkit/sf_gamma/gamma_ratios.html">
33 * Boost C++ Ratios of Gamma Functions</a>
34 * @since 1.1
35 */
36 public final class GammaRatio {
37 /** Private constructor. */
38 private GammaRatio() {
39 // intentionally empty.
40 }
41
42 /**
43 * Computes the ratio of gamma functions of two values.
44 *
45 * <p>\[ \frac{\Gamma(a)}{\Gamma(b)} \]
46 *
47 * <p>If either argument is {@code <= 0} or infinite then the result is NaN.
48 *
49 * @param a Argument a (must be positive finite).
50 * @param b Argument b (must be positive finite).
51 * @return \( \Gamma(a) / \Gamma(b) \)
52 */
53 public static double value(double a, double b) {
54 return BoostGamma.tgammaRatio(a, b);
55 }
56
57 /**
58 * Computes the ratio of gamma functions of a value and an offset value.
59 *
60 * <p>\[ \frac{\Gamma(a)}{\Gamma(a + delta)} \]
61 *
62 * <p>Note that the result is calculated accurately even when {@code delta} is
63 * small compared to {@code a}: indeed even if {@code a+delta ~ a}. The function
64 * is typically used when {@code a} is large and {@code delta} is very small.
65 *
66 * @param a Argument.
67 * @param delta Argument.
68 * @return \( \Gamma(a) / \Gamma(a + delta) \)
69 */
70 public static double delta(double a, double delta) {
71 return BoostGamma.tgammaDeltaRatio(a, delta);
72 }
73 }