Java Digamma digammaDiff(double x, int d)

Description

Compute digamma difference

License

Apache License

Parameter

Parameter	Description
x	a parameter
d	a parameter

Declaration

public static double digammaDiff(double x, int d) 

Method Source Code

//package com.java2s;
//License from project: Apache License 

public class Main {
    public static final double c1 = -0.5;
    public static final double c2 = -1.0 / 12;
    public static final double c4 = 1.0 / 120;
    public static final double c6 = -1.0 / 252;

    /**// www  .  j  a v a  2s .c o m
     * Compute digamma difference
     *
     * @param x
     * @param d
     * @return
     */
    public static double digammaDiff(double x, int d) {
        double sum = 0;
        int dcutoff = 16;
        if (d > dcutoff) {
            return (digamma(x + d) - digamma(x));
        }
        for (int i = 0; i < d; ++i) {
            sum += 1 / (x + i);
        }
        return (sum);
    }

    /**
     * Compute the value of digamma function
     *
     * @param x
     * @return
     */
    public static double digamma(double x) {
        double y, y2, sum = 0;
        for (y = x; y < 10; y++) {
            sum -= 1.0 / y;
        }
        y2 = 1.0 / (y * y);
        sum += Math.log(y) + c1 / y + y2 * (c2 + y2 * (c4 + y2 * c6));
        return sum;
    }
}

Related

1. digamma(double x)
2. digamma(double x)
3. digamma(double x)
4. digammaByDefinition(int d)