Java Digamma digammaByDefinition(int d)

Description

Compute the digamma function from first principles

License

Open Source License

Parameter

Parameter	Description
d	a parameter

Exception

Parameter	Description
Exception	an exception

Declaration

public static double digammaByDefinition(int d) throws Exception 

Method Source Code

//package com.java2s;
/*//from w ww  .j  a  v a  2  s .  com
 *  Java Information Dynamics Toolkit (JIDT)
 *  Copyright (C) 2012, Joseph T. Lizier
 *  
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *  
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *  
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

public class Main {
    private static final double EULER_MASCHERONI_CONSTANT = 0.5772156;

    /**
     * Compute the digamma function from first principles
     * 
     * @param d
     * @return
     * @throws Exception
     */
    public static double digammaByDefinition(int d) throws Exception {
        if (d < 1) {
            return Double.NaN;
        }
        double result = 0;
        for (int n = d; n > 1; n--) {
            result += 1.0 / (double) (n - 1);
        }
        // Now add in result for n == 1
        result += -EULER_MASCHERONI_CONSTANT;
        return result;
    }
}

Related

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