Here you can find the source of atanh(double z)
public static double atanh(double z)
//package com.java2s; public class Main { /** *//*from ww w. ja v a 2 s . c om*/ public static double atanh(double z) { return (StrictMath.log((1.0 + z) * StrictMath.sqrt(1.0 / (1.0 - z * z)))); } }