nd4j-common/src/main/java/org/nd4j/linalg/util/BigDecimalMath.java
static public BigDecimal acosh(final BigDecimal x) {
        if (x.compareTo(BigDecimal.ONE) < 0) {
            throw new ArithmeticException("Out of range argument cosh " + x.toString());
        } else if (x.compareTo(BigDecimal.ONE) == 0) {
            return BigDecimal.ZERO;
        } else {
            BigDecimal xhighpr = scalePrec(x, 2);
            /* arccosh(x) = log(x+sqrt(x^2-1))
             */
            BigDecimal logx = log(sqrt(xhighpr.pow(2).subtract(BigDecimal.ONE)).add(xhighpr));
            /* The absolute error in arcsinh x is err(x)/sqrt(x^2-1)
             */


            double xDbl = x.doubleValue();


            double eps = 0.5 * x.ulp().doubleValue() / Math.sqrt(xDbl * xDbl - 1.);
            MathContext mc = new MathContext(err2prec(logx.doubleValue(), eps));


            return logx.round(mc);


        }
    }
static public BigDecimal acosh(final BigDecimal x) {
        if (x.compareTo(BigDecimal.ONE) < 0) {
            throw new ArithmeticException("Out of range argument cosh " + x.toString());
        } else if (x.compareTo(BigDecimal.ONE) == 0) {
            return BigDecimal.ZERO;
        } else {
            BigDecimal xhighpr = scalePrec(x, 2);
            /* arccosh(x) = log(x+sqrt(x^2-1))
             */
            BigDecimal logx = log(sqrt(xhighpr.pow(2).subtract(BigDecimal.ONE)).add(xhighpr));
            /* The absolute error in arcsinh x is err(x)/sqrt(x^2-1)
             */


            double xDbl = x.doubleValue();


            double eps = 0.5 * x.ulp().doubleValue() / Math.sqrt(xDbl * xDbl - 1.);
            MathContext mc = new MathContext(err2prec(logx.doubleValue(), eps));


            return logx.round(mc);


        }
    }
The inverse hyperbolic cosine.

@param x The argument.
@return The arccosh(x) .