public void Evaluate(double[] x, double[] output, double[] derivative)
        {
            for (int j = 0; j < x.Length; j++)
            {
                // Kurtosis contrast function and its derivative, as given
                //  in original Hyvärinen's paper. See main references for the
                //  Independent Component Analysis class for details.

                double v = x[j];

                // g(w*x)
                output[j] = v * v * v;

                // g'(w*x)
                derivative[j] = (1.0 / 3.0) * v * v;
            }
        }

derivative[j] = 3.0 * v * v;