public Gradient gradientAt(Point point) {
double[] ax = point.toArray();
double[] gradient = new double[n];
double sqrtSum = 0.0;
double cosSum = 0.0;
for (int i = 0; i < n; i++) {
sqrtSum += ax[i] * ax[i];
cosSum += Math.cos(2 * Math.PI * ax[i]);
}
sqrtSum = Math.sqrt(sqrtSum);
cosSum /= n;
for (int i = 0; i < n; i++)
gradient[i] =
(2 * ax[i] * Math.exp(-0.1 * sqrtSum)) / sqrtSum
+ 0.5 * Math.exp(cosSum) * Math.PI * Math.sin(2 * Math.PI * ax[i]);
return Gradient.valueOf(gradient);
}
