public void ExpandReverseDistanceTest()
{
Quadratic kernel = new Quadratic(0);
var x = new double[] { 0.5, 2.0 };
var y = new double[] { 1.3, -0.2 };
var phi_x = kernel.Transform(x);
var phi_y = kernel.Transform(y);
double d = Distance.SquareEuclidean(x, y);
double phi_d = kernel.ReverseDistance(phi_x, phi_y);
Assert.AreEqual(phi_d, d);
}