public void SigmoidPrimeTest()
{
var a = new Matrix(2, 2);
a.InRandomize();
var b = a.Duplicate();
a = new Sigmoid().Backward(a);
b.InMap((x) => 1.0 / (1 + Math.Exp(-x)) * (1 - 1.0 / (1 + Math.Exp(-x))));
Assert.IsTrue(Math.Abs(a.FrobeniusNorm() - b.FrobeniusNorm()) < 0.1,
new Sigmoid().Type().ToString() + " Derivative.");
}