* @return sigmoid = 1 ./ (1 + exp(-x));
*/
@Override
public DoubleMatrix valueAt(DoubleMatrix M) {
DoubleMatrix Denom = (MatrixFunctions.exp(M.mul(-1))).addi(1);
return Denom.rdivi(1);
}
/**
* @param X input double matrix
* @return sigmoid_prime = M.*(1-M), where M = sigmoid(X);