Kernel function applied on inputs X and X2.
        In the stationary case there is an inner function depending on the
        distances from X to X2, called r.

        K(X, X2) = K_of_r((X-X2)**2)

def K(self, X, X2=None):
        """
        Kernel function applied on inputs X and X2.
        In the stationary case there is an inner function depending on the
        distances from X to X2, called r.

        K(X, X2) = K_of_r((X-X2)**2)
        """
        r = self._scaled_dist(X, X2)
        return self.K_of_r(r)