'The derivative of Ramsay\'s Ea psi function.
Notes
Used to estimate the robust covariance matrix.'

def psi_deriv(self, z):

    return (np.exp(((- self.a) * np.fabs(z))) + ((((z ** 2) * np.exp(((- self.a) * np.fabs(z)))) * (- self.a)) / np.fabs(z)))
