'The robust criterion function for Ramsay\'s Ea.
Parameters
z : array-like
1d array
Returns
rho : array
rho(z) = a**-2 * (1 - exp(-a*\|z\|)*(1 + a*\|z\|))'

def rho(self, z):

    z = np.asarray(z)
    return ((1 - (np.exp(((- self.a) * np.fabs(z))) * (1 + (self.a * np.fabs(z))))) / (self.a ** 2))
