'Integrand of the absorption distance.
Parameters
z : float or array
Input redshift.
Returns
X : float or array
The integrand for the absorption distance
References
See Hogg 1999 section 11.'

def abs_distance_integrand(self, z):

    if isiterable(z):
        zp1 = (1.0 + np.asarray(z))
    else:
        zp1 = (1.0 + z)
    return ((zp1 ** 2) * self.inv_efunc(z))
