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

def _abs_distance_integrand_scalar(self, z):

    args = self._inv_efunc_scalar_args
    return (((1.0 + z) ** 2) * self._inv_efunc_scalar(z, *args))
