docs: def _dEndfr(self):
          """Eq. 4 from Orazio and Samsing (2018)
  
          Takes f in rest frame.
  
          """
          Mc = self._chirp_mass()
          return (np.pi**(2./3.)*Mc**(5./3.)/(3.*(1.+self.z)**(1./3.)
                  * (self.freqs_orb/(1.+self.z))**(1./3.))*(2./self.n)**(2./3.)
                  * self._g_func()/self._f_func())