code: def mass_2d(self, R, Rs, rho0, r_core):
  
          
  
          x = R * Rs ** -1
          p = Rs * r_core ** -1
          gx = self._G(x, p)
  
          m_2d = 2 * np.pi * rho0 * Rs ** 3 * gx
  
          return m_2d
docs: analytic solution of the projection integral
          (convergence)
  
          :param R: projected distance
          :param Rs: scale radius
          :param rho0: central core density
          :param r_core: core radius