code: def mass_2d(self,R,Rs,rho0,r_trunc):
  
          
  
          x = R / Rs
          tau = r_trunc / Rs
          gx = self._g(x,tau)
          m_2d = 4 * rho0 * Rs * R ** 2 * gx / x ** 2 * np.pi
          return m_2d
docs: analytic solution of the projection integral
          (convergence)
  
          :param x: R/Rs
          :type x: float >0