code: def mass_2d(self, r, rho0, gamma):
          
          alpha = np.sqrt(np.pi) * special.gamma(1. / 2 * (-1 + gamma)) / special.gamma(gamma / 2.) * r ** (2 - gamma)/(3 - gamma) *np.pi * 2 * rho0
          mass_2d = alpha*r
          return mass_2d
docs: mass enclosed projected 2d sphere of radius r
          :param r:
          :param rho0:
          :param a:
          :param s:
          :return: