code: def function(self, x, y, sigma0, Rs, center_x=0, center_y=0):
          
          x_ = x - center_x
          y_ = y - center_y
          r = np.sqrt(x_**2 + y_**2)
          if isinstance(r, int) or isinstance(r, float):
              r = max(self._s, r)
          else:
              r[r < self._s] = self._s
          X = r / Rs
          f_ = sigma0 * Rs ** 2 * (np.log(X ** 2 / 4.) + 2 * self._F(X))
          return f_
docs: lensing potential
          :param x:
          :param y:
          :param sigma0: sigma0/sigma_crit
          :param a:
          :param s:
          :param center_x:
          :param center_y:
          :return: