def METHOD_NAME(self, u, x, y, q, n_sersic, R_sersic, keff, centerx, centery):
    ellip_coord = self._elliptical_coord_u(x, y, u, q)
    def_angle_circular = self._sersic.alpha_abs(
        ellip_coord, 0, n_sersic, R_sersic, keff, centerx, centery
    )
    return (
        ellip_coord * def_angle_circular * (1 - (1 - q**2) * u) ** -0.5 * u**-1
    )