@staticmethod
def kappa_r(r, sigma0, r_core):
    """
        convergence of the cored density profile. This routine is also for testing

        :param r: radius (angular scale)
        :param sigma0: convergence in the core
        :param r_core: core radius
        :return: convergence at r
        """
    return sigma0 * r_core ** 2 / (r_core ** 2 + r ** 2)
