@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 * (1 + (r / r_core) ** 2) ** (-3.0 / 2)
