  We determine the impurity-induced free energy and the impurity-induced
zero-field susceptibility of the symmetric single-impurity Kondo model from
weak-coupling perturbation theory up to third order in the Kondo coupling at
low temperatures and small magnetic fields. We reproduce the analytical
structure of the zero-field magnetic susceptibility as obtained from Wilson's
renormalization group method. This permits us to obtain analytically the first
two Wilson numbers.
