Current status of the Kondo problem

It is shown that at zero temperature the magnetic field μH >> TK does not move the system from the strong coupling to the weak coupling regime. As a result, the average of the impurity spin approaches its saturation value as a power of the small parameter (2TK/μH)2. The study of the high-temperature expansion of the free energy shows that the Kondo problem contains at least two energy scales and that these scales are separated by the coupling constant. The Hamiltonian of the Kondo problem is not renormalizable. (methodological notes)