  The s-d model describes a chain of spin-1/2 electrons interacting
magnetically with a two-level impurity. It was introduced to study the Kondo
effect, in which the magnetic susceptibility of the impurity remains finite in
the 0-temperature limit as long as the interaction of the impurity with the
electrons is anti-ferromagnetic. A variant of this model was introduced by
Andrei, which he proved was exactly solvable via Bethe Ansatz. A hierarchical
version of Andrei's model was studied by Benfatto and the authors. In the
present letter, that discussion is extended to a hierarchical version of the
s-d model. The resulting analysis is very similar to the hierarchical Andrei
model, though the result is slightly simpler.
