  In this paper, a fermionic hierarchical model is defined, inspired by the
Kondo model, which describes a 1-dimensional lattice gas of spin-1/2 electrons
interacting with a spin-1/2 impurity. This model is proved to be exactly
solvable, and is shown to exhibit a Kondo effect, i.e. that, if the interaction
between the impurity and the electrons is antiferromagnetic, then the magnetic
susceptibility of the impurity is finite in the 0-temperature limit, whereas it
diverges if the interaction is ferromagnetic. Such an effect is therefore
inherently non-perturbative. This difficulty is overcome by using the exact
solvability of the model, which follows both from its fermionic and
hierarchical nature.
