Vieri Mastropietro Small denominators and anomalous behaviour in the uncommensurate Hubbard-Holstein model (234K, TeX) ABSTRACT. A small denominator problem arising in the perturbative expansion for the Schwinger functions of a quantum model of interacting fermions in one dimension is solved by renormalization group techniques. We show that the Schwinger functions asymptotic behaviour is anomalous and described in terms of critical indices.