  We consider a system of interacting fermions on a chain in a periodic
potential incommensurate with the chain spacing. We derive a convergent
perturbative expansion, afflicted by a small denominator problem and based on
renormalization group, for the two point Schwinger function. We obtain the
large distance behavior of the Schwinger function, which is anomalous and
described by critical indices, related to the gap and the wave function
renormalization.
