Enhancement of mass anisotropy in an electron - phonon coupled model close to a Van Hove singularity

We investigate the effect of electronic mass renormalization due to the coupling of electrons to an Einstein phonon in the presence of a Van Hove singularity in the density of states, resulting from a saddle point in the bare electronic spectrum. We Show that an enhancement of mass anisotropy is produced in a wide region of parameters of our model.