Vieri Mastropietro
Non-universality in Ising models with four spin interaction.
(130K, TeX)

ABSTRACT.  We consider two bidimensional
classical Ising models,
coupled by a weak interaction bilinear in the energy
densities of the two systems; the model contains,
as limiting cases, the Ashkin-Teller and the Eight-vertex
models for certain values of their parameters. 
We write the energy-energy correlations and the specific
heat as Grassman integrals
formally describing Dirac 
1+1 dimensional interacting massive fermions
on a lattice, and an expansion based on Renormalization Group
is written for them,
convergent up to temperatures
very close to the critical temperature and for weak
coupling.
The critical behaviour is determined
by critical indices which
are continuous functions of the coupling.