Renormalisation group recursion relations using the application of generalised Callen identities to the Ashkin-Teller model

Using generalised Callen identities a sequence of appropriate renormalisation group recursion relations for Ising models are derived. They are simple to obtain and give very satisfactory results for critical and non-critical physical quantities. The technique is then used to derive the phase diagram and to study the critical properties of the Ashkin-Teller model.