ON 3 NEW CLUSTER VARIATION APPROXIMATIONS

We discuss in some detail three new cluster variation approximations w e used in a previous paper in connection with Pade approximants to inv estigate critical behaviour of classical spin models on various lattic es. The approximations are applied to the Ising model on the simple cu bic, face centered cubic, and semi-infinite simple cubic lattices. The natural iteration equations and estimates for the critical temperatur es and for the range of validity of the approximations are given.