Partially frustrated Ising models in two dimensions

We examine ordered, periodic, Ising models on a sq lattice at varying levels x of frustration. The thermodynamic singularity of the fully frustrated model (x=1) is at T=0 while those of partially frustrated lattices (0<xless than or equal to1) occur at finite T-c. The critical indices in the partially frustrated lattices that we consider-including the logarithmic specific heat-are all identical to those in the ferromagnet (x=0.) We display exact values of T-c and of ground-state energy and entropy E-o and S-o, at x=1, 2/3, 1/2, 2/5,...,0.