Key points are not available for this paper at this time.
The solution of the zero-field "eight-vertex" model is presented. This model includes the square lattice Ising, dimer, ice, F, and KDP models as special cases. It is found that in general the free energy has a branch-point singularity at a phase transition, with a continuously variable exponent.
R. J. Baxter (1971) studied this question.