The phase diagram for fisher's alternate-site lattice gas model

Exact low- and high-density double activity series (in the two appropriate sublattice activities) are given through the ninth order for Fisher's alternate-site lattice gas model. By introducing an additional interaction into the model we use these series to construct low-temperature series along the coexistence curve and to locate the tricritical temperature. On letting the additional interaction vanish, the tricritical temperature approaches absolute zero. Thus the phase diagram for the alternate-site model is a line of second order transitions (critical points).