High-temperature series expansions of susceptibility of the square-lattice Ising model with first- and second-neighbour interactions

We have calculated exactly the high-temperature series expansions of the susceptibility of the Ising model with equal first- and second-neighbour interactions on the square lattice up to 18th order by computer. Soehianie and Oitmaa computed the same series expansions up to 13th order and their result agrees with ours. We use the Padé approximants to estimate the critical exponent γ and find that 1.745<γ<1.755. Our result is consistent with the universality hypothesis which predicts that square-lattice Ising models with interactions beyond first neighbours have the same critical exponent γ=1.75 as the square-lattice Ising model with nearest-neighbour interactions.