Susceptibility calculations in periodic and quasiperiodic planar Ising models

New results are presented for the wavevector-dependent susceptibility of Z-invariant periodic and quasiperiodic Ising models in the scaling limit, generalizing old results of Tracy and McCoy for the square lattice. Explicit results are worked out for the two leading singular terms of the susceptibility on four regular isotropic lattices. The methods used provide a proof of the extended lattice–lattice scaling hypothesis for the class of models under consideration.