Universal and nonuniversal amplitude ratios for scaling corrections on Ising strips

We consider strips of Ising spins at criticality. For strips of width N sites, subdominant (additive) finite-size corrections to scaling are assumed to be of the form a(k)/N-k for the free energy, and b(k)/N-k for inverse correlation length, with integer values of k. We investigate the set {a(k), b(k)} (k >= 2) by exact evaluation and numerical transfer-matrix diagonalization techniques, and their changes upon varying anisotropy of couplings, spin quantum number S, and (finite) interaction range, in all cases for both periodic (PBCs) and free (FBCs) boundary conditions across the strip. We find that the coefficient ratios b(k)/a(k) remain constant upon varying coupling anisotropy for S = 1/2 and first-neighbor couplings, for both PBCs and FBCs (albeit at distinct values in either case). Such apparently universal behavior is not maintained upon changes in S or interaction range.