Weak-coupling selection and finite-torus entropy in the triangular Ising antiferromagnet
An antiferromagnetic second-neighbor interaction favors stripe order in the triangular-lattice Ising antiferromagnet, but uniform stripes cannot satisfy odd periodic boundaries. At each fixed odd size $L\geq5$, we minimize this interaction over the ground states of the zero-field nearest-neighbor model on an $L\times L$ torus. We prove a minimum total interface length of $4L$ and classify all $L^2(L^2-1)/2$ selected states. Under the uniform measure on these states, the area fractions of the three stripe orientations approach a uniform distribution over all nonnegative triples summing to one. Thus selection reduces an exponential degeneracy to polynomial growth, while the whole-sample orientation fractions remain broadly distributed. We also derive explicit two-sided entropy-density bounds for periodic spin ground states and honeycomb dimer coverings on $M\times N$ tori, where $M,N$ are the periods along the two primitive lattice directions. For all $M,N\geq3$, the errors relative to the bulk residual entropy are bounded by explicit constants times $\min(M,N)^{-2}$, without an aspect-ratio restriction. These bounds give finite-size accuracy guarantees and control the exponentially small fraction of unperturbed ground states that survive the selection.