Triangular Ising antiferromagnet: Boundary conditions, ground state entropy, and vortices

The ground state entropy density of the triangular Ising antiferromagnet is considered as a function of boundary conditions on domains for which the ground states do not admit a dimer covering. These domains admit a rich set of ground states that cannot be classified in the usual way in terms of nonintersecting strings. Various parametrized boundary conditions and domain shapes are identified that allow the ground state entropy density to be varied between zero and maximal degeneracy. The dependence of degeneracy on boundary spins and/or domain shape is interpreted in terms of strings that are not restricted to be nonintersecting.