Low temperature behavior of finite-size one-dimensional Ising model and the partition function zeros

In contrast to the infinite chain, the low-temperature expansion of a one-dimensional free-field Ising model has a strong dependence on boundary conditions. I derive explicit formula for the leading term of the expansion both under open and periodic boundary conditions, and show they are related to different distributions of partition function zeros on the complex temperature plane. In particular, when the periodic boundary condition is imposed, the leading coefficient of the expansion grows with size, due to the zeros approaching the origin.