Consider the 2D Ising model on a square lattice with N sites, with periodic boundary conditions in both the horizontal and vertical directions, which effectively reduces the geometry of the model to a torus. In a general case, the horizontal coupling J is not equal to the coupling in the vertical direction, J*. With an equal number of rows and columns in the lattice, there will be N of each. In terms of
Consider a configuration of spins on the square lattice . Let r and s denote the number of unlike neighbours in the vertical and horizontal directions respectively. Then the summand in corresponding to is given by
Construct a dual lattice as depicted in the diagram. For every configuration , a polygon is associated to the lattice by drawing a line on the edge of the dual lattice if the spins separated by the edge are unlike. Since by traversing a vertex of the spins need to change an even number of times so that one arrives at the starting point with the same charge, every vertex of the dual lattice is connected to an even number of lines in the configuration, defining a polygon.
This reduces the partition function to
summing over all polygons in the dual lattice, where r and s are the number of horizontal and vertical lines in the polygon, with the factor of 2 arising from the inversion of spin configuration.
defines a low temperature expansion of .
Since one has
where and . Since there are N horizontal and vertical edges, there are a total of terms in the expansion. Every term corresponds to a configuration of lines of the lattice, by associating a line connecting i and j if the term (or is chosen in the product. Summing over the configurations, using
shows that only configurations with an even number of lines at each vertex (polygons) will contribute to the partition function, giving
where the sum is over all polygons in the lattice. Since tanh K, tanh L as , this gives the high temperature expansion of .
The two expansions can be related using the Kramers-Wannier duality.