Ising-Model Reformulation. I. Fundamentals

The partition function ($Z$) for the classical Ising model of cooperative phenomena in lattice of arbitrary dimensionality and with interactions of unspecified range is expressed as a vacuum-state expectation value of a product of two operators, each constructed from boson annihilation and creation operators. In the absence of external fields, $Z$, and similarly the spin-pair correlation function $\psi \left(\mathrm{r}\right)$, are expanded thereupon into a series of Feynman diagrams. In the case of $\psi \left(\mathrm{r}\right)$, a formally exact diagram summation (1) shows how the spherical model may be recovered in low-order approximation, (2) suggests a way of introducing systematic corrections to this approximation, and (3) leads to a generalized criterion for suppression of antiferro-magnetic order in "nonfitting" lattices. A full topological-diagram reduction to restricted sets of "elementary" subdiagrams is carried out.