A direct proof of Kim's identities

TL;DR

This paper provides a direct proof of Kim's identities related to elliptic theta functions, completing a set of four identities by applying Weierstrass' factorization theorem.

Contribution

It introduces a direct proof method for Kim's identities, extending the set of known relations and clarifying their mathematical structure.

Findings

Established a fourth identity completing Kim's set

Derived all identities directly using complex analysis

Connected identities to elliptic theta functions

Abstract

As a by-product of a finite-size Bethe Ansatz calculation in statistical mechanics, Doochul Kim has established, by an indirect route, three mathematical identities rather similar to the conjugate modulus relations satisfied by the elliptic theta constants. However, they contain factors like $1 - q^{\sqrt{n}}$ and $1 - q^{n^2}$, instead of $1 - q^n$. We show here that there is a fourth relation that naturally completes the set, in much the same way that there are four relations for the four elliptic theta functions. We derive all of them directly by proving and using a specialization of Weierstrass' factorization theorem in complex variable theory.