Heat kernel expansions on the integers and the Toda lattice hierarchy

TL;DR

This paper derives explicit formulas for heat kernel coefficients related to the Toda lattice hierarchy and shows their connection to bispectral operators, providing finite formulas for the fundamental solution in special cases.

Contribution

It introduces explicit formulas for heat kernel coefficients in terms of Toda hierarchy wave functions and links these to bispectral operators, revealing new integrable structure insights.

Findings

Coefficients define a hierarchy equivalent to Toda lattice

Fundamental solution can be expressed with two Bessel functions

Finite formulas occur for bispectral operators

Abstract

We consider the heat equation $u_t=Lu$ where $L$ is a second-order difference operator in a discrete variable $n$. The fundamental solution has an expansion in terms of the Bessel functions of imaginary argument. The coefficients $\alpha_k(n,m)$ in this expansion are analogs of Hadamard's coefficients for the (continuous) Schrodinger operator. We derive an explicit formula for $\alpha_k$ in terms of the wave and the adjoint wave functions of the Toda lattice hierarchy. As a first application of this result, we prove that the values of these coefficients on the diagonals $n=m$ and $n=m+1$ define a hierarchy of differential-difference equations which is equivalent to the Toda lattice hierarchy. Using this fact and the correspondence between commutative rings of difference operators and algebraic curves we show that the fundamental solution can be summed up, giving a finite formula involving only two Bessel functions with polynomial coefficients in the time variable $t$, if and only if the operator $L$ belongs to the family of bispectral operators constructed in [18].