Heat Kernel Expansions on the Integers and the Toda Lattice Hierarchy

In the present paper we consider the heat equation ut = 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 αk(n,m) in this expansion are analogs of Hadamard’s coefficients for the (continuous) Schrödinger operator. We derive an explicit formula for αk in terms of the wave and the adjoint wave functions for 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, 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 [17].