Polynomial Toda maps are transfer matrices

Abstract

We consider entire matrix functions $A(z)$ taking values in $\mathrm{SL}(2,\mathbb{C})$. These map pairs of Herglotz functions by acting pointwise as linear fractional transformations. The main examples of such Toda maps are provided by transfer matrices of differential and difference operators and by the cocycles associated with the classical integrable systems (Toda, KdV, etc.) on these operators. Here we consider polynomial matrix functions $A(z)$. We describe these in terms of a factorization, and we then prove that if $A$ induces a Toda map, then $A$ is essentially a transfer matrix.