Positive definite functions of noncommuting contractions, Hua-Bellman matrices, and a new distance metric

Abstract

We study positive definite functions on noncommuting strict contractions. In particular, we study functions that induce positive definite Hua-Bellman matrices (i.e., matrices of the form $[\det(I-A_i^*A_j)^{-α}]_{ij}$ where $A_i$ and $A_j$ are strict contractions and $α\in\mathbb{C}$). We start by revisiting a 1959 work of \citeauthor{bellman1959} (R.~Bellman~\emph{Representation theorems and inequalities for Hermitian matrices}; Duke Mathematical J., 26(3), 1959) that studies Hua-Bellman matrices and claims a strengthening of \citeauthor{hua1955}'s representation theoretic results on their positive definiteness~(L.-K. Hua, \emph{Inequalities involving determinants}; Acta Mathematica Sinica, 5(1955), pp.~463--470). We uncover a critical error in Bellman's proof that has surprisingly escaped notice to date. We "fix" this error and provide conditions under which $\det(I-A^*B)^{-α}$ is a positive definite function; our conditions correct Bellman's claim and subsume both Bellman's and Hua's prior results. Subsequently, we build on our result and introduce a new hyperbolic-like geometry on noncommuting contractions, and remark on its potential applications.