Tridiagonal matrices with nonnegative entries

Abstract: In this paper we characterize the nonnegative irreducible tridiagonal matrices and their permutations, using certain entries in their primitive idempotents. Our main result is summarized as follows. Let $d$ denote a nonnegative integer. Let $A$ denote a matrix in $\matR$ and let ${\th_i}{i=0}^d$ denote the roots of the characteristic polynomial of $A$. We say $A$ is multiplicity-free whenever these roots are mutually distinct and contained in $\R$. In this case $E_i$ will denote the primitive idempotent of $A$ associated with $\th_i$ $(0 \leq i \leq d)$. We say $A$ is symmetrizable whenever there exists an invertible diagonal matrix $\Delta \in \matR$ such that $\Delta A \Delta^{-1}$ is symmetric. Let $\Gamma(A)$ denote the directed graph with vertex set ${0,1,...,d}$, where $i \rightarrow j$ whenever $i \neq j$ and $A{ij} \neq 0$. Theorem: Assume that each entry of $A$ is nonnegative. Then the following are equivalent for $0 \leq s,t \leq d$: (i) The graph $\Gamma(A)$ is a bidirected path with endpoints $s$, $t$: (ii) The matrix $A$ is symmetrizable and multiplicity-free. Moreover the $(s,t)$-entry of $E_i$ times $(\th_i-\th_0)...(\th_i-\th_{i-1})(\th_i-\th_{i+1})...(\th_i-\th_d)$ is independent of $i$ for $0 \leq i \leq d$, and this common value is nonzero. Recently Kurihara and Nozaki obtained a theorem that characterizes the $Q$-polynomial property for symmetric association schemes. We view the above result as a linear algebraic generalization of their theorem.