Total non-negativity of some combinatorial matrices

TL;DR

This paper provides a unified framework for understanding the total non-negativity of various combinatorial matrices, establishing conditions on sequences that guarantee this property and applying results to rook numbers and graph Stirling numbers.

Contribution

It introduces a general framework linking sequences to total non-negativity of combinatorial matrices and characterizes conditions on sequences for this property to hold.

Findings

Established necessary and sufficient conditions for total non-negativity.

Unified understanding of matrices like binomial coefficients, Stirling, and Lah numbers.

Applied results to rook numbers and graph Stirling numbers.

Abstract

Many combinatorial matrices --- such as those of binomial coefficients, Stirling numbers of both kinds, and Lah numbers --- are known to be totally non-negative, meaning that all minors (determinants of square submatrices) are non-negative. The examples noted above can be placed in a common framework: for each one there is a non-decreasing sequence $(a_1, a_2, \ldots)$, and a sequence $(e_1, e_2, \ldots)$, such that the $(m,k)$-entry of the matrix is the coefficient of the polynomial $(x-a_1)\cdots(x-a_k)$ in the expansion of $(x-e_1)\cdots(x-e_m)$ as a linear combination of the polynomials $1, x-a_1, \ldots, (x-a_1)\cdots(x-a_m)$. We consider this general framework. For a non-decreasing sequence $(a_1, a_2, \ldots)$ we establish necessary and sufficient conditions on the sequence $(e_1, e_2, \ldots)$ for the corresponding matrix to be totally non-negative. As corollaries we obtain totally non-negativity of matrices of rook numbers of Ferrers boards, and of graph Stirling numbers of chordal graphs.