Total non-negativity of some combinatorial matrices

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 $(a_1,a_2,\dots )$, and a sequence $(e_1,e_2,\dots )$, 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 $1,x-a_1,\dots ,(x-a_1)\cdots (x-a_m)$. We consider this general framework. For a non-decreasing sequence $(a_1,a_2,\dots )$ we establish necessary and sufficient conditions on the sequence $(e_1,e_2,\dots )$ 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.

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