On the class of matrices with rows that weakly decrease cyclicly from the diagonal

TL;DR

This paper studies a special class of matrices with cyclically weakly decreasing rows, linking their properties to graph structures and providing criteria for determinants and P-matrix conditions.

Contribution

It characterizes solutions to linear systems involving these matrices using graph theory and identifies conditions for the matrices to be P-matrices.

Findings

Solutions to $A^T x = ext{const}$ are linear combinations of fundamental solutions and kernel vectors.

The sign of $ ext{det} A$ depends on the number of closed strongly connected components.

Provides conditions under which the matrix $A$ is a P-matrix.

Abstract

We consider $n\times n$ real-valued matrices $A = (a_{ij})$ satisfying $a_{ii} \geq a_{i,i+1} \geq \dots \geq a_{in} \geq a_{i1} \geq \dots \geq a_{i,i-1}$ for $i = 1,\dots,n$. With such a matrix $A$ we associate a directed graph $G(A)$. We prove that the solutions to the system $A^T x = \lambda e$, with $\lambda \in \mathbb{R}$ and $e$ the vector of all ones, are linear combinations of 'fundamental' solutions to $A^T x=e$ and vectors in $\ker A^T$, each of which is associated with a closed strongly connected component (SCC) of $G(A)$. This allows us to characterize the sign of $\det A$ in terms of the number of closed SCCs and the solutions to $A^T x = e$. In addition, we provide conditions for $A$ to be a $P$-matrix.