Proof of a conjecture on the determinant of walk matrix of rooted product with a path

TL;DR

This paper proves a conjecture relating the determinant of the walk matrix of a rooted product graph with a path to the original graph's walk matrix determinant, using Chebyshev polynomial techniques.

Contribution

It confirms Mao-Wang's conjecture that the determinant formula holds for all path lengths m ≥ 2 in the rooted product graph.

Findings

The determinant formula is valid for all m ≥ 2.

Chebyshev polynomials are effective in analyzing walk matrices.

The conjecture is rigorously verified.

Abstract

The walk matrix of an $n$-vertex graph $G$ with adjacency matrix $A$, denoted by $W(G)$, is $[e,Ae,\ldots,A^{n-1}e]$, where $e$ is the all-ones vector. Let $G\circ P_m$ be the rooted product of $G$ and a rooted path $P_m$ (taking an endvertex as the root), i.e., $G\circ P_m$ is a graph obtained from $G$ and $n$ copies of $P_m$ by identifying each vertex of $G$ with an endvertex of a copy of $P_m$. Mao-Liu-Wang (2015) and Mao-Wang (2022) proved that, for $m=2$ and $m\in\{3,4\}$, respectively$$\det W(G\circ P_m)=\pm a_0^{\lfloor\frac{m}{2}\rfloor}(\det W(G))^m,$$ where $a_0$ is the constant term of the characteristic polynomial of $G$. Furthermore, Mao-Wang (2022) conjectured that the formula holds for any $m\ge 2$. In this note, we verify this conjecture using the technique of Chebyshev polynomials.