On d-Walk Regular Graphs

TL;DR

This paper characterizes d-walk regular graphs through algebraic conditions involving adjacency matrices and Schur products, providing a new criterion for identifying such graphs.

Contribution

It establishes a necessary and sufficient condition for d-walk regularity using matrix algebra and Schur products, advancing the theoretical understanding of graph regularity.

Findings

d-walk regularity characterized by matrix conditions

Equivalent algebraic condition involving adjacency matrices and Schur product

Provides a new criterion for identifying d-walk regular graphs

Abstract

Let G be a graph with set of vertices 1,...,n and adjacency matrix A of size nxn. Let d(i,j)=d, we say that f_d:N->N is a d-function on G if for every pair of vertices i,j and k>=d, we have a_ij^(k)=f_d(k). If this function f_d exists on G we say that G is d-walk regular. We prove that G is d-walk regular if and only if for every pair of vertices i,j at distance <=d and for d<=k<=n+d-1, we have that a_ij^(k) is independent of the pair i,j. Equivalently, the single condition exp(A)*A_d=cA_d holds for some constant c, where A_d is the adjacency matrix of the d-distance graph and * denotes the Schur product.