Bounds and extremal graphs for monitoring edge-geodetic sets in graphs

Abstract

A monitoring edge-geodetic set, or simply an MEG-set, of a graph $G$ is a vertex subset $M\subseteq V\left(G\right)$ such that given any edge $e$ of $G$, $e$ lies on every shortest $u$-$v$ path of $G$, for some $u,v\in M$. The monitoring edge-geodetic number of $G$, denoted by $meg\left(G\right)$, is the minimum cardinality of such an MEG-set. This notion provides a graph theoretic model of the network monitoring problem.

In this article, we compare $meg\left(G\right)$ with some other graph theoretic parameters stemming from the network monitoring problem and provide examples of graphs having prescribed values for each of these parameters. We also characterize graphs $G$ that have $V\left(G\right)$ as their minimum MEG-set, which settles an open problem due to Foucaud et al. (CALDAM 2023), and prove that some classes of graphs fall within this characterization. We also provide a general upper bound for $meg\left(G\right)$ for sparse graphs in terms of their girth, and later refine the upper bound using the chromatic number of $G$. We examine the change in $meg\left(G\right)$ with respect to two fundamental graph operations: clique-sum and subdivisions. In both cases, we provide a lower and an upper bound of the possible amount of changes and provide (almost) tight examples.