Fault-tolerant mutual-visibility: complexity and solutions for grid-like networks

TL;DR

This paper introduces and analyzes the concept of $k$-fault-tolerant mutual visibility in graphs, focusing on its complexity, properties, and solutions for grid-like networks, with applications in reliable communication systems.

Contribution

It formalizes the $k$-fault-tolerant mutual visibility concept, establishes its properties, proves NP-hardness of computing it, and provides exact formulas for specific grid-like network topologies.

Findings

Computing $ ext{f} extmu^{k}(G)$ is NP-hard for all positive $k$.

Derived exact formulas for grid-like networks such as cylinders and tori.

Established relationships between $ ext{f} extmu^{k}(G)$ and graph properties like $ ext{omega}(G)$.

Abstract

Networks are often modeled using graphs, and within this setting we introduce the notion of $k$-fault-tolerant mutual visibility. Informally, a set of vertices $X \subseteq V(G)$ in a graph $G$ is a $k$-fault-tolerant mutual-visibility set ($k$-ftmv set) if any two vertices in $X$ are connected by a bundle of $k+1$ shortest paths such that: ($i$) each shortest path contains no other vertex of $X$, and ($ii$) these paths are internally disjoint. The cardinality of a largest $k$-ftmv set is denoted by $\mathrm{f}\mu^{k}(G)$. The classical notion of mutual visibility corresponds to the case $k = 0$. This generalized concept is motivated by applications in communication networks, where agents located at vertices must communicate both efficiently (i.e., via shortest paths) and confidentially (i.e., without messages passing through the location of any other agent). The original notion of mutual visibility may fail in unreliable networks, where vertices or links can become unavailable. Several properties of $k$-ftmv sets are established, including a natural relationship between $\mathrm{f}\mu^{k}(G)$ and $\omega(G)$, as well as a characterization of graphs for which $\mathrm{f}\mu^{k}(G)$ is large. It is shown that computing $\mathrm{f}\mu^{k}(G)$ is NP-hard for any positive integer $k$, whether $k$ is fixed or not. Exact formulae for $\mathrm{f}\mu^{k}(G)$ are derived for several specific graph topologies, including grid-like networks such as cylinders and tori, and for diameter-two networks defined by Hamming graphs and by the direct product of complete graphs.