Disjoint Covering of Bipartite Graphs with $s$-Clubs

Abstract

For a positive integer $s$, an $s$-club in a graph $G$ is a set of vertices inducing a subgraph with diameter at most $s$. As generalizations of cliques, $s$-clubs offer a flexible model for real-world networks. This paper addresses the problems of partitioning and disjoint covering of vertices with $s$-clubs on bipartite graphs. First we prove that for any fixed $s \geq 6$ and fixed $k \geq 5$, determining whether the vertices of $G$ can be partitioned into at most $k$ disjoint $s$-clubs is NP-complete even for bipartite graphs. Note that our NP-completeness result is stronger than the one in Abbas and Stewart (1999), as we assume that both $s$ and $k$ are constants and not part of the input.

Additionally, we study the Maximum Disjoint $(t,s)$-Club Covering problem (MAX-DCC($t,s$)), which aims to find a collection of vertex-disjoint $(t,s)$-clubs (i.e. $s$-clubs with at least $t$ vertices) that covers the maximum number of vertices in $G$. We prove that it is NP-hard to achieve an approximation factor of $\frac{95}{94} $ for MAX-DCC($t,3$) for any fixed $t\geq 8$
and for MAX-DCC($t,2$) for any fixed $t\geq 5$ even for bipartite graphs. Previously, results were known only for MAX-DCC($3,2$). Finally, we provide a polynomial-time algorithm for MAX-DCC($2,2$) resolving an open problem from Dondi \textit{et al.} (2019).