# Cosecure Domination: Hardness Results and Algorithm

## TL;DR

This paper investigates the computational complexity of the cosecure domination problem across various graph classes, establishing NP-completeness for some and providing efficient algorithms for others, including chain and bounded tree-width graphs.

## Contribution

It proves NP-completeness for cosecure domination in multiple graph classes and offers polynomial-time algorithms for chain graphs and bounded tree-width graphs.

## Key findings

- NP-complete for circle, doubly chordal, chordal bipartite, star-convex, and comb-convex bipartite graphs
- Polynomial-time algorithm for chain graphs
- Linear-time solvability for bounded tree-width graphs

## Abstract

For a simple graph $G=(V,E)$ without any isolated vertex, a cosecure dominating set $D$ of $G$ satisfies the following two properties (i) $S$ is a dominating set of $G$, (ii) for every vertex $v \in S$ there exists a vertex $u \in V \setminus S$ such that $uv \in E$ and $(S \setminus \{v\}) \cup \{u\}$ is a dominating set of $G$. The minimum cardinality of a cosecure dominating set of $G$ is called cosecure domination number of $G$ and is denoted by $\gamma_{cs}(G)$. The Minimum Cosecure Domination problem is to find a cosecure dominating set of a graph $G$ of cardinality $\gamma_{cs}(G)$. The decision version of the problem is known to be NP-complete for bipartite, planar, and split graphs. Also, it is known that the Minimum Cosecure Domination problem is efficiently solvable for proper interval graphs and cographs. In this paper, we work on various important graph classes in an effort to reduce the complexity gap of the Minimum Cosecure Domination problem. We show that the decision version of the problem remains NP-complete for circle graphs, doubly chordal graphs, chordal bipartite graphs, star-convex bipartite graphs and comb-convex bipartite graphs. On the positive side, we give an efficient algorithm to compute the cosecure domination number of chain graphs, which is an important subclass of bipartite graphs. In addition, we show that the problem is linear-time solvable for bounded tree-width graphs. Further, we prove that the computational complexity of this problem varies from the domination problem.