Hull and Geodetic Numbers for Some Classes of Oriented Graphs

Abstract

Let $D$ be an orientation of a simple graph. Given $u,v\in V(D)$, a directed shortest $(u,v)$-path is a $(u,v)$-geodesic. $S \subseteq V(D)$ is convex if, for every $u,v \in S$, the vertices in each $(u,v)$-geodesic and in each $(v,u)$-geodesic are in $S$. For each $S \subseteq V(D)$ the (convex) hull of $S$, denoted by $[S]$, is the smallest convex set containing $S$. $S \subseteq V(D)$ is a hull set if $[S] = V(D)$. $S \subseteq V(D)$ is a geodetic set of $D$ if each vertex of $D$ lies in a $(u,v)$-geodesic, for some $u,v \in S$. The cardinality of a minimum hull set (resp. geodetic set) of $G$ is the hull number (resp. geodetic number) of $D$, denoted by $ \overrightarrow{\textrm{hn}} (D)$ (resp. $\overrightarrow{\textrm{gn}}(D)$). We first show a tight upper bound on $\overrightarrow{\textrm{hn}}(D)$. Given $k\in\mathbb{Z}_+^*$, we prove that deciding if $\overrightarrow{\textrm{hn}}\leq k$ is NP-complete when $D$ is an oriented partial cube; and if $\overrightarrow{\textrm{gn}}(D)\leq k$ is W[2]-hard parameterized by $k$ and has no $(c \cdot \ln n)$-approximation algorithm, unless P = NP, even if $D$ has an underlying graph that is bipartite or split or cobipartite. We also show polynomial-time algorithms to compute $\overrightarrow{\textrm{hn}}(D)$ and $\overrightarrow{\textrm{gn}}(D)$ when $D$ is an oriented cactus.