Deterministic Distributed Construction of $T$-Dominating Sets in Time $T$

Abstract

A $k$-dominating set is a set $D$ of nodes of a graph such that, for each node $v$, there exists a node $w\in D$ at distance at most $k$ from $v$. Our aim is the deterministic distributed construction of small $T$-dominating sets in time $T$ in networks modeled as undirected $n$-node graphs and under the $\mathcal{L}\mathcal{O}\mathcal{C}\mathcal{A}\mathcal{L}$ communication model. For any positive integer $T$, if $b$ is the size of a pairwise disjoint collection of balls of radii at least $T$ in a graph, then $b$ is an obvious lower bound on the size of a $T$-dominating set. Our first result shows that, even on rings, it is impossible to construct a $T$-dominating set of size $s$ asymptotically $b$ (i.e., such that $s/b\to 1$) in time $T$. In the range of time $T\in \Theta ({\log }^{\ast }n)$, the size of a $T$-dominating set turns out to be very sensitive to multiplicative constants in running time. Indeed, it follows from \cite{KP}, that for time $T=\gamma {\log }^{\ast }n$ with large constant $\gamma$, it is possible to construct a $T$-dominating set whose size is a small fraction of $n$. By contrast, we show that, for time $T=\alpha {\log }^{\ast }n$ for small constant $\alpha$, the size of a $T$-dominating set must be a large fraction of $n$. Finally, when $T\in o({\log }^{\ast }n)$, the above lower bound implies that, for any constant $x<1$, it is impossible to construct a $T$-dominating set of size smaller than $xn$, even on rings. On the positive side, we provide an algorithm that constructs a $T$-dominating set of size $n-\Theta (T)$ on all graphs.

13 pages