Approximation Algorithms for Connected Maximum Coverage, Minimum Connected Set Cover, and Node-Weighted Group Steiner Tree

Abstract

In the Connected Budgeted maximum Coverage problem (CBC), we are given a collection of subsets $\mathcal{S}$, defined over a ground set $X$, and an undirected graph $G=(V,E)$, where each node is associated with a set of $\mathcal{S}$. Each set in $\mathcal{S}$ has a different cost and each element of $X$ gives a different prize. The goal is to find a subcollection $\mathcal{S}'\subseteq \mathcal{S}$ such that $\mathcal{S}'$ induces a connected subgraph in $G$, the total cost of the sets in $\mathcal{S}'$ does not exceed a budget $B$, and the total prize of the elements covered by $\mathcal{S}'$ is maximized. The Directed rooted Connected Budgeted maximum Coverage problem (DCBC) is a generalization of CBC where the underlying graph $G$ is directed and in the subgraph induced by $\mathcal{S}'$ in $G$ must be an out-tree rooted at a given node. The current best algorithms achieve approximation ratios that are linear in the size of $G$ or depend on $B$. In this paper, we provide two algorithms for CBC and DCBC that guarantee approximation ratios of $O\left(\frac{\log^2|X|}{\epsilon^2}\right)$ and $O\left(\frac{\sqrt{|V|}\log^2|X|}{\epsilon^2}\right)$, resp., with a budget violation of a factor $1+\epsilon$, where $\epsilon\in (0,1]$. Our algorithms imply improved approximation factors of other related problems. For the particular case of DCBC where the prize function is additive, we improve from $O\left(\frac{1}{\epsilon^2}|V|^{2/3}\log|V|\right)$ to $O\left(\frac{1}{\epsilon^2}|V|^{1/2}\log^2|V|\right)$. For the minimum connected set cover, a minimization version of CBC, and its directed variant, we obtain approximation factors of $O(\log^3|X|)$ and $O(\sqrt{|V|}\log^3|X|)$, resp. For the Node-Weighted Group Steiner Tree and and its directed variant, we obtain approximation factors of $O(\log^3k)$ and $O(\sqrt{|V|}\log^3k)$, resp., where $k$ is the number of groups.