Approximating Source Location and Star Survivable Network Problems

TL;DR

This paper improves approximation ratios for source location and survivable network problems, especially in node-capacity variants, by developing new algorithms with better bounds than previous methods.

Contribution

It introduces improved approximation algorithms for source location and survivable network problems, extending results to node-capacity variants and star-structured edge cases.

Findings

Achieved $O( ext{min}\{ ext{ln} n, ext{ln}^2 k ight)$ ratio for star-structured survivable network.

Improved source location ratio from $O(k ext{ln} k)$ to $O( ext{ln}^2 k)$ for the case $p^*=1$.

Provided new approximation bounds for node-capacity variants and star-structured edge cases.

Abstract

In Source Location (SL) problems the goal is to select a mini-mum cost source set $S \subseteq V$ such that the connectivity (or flow) $\psi(S,v)$ from $S$ to any node $v$ is at least the demand $d_v$ of $v$. In many SL problems $\psi(S,v)=d_v$ if $v \in S$, namely, the demand of nodes selected to $S$ is completely satisfied. In a node-connectivity variant suggested recently by Fukunaga, every node $v$ gets a "bonus" $p_v \leq d_v$ if it is selected to $S$. Fukunaga showed that for undirected graphs one can achieve ratio $O(k \ln k)$ for his variant, where $k=\max_{v \in V}d_v$ is the maximum demand. We improve this by achieving ratio $\min\{p^*\lnk,k\}\cdot O(\ln (k/q^*))$ for a more general version with node capacities, where $p^*=\max_{v \in V} p_v$ is the maximum bonus and $q^*=\min_{v \in V} q_v$ is the minimum capacity. In particular, for the most natural case $p^*=1$ considered by Fukunaga, we improve the ratio from $O(k \ln k)$ to $O(\ln^2k)$. We also get ratio $O(k)$ for the edge-connectivity version, for which no ratio that depends on $k$ only was known before. To derive these results, we consider a particular case of the Survivable Network (SN) problem when all edges of positive cost form a star. We give ratio $O(\min\{\ln n,\ln^2 k\})$ for this variant, improving over the best ratio known for the general case $O(k^3 \ln n)$ of Chuzhoy and Khanna.