Approximation algorithms for Steiner Tree Augmentation Problems

TL;DR

This paper presents the first polynomial-time approximation algorithm for the Steiner Tree Augmentation Problem with a ratio better than 2, achieving a ratio of (1.5 + ε), and extends techniques to related node-weighted problems.

Contribution

It introduces a novel polynomial-time algorithm with an approximation ratio of (1.5 + ε) for STAP, improving upon the previous factor of 2, and generalizes existing decomposition methods.

Findings

Achieved a (1.5 + ε)-approximation for STAP.

Extended local search techniques to hyper-links.

Provided an O(log^2 |R|)-approximation for NW-STAP.

Abstract

In the Steiner Tree Augmentation Problem (STAP), we are given a graph $G = (V,E)$, a set of terminals $R \subseteq V$, and a Steiner tree $T$ spanning $R$. The edges $L := E \setminus E(T)$ are called links and have non-negative costs. The goal is to augment $T$ by adding a minimum cost set of links, so that there are 2 edge-disjoint paths between each pair of vertices in $R$. This problem is a special case of the Survivable Network Design Problem, which can be approximated to within a factor of 2 using iterative rounding~\cite{J2001}. We give the first polynomial time algorithm for STAP with approximation ratio better than 2. In particular, we achieve an approximation ratio of $(1.5 + \varepsilon)$. To do this, we employ the Local Search approach of~\cite{TZ2022} for the Tree Augmentation Problem and generalize their main decomposition theorem from links (of size two) to hyper-links. We also consider the Node-Weighted Steiner Tree Augmentation Problem (NW-STAP) in which the non-terminal nodes have non-negative costs. We seek a cheapest subset $S \subseteq V \setminus R$ so that $G[R \cup S]$ is 2-edge-connected. Using a result of Nutov~\cite{N2010}, there exists an $O(\log |R|)$-approximation for this problem. We provide an $O(\log^2 (|R|))$-approximation algorithm for NW-STAP using a greedy algorithm leveraging the spider decomposition of optimal solutions.