We study single-sink network design problems in undirected graphs with vertex connectivity requirements. The input to these problems is an edge-weighted undirected graph $G=(V,E)$, a sink/root vertex $r$, a set of terminals $T \subseteq V$, and integer $k$. The goal is to connect each terminal $t \in T$ to $r$ via $k$ \emph{vertex-disjoint} paths. In the {\em connectivity} problem, the objective is to find a min-cost subgraph of $G$ that contains the desired paths. There is a $2$-approximation for this problem when $k \le 2$ \cite{FleischerJW} but for $k \ge 3$, the first non-trivial approximation was obtained in the recent work of Chakraborty, Chuzhoy and Khanna \cite{ChakCK08}; they describe and analyze an algorithm with an approximation ratio of $O(k^{O(k^2)}\log^4 n)$ where $n=|V|$. In this paper, inspired by the results and ideas in \cite{ChakCK08}, we show an $O(k^{O(k)}\log |T|)$-approximation bound for a simple greedy algorithm. Our analysis is based on the dual of a natural linear program and is of independent technical interest. We use the insights from this analysis to obtain an $O(k^{O(k)}\log |T|)$-approximation for the more general single-sink {\em rent-or-buy} network design problem with vertex connectivity requirements. We further extend the ideas to obtain a poly-logarithmic approximation for the single-sink {\em buy-at-bulk} problem when $k=2$ and the number of cable-types is a fixed constant; we believe that this should extend to any fixed $k$. We also show that for the non-uniform buy-at-bulk problem, for each fixed $k$, a small variant of a simple algorithm suggested by Charikar and Kargiazova \cite{CharikarK05} for the case of $k=1$ gives an $2^{O(\sqrt{\log |T|})}$ approximation for larger $k$. These results show that for each of these problems, simple and natural algorithms that have been developed for $k=1$ have good performance for small $k > 1$.
@InProceedings{chekuri_et_al:LIPIcs.FSTTCS.2008.1747, author = {Chekuri, Chandra and Korula, Nitish}, title = {{Single-Sink Network Design with Vertex Connectivity Requirements}}, booktitle = {IARCS Annual Conference on Foundations of Software Technology and Theoretical Computer Science}, pages = {131--142}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-08-8}, ISSN = {1868-8969}, year = {2008}, volume = {2}, editor = {Hariharan, Ramesh and Mukund, Madhavan and Vinay, V}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2008.1747}, URN = {urn:nbn:de:0030-drops-17475}, doi = {10.4230/LIPIcs.FSTTCS.2008.1747}, annote = {Keywords: Network Design, Vertex Connectivity, Buy-at-Bulk, Rent-or-Buy, Approximation} }
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