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We study a generalization of the Steiner tree problem, where we are given a weighted network G together with a collection of k subsets of its vertices and a root r. We wish to construct a minimum cost network such that the network supports one unit of flow to the root from every node in a subset simultaneously. The network constructed does not need to support flows from all the subsets simultaneously. We settle an open question regarding the complexity of this problem for k=2, and give a 3/2-approximation algorithm that improves over a (trivial) known 2-approximation. Furthermore, we prove some structural results that prevent many well-known techniques from doing better than the known O(log n)-approximation. Despite these obstacles, we conjecture that this problem should have an O(1)-approximation. We also give an approximation result for a variant of the problem where the solution is required to be a path.
@InProceedings{guruganesh_et_al:LIPIcs.ESA.2017.46,
author = {Guruganesh, Guru and Iglesias, Jennifer and Ravi, R. and Sanita, Laura},
title = {{Single-Sink Fractionally Subadditive Network Design}},
booktitle = {25th Annual European Symposium on Algorithms (ESA 2017)},
pages = {46:1--46:13},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-049-1},
ISSN = {1868-8969},
year = {2017},
volume = {87},
editor = {Pruhs, Kirk and Sohler, Christian},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2017.46},
URN = {urn:nbn:de:0030-drops-78581},
doi = {10.4230/LIPIcs.ESA.2017.46},
annote = {Keywords: Network design, single-commodity flow, approximation algorithms, Steiner tree}
}