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We study several canonical decision problems arising from some well-known theorems from combinatorial geometry. Among others, we show that computing the minimum size of a Caratheodory set and a Helly set and certain decision versions of the hs cut problem are W[1]-hard (and NP-hard) if the dimension is part of the input. This is done by fpt-reductions (which are actually ptime-reductions) from the d-Sum problem. Our reductions also imply that the problems we consider cannot be solved in time n^{o(d)} (where n is the size of the input), unless the Exponential-Time Hypothesis (ETH) is false.
The technique of embedding d-Sum into a geometric setting is conceptually much simpler than direct fpt-reductions from purely combinatorial W[1]-hard problems (like the clique problem) and has great potential to show (parameterized) hardness and (conditional) lower bounds for many other problems.
@InProceedings{knauer_et_al:LIPIcs.STACS.2011.649,
author = {Knauer, Christian and Tiwary, Hans Raj and Werner, Daniel},
title = {{On the computational complexity of Ham-Sandwich cuts, Helly sets, and related problems}},
booktitle = {28th International Symposium on Theoretical Aspects of Computer Science (STACS 2011)},
pages = {649--660},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-939897-25-5},
ISSN = {1868-8969},
year = {2011},
volume = {9},
editor = {Schwentick, Thomas and D\"{u}rr, Christoph},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2011.649},
URN = {urn:nbn:de:0030-drops-30514},
doi = {10.4230/LIPIcs.STACS.2011.649},
annote = {Keywords: computational geometry, combinatorial geometry, ham-sandwich cuts, parameterized complexity.}
}