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We investigate parameterizing hard combinatorial problems by the size of the solution set compared to all solution candidates. Our main result is a uniform sampling algorithm for satisfying assignments of 2-CNF formulas that runs in expected time O^*(eps^{-0.617}) where eps is the fraction of assignments that are satisfying. This improves significantly over the trivial sampling bound of expected Theta^*(eps^{-1}), and on all previous algorithms whenever eps = Omega(0.708^n). We also consider algorithms for 3-SAT with an eps fraction of satisfying assignments, and prove that it can be solved in O^*(eps^{-2.27}) deterministic time, and in O^*(eps^{-0.936}) randomized time. Finally, to further demonstrate the applicability of this framework, we also explore how similar techniques can be used for vertex cover problems.
@InProceedings{cardinal_et_al:LIPIcs.IPEC.2017.11,
author = {Cardinal, Jean and Nummenpalo, Jerri and Welzl, Emo},
title = {{Solving and Sampling with Many Solutions: Satisfiability and Other Hard Problems}},
booktitle = {12th International Symposium on Parameterized and Exact Computation (IPEC 2017)},
pages = {11:1--11:12},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-051-4},
ISSN = {1868-8969},
year = {2018},
volume = {89},
editor = {Lokshtanov, Daniel and Nishimura, Naomi},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.IPEC.2017.11},
URN = {urn:nbn:de:0030-drops-85459},
doi = {10.4230/LIPIcs.IPEC.2017.11},
annote = {Keywords: Satisfiability, Sampling, Parameterized complexity}
}