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DOI: 10.4230/LIPIcs.MFCS.2017.62
URN: urn:nbn:de:0030-drops-81344
URL: http://drops.dagstuhl.de/opus/volltexte/2017/8134/
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Yamakami, Tomoyuki

The 2CNF Boolean Formula Satisfiability Problem and the Linear Space Hypothesis

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Abstract

We aim at investigating the solvability/insolvability of nondeterministic logarithmic-space (NL) decision, search, and optimization problems parameterized by size parameters using simultaneously polynomial time and sub-linear space on multi-tape deterministic Turing machines. We are particularly focused on a special NL-complete problem, 2SAT - the 2CNF Boolean formula satisfiability problem-parameterized by the number of Boolean variables. It is shown that 2SAT with n variables and m clauses can be solved simultaneously polynomial time and (n/2^{c sqrt{log(n)}}) polylog(m+n) space for an absolute constant c>0. This fact inspires us to propose a new, practical working hypothesis, called the linear space hypothesis (LSH), which states that 2SAT_3-a restricted variant of 2SAT in which each variable of a given 2CNF formula appears as literals in at most 3 clauses-cannot be solved simultaneously in polynomial time using strictly "sub-linear" (i.e., n^{epsilon} polylog(n) for a certain constant epsilon in (0,1)) space. An immediate consequence of this working hypothesis is L neq NL. Moreover, we use our hypothesis as a plausible basis to lead to the insolvability of various NL search problems as well as the nonapproximability of NL optimization problems. For our investigation, since standard logarithmic-space reductions may no longer preserve polynomial-time sub-linear-space complexity, we need to introduce a new, practical notion of "short reduction." It turns out that overline{2SAT}_3 is complete for a restricted version of NL, called Syntactic NL or simply SNL, under such short reductions. This fact supports the legitimacy of our working hypothesis.

BibTeX - Entry

@InProceedings{yamakami:LIPIcs:2017:8134,
  author =	{Tomoyuki Yamakami},
  title =	{{The 2CNF Boolean Formula Satisfiability Problem and the Linear Space Hypothesis}},
  booktitle =	{42nd International Symposium on Mathematical Foundations of Computer Science (MFCS 2017)},
  pages =	{62:1--62:14},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-046-0},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{83},
  editor =	{Kim G. Larsen and Hans L. Bodlaender and Jean-Francois Raskin},
  publisher =	{Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{http://drops.dagstuhl.de/opus/volltexte/2017/8134},
  URN =		{urn:nbn:de:0030-drops-81344},
  doi =		{10.4230/LIPIcs.MFCS.2017.62},
  annote =	{Keywords: sub-linear space, linear space hypothesis, short reduction, Boolean formula satisfiability problem, NL search, NL optimization, Syntactic NL}
}

Keywords: sub-linear space, linear space hypothesis, short reduction, Boolean formula satisfiability problem, NL search, NL optimization, Syntactic NL
Seminar: 42nd International Symposium on Mathematical Foundations of Computer Science (MFCS 2017)
Issue Date: 2017
Date of publication: 22.11.2017


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