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### On OBDD-Based Algorithms and Proof Systems That Dynamically Change Order of Variables

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### Abstract

In 2004 Atserias, Kolaitis and Vardi proposed OBDD-based propositional proof systems that prove unsatisfiability of a CNF formula by deduction of identically false OBDD from OBDDs representing clauses of the initial formula. All OBDDs in such proofs have the same order of variables. We initiate the study of OBDD based proof systems that additionally contain a rule that allows to change the order in OBDDs. At first we consider a proof system OBDD(and, reordering) that uses the conjunction (join) rule and the rule that allows to change the order. We exponentially separate this proof system from OBDD(and)-proof system that uses only the conjunction rule. We prove two exponential lower bounds on the size of OBDD(and, reordering)-refutations of Tseitin formulas and the pigeonhole principle. The first lower bound was previously unknown even for OBDD(and)-proofs and the second one extends the result of Tveretina et al. from OBDD(and) to OBDD(and, reordering). In 2004 Pan and Vardi proposed an approach to the propositional satisfiability problem based on OBDDs and symbolic quantifier elimination (we denote algorithms based on this approach as OBDD(and, exists)-algorithms. We notice that there exists an OBDD(and, exists)-algorithm that solves satisfiable and unsatisfiable Tseitin formulas in polynomial time. In contrast, we show that there exist formulas representing systems of linear equations over F_2 that are hard for OBDD(and, exists, reordering)-algorithms. Our hard instances are satisfiable formulas representing systems of linear equations over F_2 that correspond to some checksum matrices of error correcting codes.

### BibTeX - Entry

```@InProceedings{itsykson_et_al:LIPIcs:2017:6991,
author =	{Dmitry Itsykson and Alexander Knop and Andrey Romashchenko and Dmitry Sokolov},
title =	{{On OBDD-Based Algorithms and Proof Systems That Dynamically Change Order of Variables}},
booktitle =	{34th Symposium on Theoretical Aspects of Computer Science (STACS 2017)},
pages =	{43:1--43:14},
series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN =	{978-3-95977-028-6},
ISSN =	{1868-8969},
year =	{2017},
volume =	{66},
editor =	{Heribert Vollmer and Brigitte Vallée},
publisher =	{Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik},
address =	{Dagstuhl, Germany},
URL =		{http://drops.dagstuhl.de/opus/volltexte/2017/6991},
URN =		{urn:nbn:de:0030-drops-69914},
doi =		{10.4230/LIPIcs.STACS.2017.43},
annote =	{Keywords: Proof complexity, OBDD, error-correcting codes, Tseitin formulas, expanders}
}
```

 Keywords: Proof complexity, OBDD, error-correcting codes, Tseitin formulas, expanders Seminar: 34th Symposium on Theoretical Aspects of Computer Science (STACS 2017) Issue date: 2017 Date of publication: 06.03.2017

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