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Recognizing frozen variables in constraint satisfaction problems.

Jonsson, P. and Krokhin, A. (2004) 'Recognizing frozen variables in constraint satisfaction problems.', Theoretical computer science., 329 (1-3). pp. 93-113.


In constraint satisfaction problems over finite domains, some variables can be frozen, that is, they take the same value in all possible solutions. We study the complexity of the problem of recognizing frozen variables with restricted sets of constraint relations allowed in the instances. We show that the complexity of such problems is determined by certain algebraic properties of these relations. Under the assumption that NP≠coNP (and consequently PTIME≠NP), we characterize all tractable problems, and describe large classes of NP-complete, coNP-complete, and DP-complete problems. As an application of these results, we completely classify the complexity of the problem in two cases: (1) with domain size 2; and (2) when all unary relations are present. We also give a rough classification for domain size 3.

Item Type:Article
Keywords:Constraint satisfaction, Frozen variable, Computational complexity, Polymorphism.
Full text:(AM) Accepted Manuscript
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Date accepted:No date available
Date deposited:07 April 2010
Date of first online publication:December 2004
Date first made open access:No date available

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