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ABSTRACT
We give a complexity theoretic classification of homomorphism problems for graphs and, more generally, relational structures obtained by restricting the left hand side structure in a homomorphism. For every class C of structures, let HOM(C,−) be the problem of deciding whether a given structure A ∈C has a homomorphism to a given (arbitrary) structure ß. We prove that, under some complexity theoretic assumption from parameterized complexity theory, HOM(C,−) is in polynomial time if and only if C has bounded tree width modulo homomorphic equivalence. Translated into the language of constraint satisfaction problems, our result yields a characterization of the tractable structural restrictions of constraint satisfaction problems. Translated into the language of database theory, it implies a characterization of the tractable instances of the evaluation problem for conjunctive queries over relational databases.
REFERENCES
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Peter Golbus , Robert W. McGrail , Tomasz Przytycki , Mary Sharac , Aleksandar Chakarov, Tricolorable torus knots are NP-complete, Proceedings of the 47th Annual Southeast Regional Conference, March 19-21, 2009, Clemson, South Carolina
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