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The complexity of homomorphism and constraint satisfaction problems seen from the other side
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Volume 54 ,  Issue 1  (March 2007) table of contents
Article No. 1  
Year of Publication: 2007
ISSN:0004-5411
Author
Martin Grohe  Humboldt-Universität zu Berlin, Berlin, Germany
Publisher
ACM  New York, NY, USA
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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

Note: OCR errors may be found in this Reference List extracted from the full text article. ACM has opted to expose the complete List rather than only correct and linked references.

 
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