The homomorphism problem for relational structures is an abstract way of formulating constraint satisfaction problems (CSP) and various problems in database theory. The decision version of the homomorphism problem received a lot of attention in literature; in particular, the way the graph-theoretical structure of the variables and constraints influences the complexity of the problem is intensively studied. Here we study the problem of enumerating all the solutions with polynomial delay from a similar point of view. It turns out that the enumeration problem behaves very differently from the decision version. We give evidence that it is unlikely that a characterization result similar to the decision version can be obtained. Nevertheless, we show nontrivial cases where enumeration can be done with polynomial delay.
@InProceedings{bulatov_et_al:LIPIcs.STACS.2009.1838, author = {Bulatov, Andrei A. and Dalmau, Victor and Grohe, Martin and Marx, Daniel}, title = {{Enumerating Homomorphisms}}, booktitle = {26th International Symposium on Theoretical Aspects of Computer Science}, pages = {231--242}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-939897-09-5}, ISSN = {1868-8969}, year = {2009}, volume = {3}, editor = {Albers, Susanne and Marion, Jean-Yves}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2009.1838}, URN = {urn:nbn:de:0030-drops-18385}, doi = {10.4230/LIPIcs.STACS.2009.1838}, annote = {Keywords: } }
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