Enumerating Homomorphisms

The homomorphism problem for relational structures is an a bstr ct way of formulating constraint satisfaction problems (CSP) and various proble ms in database theory. The decision version of the homomorphism problem received a lot of attention in li terature; in particular, the way the graph-theoretical structure of the variables and constrai nts influences the complexity of the problem is intensively studied. Here we study the problem of enumera ting all the solutions with polynomial delay from a similar point of view. It turns out that the enume ration problem behaves very differently from the decision version. We give evidence that it is unlike ly 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.

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