THE COMPLEXITY OF COUNTING SURJECTIVE HOMOMORPHISMS AND COMPACTIONS

A homomorphism from a graph G to a graph H is a function from the vertices of G to the vertices of H that preserves edges. A homomorphism is surjective if it uses all of the vertices of H, and it is a compaction if it uses all of the vertices of H and all of the nonloop edges of H. Hell and Nesetril gave a complete characterization of the complexity of deciding whether there is a homomorphism from an input graph G to a fixed graph H. A complete characterization is not known for surjective homomorphisms or for compactions, though there are many interesting results. Dyer and Greenhill gave a complete characterization of the complexity of counting homomorphisms from an input graph G to a fixed graph H. In this paper, we give a complete characterization of the complexity of counting surjective homomorphisms from an input graph G to a fixed graph H, and we also give a complete characterization of the complexity of counting compactions from an input graph G to a fixed graph H. In an addendum we use our characterizations to point out a dichotomy for the complexity of the respective approximate counting problems (in the connected case).