Martin Dyer, Leslie Ann Goldberg, Michael S. Paterson

We give a dichotomy theorem for the problem of counting homomorphisms to

directed acyclic graphs. $H$ is a fixed directed acyclic graph.

The problem is, given an input digraph $G$, how many homomorphisms are there

from $G$ to $H$. We give a graph-theoretic classification, showing that

for some digraphs $H$, ...
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