A topological proof of the Hell-Nešetřil dichotomy

We provide a new proof of a theorem of Hell and Nešetřil [J. Comb. Theory B, 48(1):92-110, 1990] using tools from topological combinatorics based on ideas of Lovász [J. Comb. Theory, Ser. A, 25(3):319-324, 1978]. The Hell-Nešetřil Theorem provides a dichotomy of the graph homomorphism problem. It states that deciding whether there is a graph homomorphism from a given graph to a fixed graph H is in P if H is bipartite (or contains a self-loop), and is NP-complete otherwise. In our proof we combine topological combinatorics with the algebraic approach to constraint satisfaction problem.