Inapproximability of counting hypergraph colourings
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Recent developments in approximate counting have made startling progress in developing fast algorithmic methods for approximating the number of solutions to constraint satisfaction problems (CSPs) with large arities, using connections to the Lovasz Local Lemma. Nevertheless, the boundaries of these methods for CSPs with non-Boolean domain are not well-understood. Our goal in this paper is to fill in this gap and obtain strong inapproximability results by studying the prototypical problem in this class of CSPs, hypergraph colourings. More precisely, we focus on the problem of approximately counting q-colourings on K-uniform hypergraphs with bounded degree Δ. An efficient algorithm exists if Δ≲q^K/3-1/4^KK^2 (Jain, Pham, and Voung, 2021; He, Sun, and Wu, 2021). Somewhat surprisingly however, a hardness bound is not known even for the easier problem of finding colourings. For the counting problem, the situation is even less clear and there is no evidence of the right constant controlling the growth of the exponent in terms of K. To this end, we first establish that for general q computational hardness for finding a colouring on simple/linear hypergraphs occurs at Δ≳ Kq^K, almost matching the algorithm from the Lovasz Local Lemma. Our second and main contribution is to obtain a far more refined bound for the counting problem that goes well beyond the hardness of finding a colouring and which we conjecture that is asymptotically tight (up to constant factors). We show in particular that for all even q≥ 4 it is NP-hard to approximate the number of colourings when Δ≳ q^K/2.
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