H-colouring Pt-free graphs in subexponential time
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A graph is called Pt-free if it does not contain the path on t vertices as an induced subgraph. Let H be a multigraph with the property that any two distinct vertices share at most one common neighbour. We show that the generating function for (list) graph homomorphisms from G to H can be calculated in subexponential time 2O (√tn log(n)) for n = |V (G)| in the class of Pt-free graphs G. As a corollary, we show that the number of 3-colourings of a Pt-free graph G can be found in subexponential time. On the other hand, no subexponential time algorithm exists for 4-colourability of Pt-free graphs assuming the Exponential Time Hypothesis. Along the way, we prove that Pt-free graphs have pathwidth that is linear in their maximum degree.
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Carla Groenland, Karolina Okrasa, Paweł Rzążewski, Alex Scott, Paul Seymour, Sophie Spirkl (2019). H-colouring Pt-free graphs in subexponential time. UWSpace. http://hdl.handle.net/10012/18530
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