Computing homomorphisms in hereditary graph classes: the peculiar case of the 5-wheel and graphs with no long claws
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For graphs G and H, an H-coloring of G is an edge-preserving mapping from V(G) to V(H). In the H-Coloring problem the graph H is fixed and we ask whether an instance graph G admits an H-coloring. A generalization of this problem is H-ColoringExt, where some vertices of G are already mapped to vertices of H and we ask if this partial mapping can be extended to an H-coloring. We study the complexity of variants of H-Coloring in F-free graphs, i.e., graphs excluding a fixed graph F as an induced subgraph. For integers a,b,c ≥ 1, by S_a,b,c we denote the graph obtained by identifying one endvertex of three paths on a+1, b+1, and c+1 vertices, respectively. For odd k ≥ 5, by W_k we denote the graph obtained from the k-cycle by adding a universal vertex. As our main algorithmic result we show that W_5-ColoringExt is polynomial-time solvable in S_2,1,1-free graphs. This result exhibits an interesting non-monotonicity of H-ColoringExt with respect to taking induced subgraphs of H. Indeed, W_5 contains a triangle, and K_3-Coloring, i.e., classical 3-coloring, is NP-hard already in claw-free (i.e., S_1,1,1-free) graphs. Our algorithm is based on two main observations: 1. W_5-ColoringExt in S_2,1,1-free graphs can be in polynomial time reduced to a variant of the problem of finding an independent set intersecting all triangles, and 2. the latter problem can be solved in polynomial time in S_2,1,1-free graphs. We complement this algorithmic result with several negative ones. In particular, we show that W_5-ColoringExt is NP-hard in S_3,3,3-free graphs. This is again uncommon, as usually problems that are NP-hard in S_a,b,c-free graphs for some constant a,b,c are already hard in claw-free graphs.
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