Choosability with Separation of Cycles and Outerplanar Graphs
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We consider the following list coloring with separation problem of graphs: Given a graph G and integers a,b, find the largest integer c such that for any list assignment L of G with |L(v)|≤ a for any vertex v and |L(u)∩ L(v)|≤ c for any edge uv of G, there exists an assignment φ of sets of integers to the vertices of G such that φ(u)⊂ L(u) and |φ(v)|=b for any vertex v and φ(u)∩φ(v)=∅ for any edge uv. Such a value of c is called the separation number of (G,a,b). We also study the variant called the free-separation number which is defined analogously but assuming that one arbitrary vertex is precolored. We determine the separation number and free-separation number of the cycle and derive from them the free-separation number of a cactus. We also present a lower bound for the separation and free-separation numbers of outerplanar graphs of girth g≥ 5.
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