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Erschienen in:
10.07.2015
Upper bounds on the chromatic number of triangle-free graphs with a forbidden subtree
Erschienen in: Journal of Combinatorial Optimization | Ausgabe 1/2017
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Abstract
Gyárfás conjectured that for a given forest F, there exists an integer function f(F, x) such that \(\chi (G)\le f(F,\omega (G))\) for each F-free graph G, where \(\omega (G)\) is the clique number of G. The broom B(m, n) is the tree of order \(m+n\) obtained from identifying a vertex of degree 1 of the path \(P_m\) with the center of the star \(K_{1,n}\). In this note, we prove that every connected, triangle-free and B(m, n)-free graph is \((m+n-2)\)-colorable as an extension of a result of Randerath and Schiermeyer and a result of Gyárfás, Szemeredi and Tuza. In addition, it is also shown that every connected, triangle-free, \(C_4\)-free and T-free graph is \((p-2)\)-colorable, where T is a tree of order \(p\ge 4\) and \(T\not \cong K_{1,3}\).