# Cycle length parities and the chromatic number

In 1966 Erdös and Hajnal proved that the chromatic number of graphs whose odd cycles have lengths at most l is at most l + 1. Similarly, in 1992 Gyárfás proved that the chromatic number of graphs which have at most k odd cycle lengths is at most 2k + 2 which was originally conjectured by Bollobás and Erdös. Here we consider the in influence of the parities of the cycle lengths modulo some odd prime on the chromatic number of graphs. As our main result we prove the following: Let p be an odd prime, k \in \mathbb{N} and I \subseteq {0,1,...,p-1} with \mid I \mid \leq p - 1. If G is a graph such that the set of cycle lengths of G contains at most k elements which are not in I modulo p, then X(G) \leq (1+ \frac{\mid I \mid}{p - \mid I \mid}) k+p (p-1)(r(2p,2p)+1)+1 where r(p,q) denotes the ordinary Ramsey number.

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Löwenstein, Christian / Rautenbach, Dieter / Schiermeyer, Ingo: Cycle length parities and the chromatic number. 2008.

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