Generalized and geometric Ramsey numbers for cycles

  • Authors:
  • Gyula Károlyia;Vera Rosta

  • Affiliations:
  • Eötvös Univ. of Budapest, Budapest, Hungary;Webster Univ., Geneva, Switzerland

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2001

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Abstract

Let Cn denote the cycle of length n. The generalized Ramsey number of the pair (Cn , Ck) denoted by; R(Cn, Ck)is the smallest positive integer Rsuch that any complete graph with R vertices whose edges are coloured with two di/erent colours contains either a monochromatic cycle of length n in the 1rst colour or a monochromatic cycle of length k in the second colour. Generalized Ramsey numbers for cycles were completely determined by Faudree-Schelp and Rosta, based on earlier works of Bondy, Erd&ohuml;s and Gallai. Unfortunately, both proofs are quite involved and di9cult to follow. In the present paper we treat this problem in a uni1ed, self-contained and simpli1ed way. We also extend this study to a related geometric problem, where we colour the straight-line segments determined by a 1nite number of points in the plane. In this case, the monochromatic subgraphs are required to satisfy an additional (non-crossing) geometric condition.