Upper chromatic number of finite projective planes

Gábor Bacsó, Zsolt Tuza

Research output: Contribution to journalArticle

4 Citations (Scopus)


For a finite projective plane ∏, let χ̄(∏) denote the maximum number of classes in a partition of the point set, such that each line has at least two points in the same partition class. We prove that the best possible general estimate in terms of the order of projective planes is q 2 - q - ⊖( √ q), which is tight apart from a multiplicative constant in the third term √ q: (1) As q→∞, χ̄( ∏) ≤ q2 - q - √ q/2 + o( √ q) holds for every projective plane ∏ of order q. (2) If q is a square, then the Galois plane of order q satisfies χ̄(PG(2, q)) ≥ q2 - q - 2 √ q. Our results asymptotically solve a ten-year-old open problem in the coloring theory of mixed hypergraphs, where χ̄( ∏) is termed the upper chromatic number of ∏. Further improvements on the upper bound (1) are presented for Galois planes and their subclasses.

Original languageEnglish
Pages (from-to)221-230
Number of pages10
JournalJournal of Combinatorial Designs
Issue number3
Publication statusPublished - May 1 2008



  • 05B25
  • 05C15
  • Finite projective plane
  • Hypergraph
  • MSC 2000
  • Upper chromatic number
  • Vertex coloring

ASJC Scopus subject areas

  • Discrete Mathematics and Combinatorics

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