Triangle-Freeness is Hard to Detect

  • Authors:
  • S. Jukna;G. Schnitger

  • Affiliations:
  • Universität Frankfurt, Institut für Informatik, Robert-Mayer-Strasse 11-15, D-60054 Frankfurt, Germany (e-mail: jukna@thi.informatik.uni-frankfurt.de) and Institute of Mathematics and In ...;Universität Frankfurt, Institut für Informatik, Robert-Mayer-Strasse 11-15, D-60054 Frankfurt, Germany (e-mail: jukna@thi.informatik.uni-frankfurt.de)

  • Venue:
  • Combinatorics, Probability and Computing
  • Year:
  • 2002

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Abstract

We show that recognizing the K3-freeness andK4-freeness of graphs is hard, respectively, fortwo-player nondeterministic communication protocols usingexponentially many partitions and for nondeterministic syntacticread-r times branching programs.The key ingredient is ageneralization of a colouring lemma, due to Papadimitriou andSipser, which says that for every balanced redblue colouring of theedges of the complete n-vertex graph there is a set ofεn2 triangles, none of which ismonochromatic, such that no triangle can be formed by picking edgesfrom different triangles. We extend this lemma to exponentiallymany colourings and to partial colourings.