Triangle-Freeness Is Hard To Detect

We show that recognizing the K3-freeness and K4-freeness of graphs is hard, respectively, for two-player nondeterministic communication protocols using exponentially many partitions and for nondeterministic syntactic read-r times branching programs.The key ingredient is a generalization of a colouring lemma, due to Papadimitriou and Sipser, which says that for every balanced redblue colouring of the edges of the complete n-vertex graph there is a set of en2 triangles, none of which is monochromatic, such that no triangle can be formed by picking edges from different triangles. We extend this lemma to exponentially many colourings and to partial colourings.