English

Two questions of Erd\H{o}s on hypergraphs above the Tur{\'a}n threshold}

Combinatorics 2011-11-28 v1 Discrete Mathematics

Abstract

For ordinary graphs it is known that any graph GG with more edges than the Tur{\'a}n number of KsK_s must contain several copies of KsK_s, and a copy of Ks+1K_{s+1}^-, the complete graph on s+1s+1 vertices with one missing edge. Erd\H{o}s asked if the same result is true for Ks3K^3_s, the complete 3-uniform hypergraph on ss vertices. In this note we show that for small values of nn, the number of vertices in GG, the answer is negative for s=4s=4. For the second property, that of containing a Ks+13{K^3_{s+1}}^-, we show that for s=4s=4 the answer is negative for all large nn as well, by proving that the Tur{\'a}n density of K53{K^3_5}^- is greater than that of K43K^3_4.

Keywords

Cite

@article{arxiv.1111.5743,
  title  = {Two questions of Erd\H{o}s on hypergraphs above the Tur{\'a}n threshold}},
  author = {Klas Markström},
  journal= {arXiv preprint arXiv:1111.5743},
  year   = {2011}
}