English

On the number of 4-cycles in a tournament

Combinatorics 2015-08-24 v3

Abstract

If TT is an nn-vertex tournament with a given number of 33-cycles, what can be said about the number of its 44-cycles? The most interesting range of this problem is where TT is assumed to have cn3c\cdot n^3 cyclic triples for some c>0c>0 and we seek to minimize the number of 44-cycles. We conjecture that the (asymptotic) minimizing TT is a random blow-up of a constant-sized transitive tournament. Using the method of flag algebras, we derive a lower bound that almost matches the conjectured value. We are able to answer the easier problem of maximizing the number of 44-cycles. These questions can be equivalently stated in terms of transitive subtournaments. Namely, given the number of transitive triples in TT, how many transitive quadruples can it have? As far as we know, this is the first study of inducibility in tournaments.

Keywords

Cite

@article{arxiv.1405.1576,
  title  = {On the number of 4-cycles in a tournament},
  author = {Nati Linial and Avraham Morgenstern},
  journal= {arXiv preprint arXiv:1405.1576},
  year   = {2015}
}

Comments

11 pages, 5 figures