English

EKR sets for large $n$ and $r$

Combinatorics 2012-10-30 v1

Abstract

Let \A([n]r)\A\subset\binom{[n]}{r} be a compressed, intersecting family and let X[n]X\subset[n]. Let \A(X)=A\A:AX\A(X)={A\in\A:A\cap X\ne\emptyset} and §n,r=([n]r)(1)\S_{n,r}=\binom{[n]}{r}({1}). Motivated by the Erd\H{o}s-Ko-Rado theorem, Borg asked for which X[2,n]X\subset[2,n] do we have \A(X)§n,r(X)|\A(X)|\le|\S_{n,r}(X)| for all compressed, intersecting families \A\A? We call XX that satisfy this property EKR. Borg classified EKR sets XX such that Xr|X|\ge r. Barber classified XX, with Xr|X|\le r, such that XX is EKR for sufficiently large nn, and asked how large nn must be. We prove nn is sufficiently large when nn grows quadratically in rr. In the case where \A\A has a maximal element, we are able to sharpen this bound to n>φ2rn>\varphi^{2}r implies \A(X)§n,r(X)|\A(X)|\le|\S_{n,r}(X)|. We conclude by giving a generating function that speeds up computation of \A(X)|\A(X)| in comparison with the na\"{i}ve methods.

Keywords

Cite

@article{arxiv.1210.7470,
  title  = {EKR sets for large $n$ and $r$},
  author = {Benjamin Bond},
  journal= {arXiv preprint arXiv:1210.7470},
  year   = {2012}
}
R2 v1 2026-06-21T22:28:55.975Z