English

The overflow in the Katona Theorem

Combinatorics 2025-06-09 v1

Abstract

Let n>2r>0n>2r>0 be integers. We consider families F\mathcal{F} of subsets of an nn-element set, in which the union of any two members has size at most 2r2r. One of our results states that for n6rn\geq 6r the number of members of size exceeding rr in F\mathcal{F} is at most (n2r1)\binom{n-2}{r-1}. Another result shows that for n>3.5rn>3.5r the number of sets of size at least rr is at most (nr)\binom{n}{r}. Both bounds are best possible and the latter sharpens the classical Katona Theorem. Similar results are proved for the odd case of the Katona Theorem as well.

Keywords

Cite

@article{arxiv.2506.05704,
  title  = {The overflow in the Katona Theorem},
  author = {Peter Frankl and Jian Wang},
  journal= {arXiv preprint arXiv:2506.05704},
  year   = {2025}
}
R2 v1 2026-07-01T03:02:54.226Z