English

Avoiding secants of given size in finite projective planes

Combinatorics 2024-09-24 v1

Abstract

Let qq be a prime power and kk be a natural number. What are the possible cardinalities of point sets S{S} in a projective plane of order qq, which do not intersect any line at exactly kk points? This problem and its variants have been investigated before, in relation with blocking sets, untouchable sets or sets of even type, among others. In this paper we show a series of results which point out the existence of all or almost all possible values m[0,q2+q+1]m\in [0, q^2+q+1] for S=m|S|=m, provided that kk is not close to the extremal values 00 or q+1q+1. Moreover, using polynomial techniques we show the existence of a point set SS with the following property: for every prescribed list of numbers t1,tq2+q+1t_1, \ldots t_{q^2+q+1}, Siti|S\cap \ell_i|\neq t_i holds for the iith line i\ell_i, i{1,2,,q2+q+1}\forall i \in \{1, 2, \ldots, q^2+q+1\}.

Keywords

Cite

@article{arxiv.2409.14213,
  title  = {Avoiding secants of given size in finite projective planes},
  author = {Tamás Héger and Zoltán Lóránt Nagy},
  journal= {arXiv preprint arXiv:2409.14213},
  year   = {2024}
}