English

On the number of ordinary lines determined by sets in complex space

Combinatorics 2021-11-11 v2 Discrete Mathematics

Abstract

Kelly's theorem states that a set of nn points affinely spanning C3\mathbb{C}^3 must determine at least one ordinary complex line (a line passing through exactly two of the points). Our main theorem shows that such sets determine at least 3n/23n/2 ordinary lines, unless the configuration has n1n-1 points in a plane and one point outside the plane (in which case there are at least n1n-1 ordinary lines). In addition, when at most 2n/32n/3 points are contained in any plane, we prove a theorem giving stronger bounds that take advantage of the existence of lines with 4 and more points (in the spirit of Melchior's and Hirzebruch's inequalities). Furthermore, when the points span 4 or more dimensions, with at most 2n/32n/3 points contained in any three dimensional affine subspace, we show that there must be a quadratic number of ordinary lines.

Keywords

Cite

@article{arxiv.1611.08740,
  title  = {On the number of ordinary lines determined by sets in complex space},
  author = {Abdul Basit and Zeev Dvir and Shubhangi Saraf and Charles Wolf},
  journal= {arXiv preprint arXiv:1611.08740},
  year   = {2021}
}

Comments

Appeared in Discrete Comput. Geom. This version corrects some errors from the previous version, and clarifies the analysis