English

Circles and crossing planar compact convex sets

Metric Geometry 2018-02-20 v1 Combinatorics

Abstract

Let K0K_0 be a compact convex subset of the plane R2\mathbb R^2, and assume that whenever K1R2K_1\subseteq \mathbb R^2 is congruent to K0K_0, then K0K_0 and K1K_1 are not crossing in a natural sense due to L. Fejes-T\'oth. A theorem of L. Fejes-T\'oth from 1967 states that the assumption above holds for K0K_0 if and only if K0K_0 is a disk. In a paper appeared in 2017, the present author introduced a new concept of crossing, and proved that L. Fejes-T\'oth's theorem remains true if the old concept is replaced by the new one. Our purpose is to describe the hierarchy among several variants of the new concepts and the old concept of crossing. In particular, we prove that each variant of the new concept of crossing is more restrictive then the old one. Therefore, L. Fejes-T\'oth's theorem from 1967 becomes an immediate consequence of the 2017 characterization of circles but not conversely. Finally, a mini-survey shows that this purely geometric paper has precursor in combinatorics and, mainly, in lattice theory.

Cite

@article{arxiv.1802.06457,
  title  = {Circles and crossing planar compact convex sets},
  author = {Gábor Czédli},
  journal= {arXiv preprint arXiv:1802.06457},
  year   = {2018}
}

Comments

14 pages, 9 figures

R2 v1 2026-06-23T00:25:54.744Z