English

Pentagonal geometries with block sizes 3, 4 and 5

Combinatorics 2020-07-28 v3

Abstract

A pentagonal geometry PENT(kk, rr) is a partial linear space, where every line, or block, is incident with kk points, every point is incident with rr lines, and for each point xx, there is a line incident with precisely those points that are not collinear with xx. An opposite line pair in a pentagonal geometry consists of two parallel lines such that each point on one of the lines is not collinear with precisely those points on the other line. We give a direct construction for an infinite sequence of pentagonal geometries with block size 3 and connected deficiency graphs. Also we present 39 new pentagonal geometries with block size 4 and five with block size 5, all with connected deficiency graphs. Consequentially we determine the existence spectrum up to a few possible exceptions for PENT(4, rr) that do not contain opposite line pairs and for PENT(4, rr) with one opposite line pair. More generally, given jj we show that there exists a PENT(4, rr) with jj opposite line pairs for all sufficiently large admissible rr. Using some new group divisible designs with block size 5 (including types 2352^{35}, 2712^{71} and 102310^{23}) we significantly extend the known existence spectrum for PENT(5, rr).

Keywords

Cite

@article{arxiv.2006.15734,
  title  = {Pentagonal geometries with block sizes 3, 4 and 5},
  author = {Anthony D. Forbes},
  journal= {arXiv preprint arXiv:2006.15734},
  year   = {2020}
}

Comments

42 pages. Abstract changed. Lemma 4.1, Theorem 4.2 and Table 8 updated. The paper will be submitted to a journal

R2 v1 2026-06-23T16:41:07.804Z