English

Cliques in graphs constructed from Strongly Orthogonal Subsets in exceptional root systems

Combinatorics 2026-04-06 v1

Abstract

Given a root system RR, two roots are said to be \emph{strongly orthogonal} if neither their sum nor difference is a root. Gashi defined a family of graphs with vertices labelled by sums of kk-element strongly orthogonal subsets of roots, and edges connect vertices whose difference is also a vertex. Gashi and the current authors established Erd\H{o}s--Ko--Rado type results for graphs developed from Type AA root systems. In this paper, we study graphs developed from the exceptional root systems G2G_2, F4F_4, E6E_6, E7E_7, and E8E_8. We compute graph-theoretic invariants including regularity, connectivity, and clique numbers, and analyze clique structures with respect to sunflower properties. The automorphism group contains the Weyl group; we use these symmetries to obtain complete counts of maximum cliques and maximum sunflowers. Unlike type AA, where all maximal cliques are sunflowers for large rank, sunflower cliques comprise at most 11\% of maximum cliques in the simply-laced exceptional types E6E_6, E7E_7, and E8E_8.

Keywords

Cite

@article{arxiv.2604.02983,
  title  = {Cliques in graphs constructed from Strongly Orthogonal Subsets in exceptional root systems},
  author = {Patrick J. Browne and Pádraig Ó Catháin},
  journal= {arXiv preprint arXiv:2604.02983},
  year   = {2026}
}

Comments

12 pages

R2 v1 2026-07-01T11:52:45.843Z