English

Examples of diameter-2 graphs with no triangle or $K_{2,t}$

Combinatorics 2026-02-17 v1

Abstract

For each t1t \ge 1 let WtW_t denote the class of graphs other than stars that have diameter 22 and contain neither a triangle nor a K2,tK_{2,t}. The famous Hoffman--Singleton Theorem implies that W2W_2 is finite. Recently Wood suggested the study of WtW_t for t>2t > 2 and conjectured that WtW_t is finite for all t2t \ge 2. In this note we show that (1) W3W_3 is infinite, (2) W5W_5 contains infinitely many regular graphs, and (3) W7W_7 contains infinitely many Cayley graphs. Our W3W_3 and W5W_5 examples are based on so-called crooked graphs, first constructed by de Caen, Mathon, and Moorhouse. Our W7W_7 examples are Cayley graphs with vertex set Fp2\mathbb{F}_p^2 for prime p11(mod12)p \equiv 11 \pmod {12}.

Keywords

Cite

@article{arxiv.2508.19646,
  title  = {Examples of diameter-2 graphs with no triangle or $K_{2,t}$},
  author = {Sean Eberhard and Vladislav Taranchuk and Craig Timmons},
  journal= {arXiv preprint arXiv:2508.19646},
  year   = {2026}
}

Comments

8 pages

R2 v1 2026-07-01T05:08:00.019Z