English

Chromatic roots at 2 and at the Beraha number $B_{10}$

Combinatorics 2018-07-24 v1

Abstract

By the construction of suitable graphs and the determination of their chromatic polynomials, we resolve two open questions concerning real chromatic roots. First we exhibit graphs for which the Beraha number B10=(5+5)/2B_{10} = (5 + \sqrt{5})/2 is a chromatic root. As it was previously known that no other non-integer Beraha number is a chromatic root, this completes the determination of precisely which Beraha numbers can be chromatic roots. Next we construct an infinite family of 33-connected graphs such that for any k1k \geqslant 1, there is a member of the family with q=2q=2 as a chromatic root of multiplicity at least kk. The former resolves a question of Salas and Sokal [J. Statist. Pys. 104 (2001) pp. 609--699] and the latter a question of Dong and Koh [J. Graph Theory 70 (2012) pp. 262--283].

Keywords

Cite

@article{arxiv.1807.08397,
  title  = {Chromatic roots at 2 and at the Beraha number $B_{10}$},
  author = {Daniel J. Harvey and Gordon F. Royle},
  journal= {arXiv preprint arXiv:1807.08397},
  year   = {2018}
}

Comments

This first section of this paper was previously uploaded as arxiv:1611.01941