English

Average crosscap number of a 2-bridge knot

Geometric Topology 2025-08-07 v2

Abstract

We determine a simple condition on a particular state graph of an alternating knot or link diagram that characterizes when the unoriented genus and crosscap number coincide, extending work of Adams and Kindred. Building on this same work and using continued fraction expansions, we provide a new formula for the unoriented genus of a 2-bridge knot or link. We use recursion to obtain exact formulas for the average unoriented genus Γ(c)\overline{\Gamma}(c) and average crosscap number γ(c)\overline{\gamma}(c) of all 2-bridge knots with crossing number cc, and in particular we show that limc(c3+19Γ(c))=limc(c3+19γ(c))=0\displaystyle{\lim_{c\to\infty} \left(\frac{c}{3}+\frac{1}{9} - \overline{\Gamma}(c)\right) = \lim_{c\to\infty} \left(\frac{c}{3}+\frac{1}{9} - \overline{\gamma}(c)\right) = 0}.

Keywords

Cite

@article{arxiv.2501.03099,
  title  = {Average crosscap number of a 2-bridge knot},
  author = {Moshe Cohen and Thomas Kindred and Adam M. Lowrance and Patrick D. Shanahan and Cornelia A. Van Cott},
  journal= {arXiv preprint arXiv:2501.03099},
  year   = {2025}
}

Comments

Minor revisions of exposition

R2 v1 2026-06-28T20:57:41.420Z