Sparse Graphs of High Gonality

Abstract

By considering graphs as discrete analogues of Riemann surfaces, Baker and Norine [Adv. Math., 215 (2007), pp. 766--788] developed a concept of linear systems of divisors for graphs. Building on this idea, a concept of gonality for graphs has been defined and has generated much recent interest. We show that there are connected graphs of treewidth 2 of arbitrarily high gonality. We also show that there exist pairs of connected graphs $\{G,H\}$ such that $H\subseteq G$ and $H$ has strictly lower gonality than $G$. These results resolve three open problems posed in a recent survey by Norine [in Surveys in Combinatorics, London Math. Soc. Lecture Note Ser. 424, Cambridge University Press, Cambridge, 2015, pp. 221--260].