On gonality-tight graphs

Abstract

We address a question posed by Fessler--Jensen--Kelsey--Owen regarding graphs whose second gonality is greater than the first by exactly 1. We answer the question affirmatively under a stronger condition, thereby characterising the entire gonality sequence for a large family of graphs, that we refer to as quasi-banana graphs. We prove a structure theorem for those graphs, and show that they are all obtained via an inductive process by gluing together complete and banana graphs under certain rules.