Computing graph gonality is hard
Dion Gijswijt (TU Delft - Discrete Mathematics and Optimization)
Harry J. Smit (Universiteit Utrecht)
Marieke van der Wegen (Universiteit Utrecht)
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Abstract
There are several notions of gonality for graphs. The divisorial gonality dgon(G) of a graph G is the smallest degree of a divisor of positive rank in the sense of Baker-Norine. The stable gonality sgon(G) of a graph G is the minimum degree of a finite harmonic morphism from a refinement of G to a tree, as defined by Cornelissen, Kato and Kool. We show that computing dgon(G) and sgon(G) are NP-hard by a reduction from the maximum independent set problem and the vertex cover problem, respectively. Both constructions show that computing gonality is moreover APX-hard.
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