Cross-ratio degrees and perfect matchings

Abstract

Cross-ratio degrees count configurations of points $z_1,\dots ,z_n\in {\mathbb{P}}^1$ satisfying $n-3$ cross-ratio constraints, up to isomorphism. These numbers arise in multiple contexts in algebraic and tropical geometry, and may be viewed as combinatorial invariants of certain hypergraphs. We prove an upper bound on cross-ratio degrees in terms of the theory of perfect matchings on bipartite graphs. We also discuss several of the many perspectives on cross-ratio degrees -- including a connection to Gromov-Witten theory -- and give many example computations.

11 pages, significantly restructured, introduction has been expanded, more example computations added