English

Counting point configurations in projective space

Algebraic Geometry 2026-02-09 v2 Combinatorics

Abstract

We investigate the enumerative geometry of point configurations in projective space. We define "projective configuration counts": these enumerate configurations of points in projective space such that certain specified subsets are in fixed relative positions. The P1\mathbb{P}^1 case recovers cross-ratio degrees, which arise naturally in numerous contexts. We establish two main results. The first is a combinatorial upper bound given by the number of weighted transversals of a bipartite graph. The second is a recursion that relates counts associated to projective spaces of different dimensions, by projecting away from a given point. Key inputs include the Gelfand-MacPherson correspondence, the Jacobi-Trudi and Thom-Porteous formulae, and the notion of surplus from matching theory of bipartite graphs.

Keywords

Cite

@article{arxiv.2601.15421,
  title  = {Counting point configurations in projective space},
  author = {Alex Fink and Navid Nabijou and Rob Silversmith},
  journal= {arXiv preprint arXiv:2601.15421},
  year   = {2026}
}

Comments

27 pages. Includes ancillary Mathematica code. Fixed a typography issue where some references were mislabelled. Comments welcome

R2 v1 2026-07-01T09:14:51.533Z