Equations for point configurations to lie on a rational normal curve

TL;DR

This paper investigates the defining equations of point configurations on rational normal curves, revealing determinantal structures for conics, using Gale transforms for twisted cubics, and proposing conjectures for higher degrees.

Contribution

It provides explicit equations for point configurations on rational normal curves for conics and twisted cubics, and offers conjectures and partial results for higher degrees.

Findings

Determinantal equations for conics reveal geometric properties.

Gale transform helps describe configurations on twisted cubics.

Partial results and conjectures for equations in higher degrees.

Abstract

The parameter space of $n$ ordered points in projective $d$-space that lie on a rational normal curve admits a natural compactification by taking the Zariski closure in $(\mathbb{P}^d)^n$. The resulting variety was used to study the birational geometry of the moduli space $\overline{\mathrm{M}}_{0,n}$ of $n$-tuples of points in $\mathbb{P}^1$. In this paper we turn to a more classical question, first asked independently by both Speyer and Sturmfels: what are the defining equations? For conics, namely $d=2$, we find scheme-theoretic equations revealing a determinantal structure and use this to prove some geometric properties; moreover, determining which subsets of these equations suffice set-theoretically is equivalent to a well-studied combinatorial problem. For twisted cubics, $d=3$, we use the Gale transform to produce equations defining the union of two irreducible components, the compactified configuration space we want and the locus of degenerate point configurations, and we explain the challenges involved in eliminating this extra component. For $d \ge 4$ we conjecture a similar situation and prove partial results in this direction.