Tri-linear birational maps in dimension three

Abstract

A tri-linear rational map in dimension three is a rational map $\phi: (\mathbb{P}_\mathbb{C}^1)^3 \dashrightarrow \mathbb{P}_\mathbb{C}^3$ defined by four tri-linear polynomials without a common factor. If $\phi$ admits an inverse rational map $\phi^{-1}$, it is a tri-linear birational map. In this paper, we address computational and geometric aspects about these transformations. We give a characterization of birationality based on the first syzygies of the entries. More generally, we describe all the possible minimal graded free resolutions of the ideal generated by these entries. With respect to geometry, we show that the set $\mathfrak{Bir}_{(1,1,1)}$ of tri-linear birational maps, up to composition with an automorphism of $\mathbb{P}_\mathbb{C}^3$, is a locally closed algebraic subset of the Grassmannian of $4$-dimensional subspaces in the vector space of tri-linear polynomials, and has eight irreducible components. Additionally, the group action on $\mathfrak{Bir}_{(1,1,1)}$ given by composition with automorphisms of $(\mathbb{P}_\mathbb{C}^1)^3$ defines 19 orbits, and each of these orbits determines an isomorphism class of the base loci of these transformations.