Degenerations of the generic square matrix. Polar map and determinantal structure

TL;DR

This paper investigates degenerations of generic square matrices, analyzing their determinants, polar maps, and minors using commutative algebra, revealing structural properties and answering a question about dual varieties.

Contribution

It fully characterizes the structure of polar maps and submaximal minors for degenerated matrices, providing new insights into their algebraic and geometric properties.

Findings

The structure of the polar map is completely identified.

Degenerated determinants can have non-vanishing Hessians with predictable factors.

Confirmed a conjecture of F. Russo regarding the codimension in the polar image.

Abstract

One studies certain degenerations of the generic square matrix over a field $k$ along with its main related structures, such as the determinant of the matrix, the ideal generated by its partial derivatives, the polar map defined by these derivatives, the Hessian matrix and the ideal of the submaximal minors of the matrix. The main tool comes from commutative algebra, with emphasis on ideal theory and syzygy theory. The structure of the polar map is completely identified and the main properties of the ideal of submaximal minors are determined. Cases where the degenerated determinant has non-vanishing Hessian determinant show that the former is a factor of the latter with the (Segre) expected multiplicity, a result treated by Landsberg-Manivel-Ressayre by geometric means. Another byproduct is an affirmative answer to a question of F. Russo concerning the codimension in the polar image of the dual variety to a hypersurface.