Apolarity, border rank and multigraded Hilbert scheme

TL;DR

This paper presents a new elementary method based on apolarity to analyze the border rank of polynomials and tensors, including those with high symmetry, using multigraded Hilbert schemes on toric varieties.

Contribution

It introduces a systematic approach to study border rank uniformly, extending to arbitrary smooth toric varieties and applying to tensors like matrix multiplication.

Findings

Provides lower bounds for border rank of key tensors

Defines a border rank variant of the variety of sums of powers

Utilizes multigraded Hilbert schemes for analysis

Abstract

We introduce an elementary method to study the border rank of polynomials and tensors, analogous to the apolarity lemma. This can be used to describe the border rank of all cases uniformly, including those very special ones that resisted a systematic approach. We also define a border rank version of the variety of sums of powers and analyse its usefulness in studying tensors and polynomials with large symmetries. In particular, it can be applied to provide lower bounds for the border rank of some very interesting tensors, such as the matrix multiplication tensor. We work in a general setting, where the base variety is not necessarily a Segre or Veronese variety, but an arbitrary smooth toric projective variety. A critical ingredient of our work is an irreducible component of a multigraded Hilbert scheme related to the toric variety in question.