Tensor Rank, Invariants, Inequalities, and Applications

TL;DR

This paper studies tensors of size n×n×n with rank n over complex numbers, constructing polynomial invariants to distinguish their orbit, and explores applications in topology, probability models, and tensor rank approximation.

Contribution

It introduces explicit polynomial invariants generalizing Cayley's hyperdeterminant for tensors of rank n, providing a semialgebraic description of these tensors and their applications.

Findings

Polynomial invariants distinguish tensors of rank n from their closure.

The invariants provide a semialgebraic description of tensors with specific ranks.

Examples of tensors with rank 2n-1 in the closure of rank n tensors.

Abstract

Though algebraic geometry over $\mathbb C$ is often used to describe the closure of the tensors of a given size and complex rank, this variety includes tensors of both smaller and larger rank. Here we focus on the $n\times n\times n$ tensors of rank $n$ over $\mathbb C$, which has as a dense subset the orbit of a single tensor under a natural group action. We construct polynomial invariants under this group action whose non-vanishing distinguishes this orbit from points only in its closure. Together with an explicit subset of the defining polynomials of the variety, this gives a semialgebraic description of the tensors of rank $n$ and multilinear rank $(n,n,n)$. The polynomials we construct coincide with Cayley's hyperdeterminant in the case $n=2$, and thus generalize it. Though our construction is direct and explicit, we also recast our functions in the language of representation theory for additional insights. We give three applications in different directions: First, we develop basic topological understanding of how the real tensors of complex rank $n$ and multilinear rank $(n,n,n)$ form a collection of path-connected subsets, one of which contains tensors of real rank $n$. Second, we use the invariants to develop a semialgebraic description of the set of probability distributions that can arise from a simple stochastic model with a hidden variable, a model that is important in phylogenetics and other fields. Third, we construct simple examples of tensors of rank $2n-1$ which lie in the closure of those of rank $n$.