The Generalized Schur Decomposition and the rank-$R$ set of real $I\times J\times 2$ arrays

TL;DR

This paper extends the understanding of the rank-$R$ set of real $I\times J\times 2$ arrays by showing the equivalence of the Generalized Schur Decomposition solutions with the interior and boundary of this set, removing previous restrictions.

Contribution

It proves that the set of GSD solutions matches the interior and boundary of the rank-$R$ set without restrictions on the matrices, providing a complete classification of points.

Findings

GSD solutions correspond to interior and boundary points of the rank-$R$ set.

Complete classification of interior, boundary, and exterior points of the rank-$R$ set.

The equivalence holds even without the previous nonsingular upper triangular restriction.

Abstract

It is known that a best low-rank approximation to multi-way arrays or higher-order tensors may not exist. This is due to the fact that the set of multi-way arrays with rank at most $R$ is not closed. Nonexistence of the best low-rank approximation results in diverging rank-1 components when an attempt is made to compute the approximation. Recently, a solution to this problem has been proposed for real $I\times J\times 2$ arrays. Instead of a best rank-$R$ approximation the best fitting Generalized Schur Decomposition (GSD) is computed. Under the restriction of nonsingular upper triangular matrices in the GSD, the set of GSD solutions equals the interior and boundary of the rank-$R$ set. Here, we show that this holds even without the restriction. We provide a complete classification of interior, boundary, and exterior points of the rank-$R$ set of real $I\times J\times 2$ arrays, and show that the set of GSD solutions equals the interior and boundary of this set.