Tubal Matrices

TL;DR

This paper generalizes the concept of s-diagonal tensors in T-SVD to any real invertible linear transformation, establishing Eckart-Young like theorems for third-order tensors using tubal matrices.

Contribution

It extends the theory of s-diagonal tensors to broader transformations and proves Eckart-Young theorems for tensors under doubly real-preserving unitary transformations.

Findings

Eckart-Young like theorems hold for third-order tensors under certain transformations.

Doubly real-preserving unitary transformations include DFT and orthogonal matrices.

Tubal matrices provide a natural framework for this tensor analysis.

Abstract

It was shown recently that the f-diagonal tensor in the T-SVD factorization must satisfy some special properties. Such f-diagonal tensors are called s-diagonal tensors. In this paper, we show that such a discussion can be extended to any real invertible linear transformation. We show that two Eckart-Young like theorems hold for a third order real tensor, under any doubly real-preserving unitary transformation. The normalized Discrete Fourier Transformation (DFT) matrix, an arbitrary orthogonal matrix, the product of the normalized DFT matrix and an arbitrary orthogonal matrix are examples of doubly real-preserving unitary transformations. We use tubal matrices as a tool for our study. We feel that the tubal matrix language makes this approach more natural.