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  Discrete Fourier Transformation matrix and any orthogonal matrices are doubly real-preserving unitary transformations . In particular, Discrete Cosine Transformation is in this category . We use tubal matrices as a tool for our study. We feel that the tubal matrix language makes this approach more natural.
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.
A
normalized
R
matrix and any orthogonal matrices are
(DFT) matrix, an arbitrary orthogonal matrix, the product of the normalized DFT matrix and an arbitrary orthogonal matrix are examples of
D
. In particular, Discrete Cosine Transformation is in this category