Related to this question as far as concerned the paper discussed.
In the following theorem the author is not very clear about who $(W,W-)$ and $M$ should be. Morover, the notation orthonormal pair I'm not sure what it means. This blocked my understanding of the conclusion: It's not clear to me how the theorem follows on taking norm. Which properties are used of Frobenius norm and of matrices involved in order to verify the last sentence of the page?
Any help on understanding notation and matrices involved would be appreciated, I'm adding an image to clarify the background.
