# Two Strange Facts

August 25, 2016 2 Comments

Here are two strange facts about matrices, which I can prove but not in a satisfying way.

- If and are symmetric matrices satisfying , then , and , but it is NOT necessarily the case that . Is there a nice way to see why the first two properties should hold but not necessarily the third? In general, do we have if ?
- Given a rectangular matrix , and a set , let be the submatrix of with rows in , and let denote the nuclear norm (sum of singular values) of . Then the function is submodular, meaning that for all sets . In fact, this is true if we take , defined as the sum of the th powers of the singular values of , for any . The only proof I know involves trigonometric integrals and seems completely unmotivated to me. Is there any clean way of seeing why this should be true?

If anyone has insight into either of these, I’d be very interested!

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Hi Jacob,

I was just reading this post by Terrance Tao https://terrytao.wordpress.com/2017/05/22/quantitative-continuity-estimates/

and the last few paragraphs reminded me of the first fact stated here. My gut feeling is the Sylvester equation based approach might be helpful in establishing true/false conclusion of the ‘In general…’ question you asked in the end.

Thanks a lot! I’ll take a look.

Best,

Jacob