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How can one simply see that Ky Fan $k$-norm satisfies the triangle inequality? (The Ky Fan $k$-norm of a matrix is the sum of the $k$ largest singular values of the matrix)

Thanks.

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    Could you please define the Ky Fan $k$-norm and let us know what you already tried? Also, is this homework?2011-03-21
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    The Ky Fan $k$-norm of a matrix is the sum of the $k$ largest singular values of the matrix2011-03-21
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    @user4727 Thanks for the edit. Are you aware of the original papers by Ky Fan (~1950)? The reason why it is called the "Ky Fan norm" is because he proved it is a norm first. This might be a good opportunity for some "research"! :)2011-03-21

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