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Regarding the matrix inner product based on singular values, Lewis (1995) "The convex analysis of unitarily invariant matrix functions" states the result by von Neumann that $\langle X,Y \rangle \leq \langle \sigma_X ,\sigma_Y \rangle$. Does anyone know any easy proof or reference for it. I couldn't understand the reference cited in the paper.

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For a "pedagogical" proof, see A Note on von Neumann's Trace Inequality by Rolf Dieter Grigorieff.

It has been remarked in the literature that "unexpectedly, finding a decent proof of this seemingly simple result turns out to be anything but trivial". The aim of the present note is to present still a further proof which seems to be elementary enough to correspond to the simplicity of the statement in von Neumann's Theorem.

The original proof by von Neumann is in "Some matrix-inequalities and metrization of metric-space," Tomsk Univ. Rev. 1 (1937) 286-300 -- which I have not found online.

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