Bounds for Matrix Inner Product based on singular values

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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 couldnt understand the reference cited in the paper.

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This is called the von-Neumann trace inequality. For a reference: this is given in Horn and Johnson's Matrix Analysis as Theorem 7.4.1.1. For a more powerful result, you could use corollary IV.2.6 of Bhatia's Matrix Analysis.