Extension of the block-matrix majorization inequality to normal off-diagonal blocks

Let a positive semidefinite matrix in M2n+\mathbb{M}_{2n}^{+} be written in blocks of the same size, with normal off-diagonal blocks. Normal-block majorization conjecture. The inequality from Corollary 1.3 should still hold: for every symmetric norm,

[AXXB]A+B.\left\|\begin{bmatrix} A & X \\ X & B\end{bmatrix}\right\|\leq \|A+B\|.

The preceding result establishes this inequality when the off-diagonal blocks are Hermitian; the conjecture asks whether normality suffices. The paper gives no resolution, so the question remains open.

Sources & referencesView supporting material

Primary source

Jean-Christophe Bourin, Eun-Young Lee and Minghua Lin, “On a decomposition lemma for positive semi-definite block-matrices”, arXiv:1202.0473 (2012).

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