Noncommutative Clarkson inequality for Schatten norms

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Let H\mathscr{H} be a Hilbert space and let Bp(H)\mathbb{B}_p(\mathscr{H}) denote the Schatten pp-class on H\mathscr{H}. For A,B∈Bp(H)A,B\in\mathbb{B}_p(\mathscr{H}), the noncommutative Clarkson conjecture. For p≥2p\ge2,

2(∥A∥p2+∥B∥p2)p/2≤∥A+B∥pp+∥A−B∥pp,2\bigl(\|A\|_p^2+\|B\|_p^2\bigr)^{p/2}\le\|A+B\|_p^p+\|A-B\|_p^p,

equivalently,

∥A∥p2+∥B∥p2≤(∥A+B∥pp+∥A−B∥pp2)2/p.\|A\|_p^2+\|B\|_p^2\le\left(\frac{\|A+B\|_p^p+\|A-B\|_p^p}{2}\right)^{2/p}.

For 0<p≤20<p\le2, the inequality is reversed. It is motivated by the scalar convexity inequality and its known operator analogue; the supplied text does not state whether this conjecture has been resolved.

References

Primary source

Teng Zhang, “Clarkson–McCarthy type inequalities, part I: _p–_p and _q–_p Schatten p-estimates”, arXiv:2410.21961 (2026).

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