Al-Rashed–Zegarliński monotonicity conjecture below Schatten exponent two

Let HH be a complex Hilbert space, let B(H)B(H) denote the linear operators on HH, and let B(H)++B(H)_{++} denote the invertible positive operators. For βR\beta\in\mathbb{R}, α[1,0]\alpha\in[-1,0] with α+β=1\alpha+\beta=-1, 1p<1\leq p<\infty, and PB(H)++P\in B(H)_{++}, define

Λα,β,p(P,X):=PαpXPβppp=TrPαpXPβpp.\Lambda_{\alpha,\beta,p}(P,X):=\|P^{\frac{\alpha}{p}}XP^{\frac{\beta}{p}}\|_p^p=\operatorname{Tr}|P^{\frac{\alpha}{p}}XP^{\frac{\beta}{p}}|^p.

Al-Rashed–Zegarliński's conjecture. If p[1,2)p\in[1,2), then for every XB(H)X\in B(H) and every unital completely positive trace-preserving map ϕ\phi on B(H)B(H),

Λα,β,p(ϕ(P),ϕ(X))Λα,β,p(P,X).\Lambda_{\alpha,\beta,p}(\phi(P),\phi(X))\leq\Lambda_{\alpha,\beta,p}(P,X).

This proposed extension below p=2p=2 is refuted in the paper. Monotonicity would imply joint convexity of Λα,β,p\Lambda_{\alpha,\beta,p}, but the corresponding scalar function xpy1x^p y^{-1} is jointly convex only when p(p2)0p(p-2)\geq0, which fails for p[1,2)p\in[1,2).

Sources & referencesView supporting material

Primary source

Haonan Zhang, “Some convexity and monotonicity results of trace functionals”, arXiv:2108.05785 (2023).

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