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On a correspondence between regular and non-regular operator monotone functions. (English) Zbl 1178.47011

Let \(M_n ( \mathbb{C})\) be the set of all \(n\times n\) complex matrices. A function \(f: (0, \infty) \to \mathbb{R}\) is said to be operator monotone if, for any \(n \in \mathbb{N}\) and any pair of selfadjoint matrices \(A, B \in M_n ( \mathbb{C})\) such that \(0 < A \leq B\), the inequality \(f(A) \leq f(B)\) holds. The order \({}\leq{}\) denotes the Löwner partial order, that is \(A \leq B\) if and only if \(B - A\) is a positive-semidefinite matrix. Let \({\mathcal{F}}_{\text{op}}\) denote the class of all functions \(f : (0, \infty) \to (0, \infty)\) such that (i) \(f(1) =1\), (ii) \(x f(x^{-1} ) = f(x), \;x > 0\), (iii) \(f\) is operator monotone. Further, let \[ {\mathcal{F}}_{\text{op}}^{\text{r}} = \{ f \in {\mathcal{F}}_{\text{op}} \mid f(0) \neq 0 \} \qquad \text{and} \qquad {\mathcal{F}}_{\text{op}}^{\text{n}} = \{ f \in {\mathcal{F}}_{\text{op}} \mid f(0) = 0 \}. \] For \(f \in {\mathcal{F}}_{\text{op}}^{\text{r}}\), let \[ \bar{f} (x) = {1 \over 2} \left [ (x+1) - (x-1)^2 {{f(0)} \over {f(x)}} \right ] , \quad x > 0. \] The main result of this paper is the following Theorem. The correspondence \(f \mapsto \bar{f}\) is a bijection between the sets \({\mathcal{F}}_{\text{op}}^{\text{r}}\) and \({\mathcal{F}}_{\text{op}}^{\text{n}}\). As an application, the authors give a new proof of operator monotonicity of certain functions related to the Wigner-Yanse-Dyson skew information.

MSC:

47A64 Operator means involving linear operators, shorted linear operators, etc.
62B10 Statistical aspects of information-theoretic topics
46L30 States of selfadjoint operator algebras
46L60 Applications of selfadjoint operator algebras to physics
47N50 Applications of operator theory in the physical sciences
94A17 Measures of information, entropy
26A48 Monotonic functions, generalizations

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