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On a conjecture of $λ$-Aluthge transforms and Hilbert--Schmidt self-commutators

Teng Zhang

Abstract

Let $A$ be a complex square matrix, and write its polar decomposition as $A=U|A|$. For $0<λ<1$, the $λ$-Aluthge transform of $A$ is defined by $$ Δ_λ(A)=|A|^λU|A|^{1-λ}. $$ In 2007, Huang and Tam conjectured that the Frobenius norm of the self-commutator is contractive under $Δ_λ$: for every $0<λ<1$, $$ \|A^*A-AA^*\|_{F} \ \ge\ \|Δ_λ(A)^*Δ_λ(A)-Δ_λ(A)Δ_λ(A)^*\|_{F}. $$ If this inequality held, then the iterated self-commutator norms $$ \Bigl\{\bigl\|Δ_λ^{\,m}(A)^*Δ_λ^{\,m}(A) -Δ_λ^{\,m}(A)Δ_λ^{\,m}(A)^*\bigr\|_F\Bigr\}_{m\in\mathbb N} $$ would form a nonincreasing sequence and necessarily converge to $0$. In this paper we provide a counterexample, thereby disproving the conjecture. We also obtain the quantitative bounds $$ \sqrt{\frac32}\ \le\ \sup_{\substack{A\in\mathbb{M}_n(\mathbb{C}),\ A^*A\neq AA^*\\ 0<λ<1}} \frac{\|Δ_λ(A)^*Δ_λ(A)-Δ_λ(A)Δ_λ(A)^*\|_F}{\|A^*A-AA^*\|_F} \ \le\ 2. $$

On a conjecture of $λ$-Aluthge transforms and Hilbert--Schmidt self-commutators

Abstract

Let be a complex square matrix, and write its polar decomposition as . For , the -Aluthge transform of is defined by In 2007, Huang and Tam conjectured that the Frobenius norm of the self-commutator is contractive under : for every , If this inequality held, then the iterated self-commutator norms would form a nonincreasing sequence and necessarily converge to . In this paper we provide a counterexample, thereby disproving the conjecture. We also obtain the quantitative bounds
Paper Structure (4 sections, 7 theorems, 70 equations)

This paper contains 4 sections, 7 theorems, 70 equations.

Table of Contents

  1. Introduction
  2. A counterexample to Conjecture \ref{['conj:selfcomm']} for $\lambda=\tfrac{1}{2}$
  3. Sharp lower bounds for the optimal constants
  4. A uniform upper bound via a Heinz-type inequality

Key Result

Proposition 2.1

Let Then $A$ is invertible and so the inequality in eq:main fails (already for the usual Aluthge transform).

Theorems & Definitions (16)

  • Conjecture 1.1: Zha08
  • Conjecture 1.2: Zha08
  • Proposition 2.1
  • proof
  • Proposition 3.1
  • proof
  • Theorem 3.2
  • proof
  • Theorem 3.3
  • proof
  • ...and 6 more