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| Format: | Preprint |
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2026
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| Online Access: | https://arxiv.org/abs/2603.04655 |
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| _version_ | 1866911486177181696 |
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| author | Zhang, Teng |
| author_facet | Zhang, Teng |
| contents | 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 $$ \sqrt{\frac32}\ \le\ \sup_{\substack{A\in\mathbb{M}_n(\mathbb{C}),\ A^*A\neq AA^*\\ 0<λ<1}} \frac{\|Δ_λ(A)^*Δ_λ(A)-Δ_λ(A)Δ_λ(A)^*\|_F}{\|A^*A-AA^*\|_F} \ \le\ 2. $$ |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_04655 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | On a conjecture of $λ$-Aluthge transforms and Hilbert--Schmidt self-commutators Zhang, Teng Functional Analysis 47B20, 15A60, 47A30, 47A63 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 $$ \sqrt{\frac32}\ \le\ \sup_{\substack{A\in\mathbb{M}_n(\mathbb{C}),\ A^*A\neq AA^*\\ 0<λ<1}} \frac{\|Δ_λ(A)^*Δ_λ(A)-Δ_λ(A)Δ_λ(A)^*\|_F}{\|A^*A-AA^*\|_F} \ \le\ 2. $$ |
| title | On a conjecture of $λ$-Aluthge transforms and Hilbert--Schmidt self-commutators |
| topic | Functional Analysis 47B20, 15A60, 47A30, 47A63 |
| url | https://arxiv.org/abs/2603.04655 |