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arXiv · 2610.00443

Dilation Theory for Scaled $q$-Numerical Ranges

Matias Kaunisto·Ryan O'Loughlin·Jani Virtanen
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Abstract

We generalise Berger's dilation theorem from numerical ranges to scaled $q$-numerical ranges. Specifically, we prove that if the scaled $q$-numerical range $\frac{W_q(A)}{q}$ is contained in the closed unit disk, then $A$ admits a unitary $ρ_q$-dilation, where $$ ρ_q=\max\left\{1,\frac{2|q|}{1+\sqrt{1-|q|^2}}\right\}. $$ As a corollary we show that every disk containing $Ω_q(A)$ is a complete $ρ_q$-spectral set for $A$. At $q=1$, this recovers the classical fact that any disk containing $W(A)$ is a complete $2$-spectral set for $A$.

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Matias Kaunisto, Ryan O'Loughlin, Jani Virtanen. 2026-10-04. Dilation Theory for Scaled $q$-Numerical Ranges. https://arxiv.org/abs/2610.00443

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