From Clouatre-Ostermann-Ransford to Okubo-Ando

Michael Hartz — 2026-08-28

Let $\mathcal A$ be a unital operator algebra and let $\theta: \mathcal A \to B(\mathcal H)$

be a continuous unital homomorphism.

We prove that for all $n \in \mathbb{N}$ and all $\beta$ in the dual space of $\mathcal{A}$,

 \begin{equation*}

 \|\theta^{(n)}\| \le \max(1, \|\theta^{(n)} + \beta^{(n)} I \|).

 \end{equation*}

 Here, $\theta^{(n)}: M_n(\mathcal A) \to B(\mathcal H^n)$ denotes the $n$-th matrix ampliation

 of $\theta$.

This extends a result of Clou\^atre, Ostermann and Ransford (the case $n=1$), who were motivated by Crouzeix's conjecture.

Our result allows us to control the completely bounded norm of $\theta$,

which in turn has dilation theoretic consequences.

As an application,

we obtain a new proof of the similarity theorem of Okubo and Ando,

which says that if $T \in B(\mathcal H)$ is an operator of class $C_\rho$, where $\rho \ge 1$,

then there exists an invertible operator $S$ such that $\|S^{-1} T S \| \le 1$ and such that $\|S\| \|S^{-1}\| \le \rho$. The conclusion of the Okubo--Ando theorem implies that the inequality

\[

 \|f(T)\| \le \rho \|f\|_{\overline{\mathbb D}}

\]

holds for all polynomials $f$. A direct proof of this inequality was recently given by Clou\^atre, Ostermann and Ransford. Our main result shows that this inequality in fact holds for matrix-valued polynomials, so that the existence of the similarity $S$

follows from Paulsen's similarity theorem, which says that an operator $T$ is completely polynomially bounded with constant $\rho$ if and only if there exists an invertible operator $S$ such that $\|S^{-1} T S \| \le 1$ and such that $\|S\| \|S^{-1}\| \le \rho$.

The key to proving our main result is to adapt a variational argument of Caldwell, Greenbaum and Li to matrix-valued holomorphic functions. Rather than working with biholomorphic automorphisms of the disc, we work with Potapov--M\"obius transforms $m_A$, where $A$ belongs to the unit ball of $M_n(\mathbb C)$. A slightly simplified version of our key lemma, which is shown using this technique,

then reads as follows:

If $R \in M_n(B(\mathcal H))$ satisfies $\|R\| > 1$

and $\|m_{A \otimes I}(R)\| \le \|R\|$ for all $A$ close to $0$ and if

$x \in \mathcal H^n$ with $\|x\| = 1$ and $\|R x\| = \|R\|$, then

\[ 
\langle R x, (A \otimes I) x \rangle = 0

\]

for all $A \in M_n(\mathcal C)$.

Visnyk of V. N. Karazin Kharkiv National University. Ser. Mathematics, Applied Mathematics and Mechanics

From Clouatre-Ostermann-Ransford to Okubo-Ando