Clouâtre–Ostermann–Ransford conjecture on contractive homomorphisms

From papers

Let A{\mathcal A} be a uniform algebra, and let α:AA\alpha:{\mathcal A}\to{\mathcal A} be a unital anti-linear contraction. Let θ:AMn(C)\theta:{\mathcal A}\to{\mathcal M}_n(\mathbb C) be a continuous unital algebra homomorphism, and define the linear map

Λ:AMn(C),fθ(f)+θ(α(f)).\Lambda:{\mathcal A}\to{\mathcal M}_n(\mathbb C),\qquad f\mapsto\theta(f)+\theta(\alpha(f))^*.

Clouâtre–Ostermann–Ransford conjecture. (i) If Λ2\|\Lambda\|\leq 2, then θ2\|\theta\|\leq 2. (ii) If α\alpha is a complete contraction, θ\theta is completely bounded, and Λcb2\|\Lambda\|_{cb}\leq 2, then θcb2\|\theta\|_{cb}\leq 2.

The source states that this conjecture was motivated by proofs related to the Crouzeix conjecture and that, if true, it would imply the Crouzeix conjecture. The supplied text gives no resolution status.

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Sources & referencesView supporting material

Primary source

Michael Hartz and John E. McCarthy, “From Clouatre-Ostermann-Ransford to Okubo-Ando”, arXiv:2606.02922 (2026).

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