Clouâtre–Ostermann–Ransford conjecture on contractive homomorphisms

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

Λ:A→Mn(C),f↦θ(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 ∥Λ∥≤2\|\Lambda\|\leq 2, then ∥θ∥≤2\|\theta\|\leq 2. (ii) If α\alpha is a complete contraction, θ\theta is completely bounded, and ∥Λ∥cb≤2\|\Lambda\|_{cb}\leq 2, then ∥θ∥cb≤2\|\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.

References

Primary source

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

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