The completely bounded unital contraction conjecture for operator algebras

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Let A⊂B(H)A\subset B(\mathcal H) be a unital operator algebra. Let θ:A→Mn(C)\theta:A\to M_n(\mathbb{C}) be a unital completely bounded homomorphism, and let α:A→A\alpha:A\to A be a unital antilinear complete contraction. Define the linear map

θα(f)=12(θ(f)+θ(α(f))∗)(f∈A).\theta_\alpha(f)=\frac{1}{2}\bigl(\theta(f)+\theta(\alpha(f))^*\bigr) \quad (f\in A).

Here ∥ϕ∥cb=sup⁡k∥ϕ(k)∥\|\phi\|_{cb}=\sup_k\|\phi^{(k)}\| denotes the completely bounded norm. Completely bounded conjecture. If ∥θα∥cb≤1\|\theta_\alpha\|_{cb}\leq 1, then

∥θ∥cb≤2.\|\theta\|_{cb}\leq 2.

This is the completely bounded refinement of the preceding conjecture and brings operator-algebraic and dilation-theoretic methods into the problem. It is presented as a conjecture in the paper and remains open.

References

Primary source

Raphaël Clouâtre, Maëva Ostermann and Thomas Ransford, “An abstract approach to the Crouzeix conjecture”, arXiv:2011.10422 (2022).

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