Level set Crouzeix conjecture for finite Blaschke products

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Let Θ\Theta and BB be finite Blaschke products with deg⁡B<deg⁡Θ\deg B<\deg\Theta. Let MΘM_\Theta be the matrix associated with Θ\Theta, and define the 1/21/2-level set

Ω1/2B={z∈C:∣B(z)∣<1/2}.\Omega^B_{1/2}=\{z\in\mathbb{C}:|B(z)|<1/2\}.

Level set Crouzeix conjecture. One has

W(MΘ)⊈Ω1/2B.W(M_\Theta)\not\subseteq\Omega^B_{1/2}.

This is a weak version of Crouzeix's conjecture for finite compressions of the shift. It is known for certain classes of pairs (B,Θ)(B,\Theta), but most cases remain open.

References

Primary source

Kelly Bickel, Georgia Corbett, Annie Glenning, Changkun Guan and Martin Vollmayr-Lee, “Crouzeix's conjecture, compressions of shifts, and classes of nilpotent matrices”, arXiv:2312.04537 (2023).

Additional references

2 papers in this index state this conjecture (2021–2023). The statement above is taken from the most recent of them; the others are arXiv:2112.06321.

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