A structured numerical-range realization conjecture

From papers

Let BCd×dB\in \mathbb{C}^{d \times d}, and suppose that its numerical range W(B)\mathcal{W}(B) is invariant under multiplication by e2πi/ne^{2\pi i/n}. Let CC(n,d)\mathcal{C}_{\mathbb{C}}(n,d) denote the class of cyclic weighted shift matrices of size dd with cyclic structure parameter nn. Structured numerical-range realization conjecture. There exists ACC(n,d)A\in \mathcal{C}_{\mathbb{C}}(n,d) such that

W(A)=W(B).\mathcal{W}(A)=\mathcal{W}(B).

If W(B)\mathcal{W}(B) is also invariant under conjugation, the entries of AA can be chosen real. The preceding theorem establishes the analogous realization after enlarging the matrix size to nd/nn\lceil d/n\rceil; the conjecture asks whether the original size dd always suffices.

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Sources & referencesView supporting material

Primary source

Faye Pasley Simon and Cynthia Vinzant, “Invariant hyperbolic curves: determinantal representations and applications to the numerical range”, arXiv:2102.01726 (2021).

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