Maximizing-pattern conjecture for numerical ranges of cyclic shift matrices

From papers

Let S(a1,,an)S(a_1,\dots,a_n) denote the n×nn\times n cyclic shift matrix with weights a1,,ana_1,\dots,a_n. For n2n\geq 2, let 0a1an0\leq a_1\leq\cdots\leq a_n and set

A=S(a1,a3,a5,,a6,a4,a2).A=S(a_1,a_3,a_5,\dots,a_6,a_4,a_2).

For a permutation σSn\sigma\in S_n, let AσA_\sigma be the cyclic shift matrix obtained by permuting the entries of AA, and let W()W(\cdot) denote numerical range. Maximizing-pattern conjecture. For every σSn\sigma\in S_n,

W(Aσ)W(A).W(A_\sigma)\subseteq W(A).

The stated pattern is known to maximize the numerical radius among permutations, and the conjecture asks whether it also maximizes the numerical range by set inclusion. The abstract reports this as established for n6n\leq 6 and conjectures that the pattern extends to general nn; the general case remains open.

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Sources & referencesView supporting material

Primary source

Mao-Ting Chien, Steve Kirkland, Chi-Kwong Li and Hiroshi Nakazato, “Numerical ranges of cyclic shift matrices”, arXiv:2304.06050 (2023).

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