Maximizing-pattern conjecture for numerical ranges of cyclic shift matrices

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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 n≥2n\geq 2, let 0≤a1≤⋯≤an0\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 σ∈Sn\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 σ∈Sn\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 n≤6n\leq 6 and conjectures that the pattern extends to general nn; the general case remains open.

References

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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