Reality conjecture for the limiting spectrum of real block Toeplitz matrices

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Let B(z)B(z) be a real k×kk\times k matrix-valued symbol. Define

Γ(B)={z∈C:∃λ∈R such that f(z,λ)=0},\Gamma(B)=\{z\in\mathbb{C}:\exists\lambda\in\mathbb{R}\text{ such that }f(z,\lambda)=0\},

where f(z,λ)f(z,\lambda) is the characteristic function of BB, and let Λ0(B)\Lambda_0(B) denote the set of limiting spectral points associated with the qq-th and (q+1)(q+1)-st roots of zqf(z,λ)=0z^qf(z,\lambda)=0 having equal modulus. Reality conjecture. The set Λ0(B)\Lambda_0(B) belongs to R\mathbb{R} if and only if Γ(B)\Gamma(B) contains kk Jordan curves (ovals) having 00 in their interior. This conjecture generalizes the scalar Laurent-polynomial case, where Γ(B)\Gamma(B) is the inverse image of R\mathbb{R} under the symbol and the corresponding curve has been studied previously. The statement is presented as a general conjecture for real banded block Toeplitz matrices; no resolution is supplied in the source.

References

Primary source

Dario Giandinoto, “On reality of eigenvalues of banded block Toeplitz matrices”, arXiv:2411.16266 (2024).

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