Gross–Witten sum-rule conjecture for coupling below minus one

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Let μ∈M1(T)\mu\in\mathcal M_1(\mathbb T) have Verblunsky coefficients (αk)k≥0∈DN(\alpha_k)_{k\geq 0}\in\mathbb D^{\mathbb N}. Let θg\theta_{\tt g}, N−N^-, N+N^+, λi+\lambda_i^+, λi−\lambda_i^-, and Fg±\mathcal F_{\tt g}^{\pm} be the quantities associated with the Gross–Witten model, and let S1T(π−θg,π+θg)\mathcal S_1^{\mathbb T}(\pi-\theta_{\tt g},\pi+\theta_{\tt g}) denote the stated admissible class. For μ∈S1T(π−θg,π+θg)\mu\in\mathcal S_1^{\mathbb T}(\pi-\theta_{\tt g},\pi+\theta_{\tt g}) and any g<−1{\tt g}<-1, Gross–Witten sum-rule conjecture.

K(GW⁡g∣μ)+∑i=1N−Fg+(λi+)+∑i=1N+Fg−(λi−)=H(g)−g ℜ(α0−∑k=1∞αkαˉk−1)−∑k=0∞log⁡(1−∣αk∣2),\mathcal K(\operatorname{GW}_{\tt g}\mid\mu)+\sum_{i=1}^{N^-}\mathcal F_{\tt g}^+(\lambda_i^+)+\sum_{i=1}^{N^+}\mathcal F_{\tt g}^-(\lambda_i^-)=H({\tt g})-{\tt g}\,\Re\left(\alpha_0-\sum_{k=1}^{\infty}\alpha_k\bar\alpha_{k-1}\right)-\sum_{k=0}^{\infty}\log(1-|\alpha_k|^2),

where HH is defined by the formula given in the source. If μ∉S1T(π−θg,π+θg)\mu\notin\mathcal S_1^{\mathbb T}(\pi-\theta_{\tt g},\pi+\theta_{\tt g}), the right-hand side is +∞+\infty. This conjectures the extension of the Gross–Witten sum rule beyond the regime ∣g∣≤1|{\tt g}|\leq 1; the supplied text does not state whether it has been proved or disproved.

References

Primary source

Fabrice Gamboa, Jan Nagel and Alain Rouault, “Sum rules and large deviations for spectral measures on the unit circle”, arXiv:1604.06934 (2017).

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