Simon's higher-order Szegő sum-rule conjecture

Let ww be the density of the absolutely continuous part of a measure on the unit circle, let α=(αn)n0\alpha=(\alpha_n)_{n\geq 0} be its Verblunsky-coefficient sequence, and let SS be the left shift operator, (Sα)n=αn+1(S\alpha)_n=\alpha_{n+1}. For distinct θ1,θ2,,θk[0,2π)\theta_1,\theta_2,\ldots,\theta_k\in[0,2\pi) and strictly positive integers m1,m2,,mkm_1,m_2,\ldots,m_k, set m=max1jkmjm=\max_{1\leq j\leq k}m_j. Simon's higher-order Szegő conjecture. The condition

02πj=1k(1cos(θθj))mjlogw(θ)dθ2π>\int_{0}^{2\pi}\prod_{j=1}^{k}\bigl(1-\cos(\theta-\theta_j)\bigr)^{m_j}\log w(\theta)\frac{d\theta}{2\pi}>-\infty

should be equivalent to

n=0({j=1k(Seiθj)mjα}n2+αn2m+2)<.\sum_{n=0}^{\infty}\left(\left|\left\{\prod_{j=1}^{k}\bigl(S-e^{-i\theta_j}\bigr)^{m_j}\alpha\right\}_n\right|^{2}+|\alpha_n|^{2m+2}\right)<\infty.

This conjecture was subsequently shown to be false: for θ1=0\theta_1=0, θ2=π\theta_2=\pi, and m1=m2=1m_1=m_2=1, Lukic constructed a counterexample for which the sequence condition holds but the integral equals -\infty.

Sources & referencesView supporting material

Primary source

Zhihua Du, “Sum rules and Simon spectral gem problem on higher order Szegő theorems”, arXiv:2312.01323 (2023).

Additional references

2 papers in this index state this conjecture (2012–2023). The statement above is taken from the most recent of them; the others are arXiv:1210.6953.

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