Bourgain's conjecture for trigonometric polynomials on the Bohr compactification of R\mathbb R

Let P(R)\mathcal P(\mathbb R) be the space of trigonometric polynomials on R\mathbb R, and let (ωj)jN(\omega_j)_{j\in\mathbb N} be an increasing sequence of real numbers. Define

Jω,n={PP(R):P(t)=k=0nakeiωkt, a0=1, ak{0,1}}.J_{\omega,n}=\left\{P\in\mathcal P(\mathbb R):P(t)=\sum_{k=0}^{n}a_ke^{i\omega_kt},\ a_0=1,\ a_k\in\{0,1\}\right\}.

For P(t)=k=0n1akeiωktP(t)=\sum_{k=0}^{n-1}a_ke^{i\omega_kt}, its Bohr L2L^2 norm satisfies

P(eit)22=RBohrk=0n1akeiωkt2dt=1+k=1n1ak2.\|P(e^{it})\|_2^2=\int_{\mathbb R}^{\mathrm{Bohr}}\left|\sum_{k=0}^{n-1}a_ke^{i\omega_kt}\right|^2dt=1+\sum_{k=1}^{n-1}a_k^2.

Bourgain's conjecture in the Bohr compactification. For any increasing sequence (ωj)jN(\omega_j)_{j\in\mathbb N} of real numbers,

supn1{supPJω,nP(eit)1P(eit)2}<1.\sup_{n\geq1}\left\{\sup_{P\in J_{\omega,n}}\frac{\|P(e^{it})\|_1}{\|P(e^{it})\|_2}\right\}<1.

This is the formulation of Bourgain's conjecture for frequencies on the Bohr compactification of R\mathbb R, connecting the polynomial flatness problem with questions about simple Lebesgue spectrum. The source gives no resolution, so the conjecture remains open.

Sources & referencesView supporting material

Primary source

El Houcein El Abdalaoui, “Generalized Riesz Products on the Bohr compactification of”, arXiv:1206.0493 (2014).

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