Dice Question Streamline Icon: https://streamlinehq.com

Existence of sets with finite extended Fourier spectrum strictly above the ambient dimension

Determine whether there exists a compact set X ⊆ R^d and a parameter θ ∈ [0,1] such that the extended Fourier spectrum E_F^θ(X), defined as the supremum over all finite Borel measures μ supported on X of the Fourier spectrum dim_F^θ(μ), satisfies d < E_F^θ(X) < ∞ (equivalently, d < E_F^θ(X) < 2d as permitted by general bounds).

Information Square Streamline Icon: https://streamlinehq.com

Background

The paper introduces the extended Fourier spectrum E_Fθ(X) of a set X ⊆ Rd as the supremum of the Fourier spectra dim_Fθ(μ) over all finite Borel measures μ supported on X. For sets of Lebesgue measure zero, E_Fθ(X) coincides with the usual Fourier spectrum, while for sets with non-empty interior E_Fθ(X)=∞ for all θ.

General bounds imply E_Fθ(X), E_Fθ(X) ∈ [0,2d] ∪ {∞}. The authors point out that they do not know any example where E_Fθ(X) is strictly larger than d but still finite, prompting the existence question below.

References

We do not know how to construct sets with extended Fourier dimension strictly exceeding $d$ but finite.

\begin{ques} Does there exist a (compact) set $X \subseteq R$ with $d<E X 2d$ for some $\theta \in [0,1]$? \end{ques}

Fourier decay of product measures (2405.05878 - Fraser, 9 May 2024) in Section 3 (Dimensions of product sets), preceding and including Question