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Erdős problem 996

Let n1<n2<⋯n_1<n_2<\cdots be a lacunary sequence of integers and f∈L2([0,1])f\in L^2([0,1]) with nnth Fourier partial sum fnf_n. Is there an absolute constant C>0C>0 such that if ∥f−fn∥2≪(log⁡log⁡log⁡n)−C\| f-f_n\|_2 \ll (\log\log\log n)^{-C} then 1N∑k≤Nf({αnk})→∫01f\frac{1}{N}\sum_{k\leq N}f(\{\alpha n_k\})\to\int_0^1 f for almost every α\alpha? A preprint answers this negatively via a dyadic spike-block counterexample.

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Let $n_1<n_2<\cdots$ be a lacunary sequence of integers and $f\in L^2([0,1])$ with $n$th Fourier partial sum $f_n$. Is there an absolute constant $C>0$ such that if $\| f-f_n\|_2 \ll (\log\log\log n)^{-C}$ then $\frac{1}{N}\sum_{k\leq N}f(\{\alpha n_k\})\to\int_0^1 f$ for almost every $\alpha$? A preprint answers this negatively via a dyadic spike-block counterexample.

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Erdős problem 996 · problems.science