Konyagin's lacunary Walsh-Fourier convergence conjecture

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Let {nj:j≥1}\{n_j:j\ge 1\} be a lacunary sequence of integers, meaning that

inf⁡j≥1nj+1nj>1.\inf_{j\ge 1}\frac{n_{j+1}}{n_j}>1.

For a function ff on T\mathbb T, let SnjfS_{n_j}f denote its Walsh-Fourier partial sums, and write log⁡+x=27+max⁡(0,log⁡x)\log_+x=27+\max(0,\log x). Konyagin's conjecture. If

∫∣f(x)∣log⁡log⁡+∣f(x)∣ dx<∞,\int |f(x)|\log\log_+|f(x)|\,dx<\infty,

then

Snjf(x)⟶f(x)S_{n_j}f(x)\longrightarrow f(x)

for almost every x∈Tx\in\mathbb T. This conjecture identifies the near-sharp integrability condition for almost-everywhere convergence along lacunary subsequences; the paper's main result comes close to resolving it, but the supplied text does not establish a complete resolution.

References

Primary source

Yen Do and Michael T. Lacey, “On the Convergence of Lacunary Walsh-Fourier Series”, arXiv:1101.2461 (2011).

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