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arXiv · 2606.21637

A Critical-Scale Extension of Zhizhiashvili's Theorem for Rectangular Fourier Series

Ushangi Goginava
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Abstract

We address a long-standing endpoint problem arising from Zhizhiashvili's logarithmic modulus theorem for multiple Fourier series. We prove an endpoint Dini criterion for almost-everywhere Pringsheim convergence of ordinary rectangular partial sums. In Zhizhiashvili's theorem the logarithmic modulus is assumed with an exponent strictly above the critical value; here this strict power margin is replaced by a summable endpoint Dini condition. As a consequence, one obtains double-logarithmic endpoint classes lying outside the range of the classical theorem. The proof reduces the endpoint smoothness assumption to the Kaczmarz--Kojima product-logarithmic coefficient criterion by weighted translation-difference estimates.

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Ushangi Goginava. 2026-06-19. A Critical-Scale Extension of Zhizhiashvili's Theorem for Rectangular Fourier Series. https://arxiv.org/abs/2606.21637

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