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Pointwise universal trigonometric series

Functional Analysis 2012-09-07 v1

Abstract

A series Sa=n=anznS_a=\sum\limits_{n=-\infty}^\infty a_nz^n is called a {\it pointwise universal trigonometric series} if for any fC(\T)f\in C(\T), there exists a strictly increasing sequence {nk}kN\{n_k\}_{k\in\N} of positive integers such that j=nknkajzj\sum\limits_{j=-n_k}^{n_k} a_jz^j converges to f(z)f(z) pointwise on \T\T. We find growth conditions on coefficients allowing and forbidding the existence of a pointwise universal trigonometric series. For instance, if an=O(\enln1ϵn)|a_n|=O(\e^{\,|n|\ln^{-1-\epsilon}|n|}) as n|n|\to\infty for some ϵ>0\epsilon>0, then the series SaS_a can not be pointwise universal. On the other hand, there exists a pointwise universal trigonometric series SaS_a with an=O(\enln1n)|a_n|=O(\e^{\,|n|\ln^{-1}|n|}) as n|n|\to\infty.

Cite

@article{arxiv.1209.1216,
  title  = {Pointwise universal trigonometric series},
  author = {Stanislav Shakrin},
  journal= {arXiv preprint arXiv:1209.1216},
  year   = {2012}
}
R2 v1 2026-06-21T22:00:44.843Z