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Distributional Point Values and Convergence of Fourier Series and Integrals
Authors:Jasson Vindas  Ricardo Estrada
Institution:(1) Mathematics Department, Louisiana State University, Baton Rouge, Louisiana, USA
Abstract:In this article we show that the distributional point values of a tempered distribution are characterized by their Fourier transforms in the following way: If $f\in\mathcal{S}^{\prime}\left(\mathbb{R}\right)$ and $x_{0}\in\mathbb{R}$ , and $\widehat{f}$ is locally integrable, then $f(x_{0})=\gamma\ $ distributionally if and only if there exists k such that $\frac{1}{2\pi}\lim_{x\rightarrow\infty}\int_{-x}^{ax} \hat{f}(t)e^{-ix_{0}t}\,\mathrm{d}t=\gamma\ \ (\mathrm{C},k)\,$ , for each a > 0, and similarly in the case when $\widehat{f}$ is a general distribution. Here $(\mathrm{C},k)$ means in the Cesaro sense. This result generalizes the characterization of Fourier series of distributions with a distributional point value given in 5] by $\lim_{x\rightarrow\infty}\sum_{-x\leq n\leq ax}a_{n}e^{inx_{0}}=\gamma\ (\mathrm{C},k)\,$ . We also show that under some extra conditions, as if the sequence $\left\{a_{n}\right\}  _{n=-\infty}^{\infty}$ belongs to the space $l^{p}$ for some $p\in\lbrack1,\infty)$ and the tails satisfy the estimate $\sum_{\left\vert n\right\vert \geq N}^{\infty}\left\vert a_{n}\right\vert ^{p}=O\left(N^{1-p}\right)  $ ,\ as $N\rightarrow\infty$ , the asymmetric partial sums\ converge to $\gamma$ . We give convergence results in other cases and we also consider the convergence of the asymmetric partial integrals. We apply these results to lacunary Fourier series of distributions.
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