Let $\mathcal S^2$ be the Stepanov space and let $ λ_n\uparrow\infty$. Let $(a_n)_{n\ge 1}$ be satisfying Wiener's condition $A:= \sum_{n\ge 1} \big(\sum_{k\, :\, n\le λ_k \le n+1}|a_k|\big)^2 <\infty$. We prove that $\big\| \sup_{N\ge 1} \big|\sum_{n=1}^Na_n{\rm e}^{iλ_n t}\big| \, \big\|_{\mathcal S^2}\le C\, A^{1/2} $ where $C>0$ denotes a universal constant. Moreover, the series $\sum_{n\ge 1} a_n{\rm e}^{itλ_n }$ converges for $λ$-a.e. $t\in \mathbb R$. This contains as a special case Hedenmalm and Saksman result for Dirichlet series. We also obtain maximal inequalities for corresponding series of dilates. Let $1\le p,q\le 2$ be such that $1/p+1/q=3/2$. Then for any sequence $(α_n)_{n\ge 1}$ and $(β_n)_{n\ge 1}$ of complex numbers such that $K:=\sum_{n\ge 1} \big(\sum_{k\,:\, n\le λ_k< n+1}|α_k|\,\big)^p <\infty$ and $L:=\sum_{n\ge 1} \big(\sum_{k\,:\, n\le μ_k< n+1} |β_k|\,\big)^q <\infty$, we have $$ \Big\|\sup_{N\ge 1} \big|\sum_{n=1}^N α_n D(λ_n t)\big|\, \Big\|_{\mathcal S^2} \le C\, K^{1/p}\, L^{1/q }
$$ where $D(t)= \sum_{n\ge 1}β_n {\rm e}^{iμ_n t}$ is defined in $\mathcal S^2$. Moreover, the series $\sum_{n\ge 1} α_n D(λ_nt)$ converges in $\mathcal S^2$ and for $λ$-a.e. $t\in \mathbb R$. We further show that if $\{λ_k, k\ge 1\}$ satisfies the following condition $$\sum_{ k\not=\ell\,,\, k'\not=\ell'\atop (k,\ell)\neq(k',\ell')}\big(1-|(λ_k-λ_\ell)-(λ_{k'}-λ_{\ell'}) |\big)_+^2 \, <\infty,$$ then the series $\sum_{k} a_k {\rm e}^{iλ_kt}$ converges on a set of positive Lebesgue measure, only if the series $\sum_{k=1}^\infty |a_k|^2$ converges. The above condition is in particular fulfilled when $\{λ_k, k\ge 1\}$ is a Sidon sequence.
Pointwise convergence of Walsh--Fourier series of vector-valued functions
我们证明了向量值函数Walsh模型中Carleson定理的一个版本:对于$1<p< \infty$和一个UMD空间$Y$,$f \in L ^{p}(0,1;Y)$的Walsh-傅里叶级数逐点收敛,条件是对于某些$θ\in(0,1)$,$Y$是另一个UMD空间$X$和一个希尔伯特空间$H$之间的复插值空间$Y=[X,H]_θ$.显然,所有已知的UMD空间的例子都满足这个条件。
Pointwise strong (H, Phi) approximation by Fourier series of L^{Psi} integrable functions
我们本质上推广和改进了G. H. Hardy和J.E. Littlewood关于傅里叶级数的强可和性。我们将给出广义强平均(H; Phi)的一个估计,作为G. H. Hardy,J. E. Littlewood,在从L^{Psi}可积函数的情况下。作为这种逼近的一种度量,我们将在定义L^{Psi}点的基础上,使用与Φ互补的函数Psi构造的函数。并给出一些推论和说明。
The pointwise convergence of Fourier Series (II). Strong $L^1$ case for the lacunary Carleson operator
证明了缺项Carleson算子是从$L \log L$到$L^{1}$有界的.这个结果是尖锐的。证明是基于两个新引入的概念:1)\emph{time-frequency regularization of a measurable set}和2)的\emph{set-resolution of the time-frequency plane at $0-$frequency}。这两个概念将发挥核心作用,在提供一个特殊的瓦片分解之间的相互作用,适用于\emph{structure}的缺Carleson运营商和相应的\emph{structure}的固定可测集。我们的论文的另一个关键的见解是,它提供了第一次同时治疗的瓷砖与\emph{distinct}质量参数的家庭。这应该被视为一个基本特征/困难的问题的逐点收敛的傅里叶级数附近的$L^1$,上下文中,不像标准的$L^p,\:p>1$的情况下,\emph{no decay}的质量参数是可能的。
Polynomial ergodic theorems in the spirit of Dunford and Zygmund
Wrocław University of Science and Technology(弗罗茨瓦夫科技大学);Franciscan University of Steubenville(斯蒂博芬大学);Rutgers University(罗格斯大学);Uniwersytet Wrocławski(弗罗茨瓦夫大学);Politecnico di Torino(都灵理工大学)