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arXiv 1607.02041math.CA
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概周期傅里叶级数及相关扩张级数的点态收敛性

Pointwise convergence of almost periodic Fourier series and associated series of dilates

Christophe Cuny, Michel Weber

摘要:

设$\mathcal S^2$是Stepanov空间,并设$ λ_n\uparrow\infty$。设$(a_n)_{n\ge 1}$满足Wiener条件$A:= \sum_{n\ge 1} \big(\sum_{k\, :\, n\le λ_k \le n+1}|a_k|\big)^2 <\infty$ .我们证明了$\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} $,其中$C>0$表示一个通用常数。此外,序列$\sum_{n\ge 1} a_n{\rm e}^{itλ_n }$收敛$λ$ -a.e. $t\in \mathbb R$。这包含作为特殊情况Hedenmalm和Saksman结果Dirichlet级数。我们还得到了相应的系列扩张的极大不等式。设$1\le p,q\le 2$为$1/p+1/q=3/2$。然后对于任何复数序列$(α_n)_{n\ge 1}$和$(β_n)_{n\ge 1}$,使得$K:=\sum_{n\ge 1} \big(\sum_{k\,:\, n\le λ_k< n+1}|α_k|\,\big)^p <\infty$和$L:=\sum_{n\ge 1} \big(\sum_{k\,:\, n\le μ_k< n+1} |β_k|\,\big)^q <\infty$,我们有$$ \Big\|\sup_{N\ge 1} \big|\sum_{n=1}^N α_n D(λ_n t)\big|\, \Big\|_{\mathcal S^2} \le C\, K^{1/p}\, L^{1/q } $$,其中$D(t)= \sum_{n\ge 1}β_n {\rm e}^{iμ_n t}$在$\mathcal S^2$中定义。此外,系列$\sum_{n\ge 1} α_n D(λ_nt)$收敛于$\mathcal S^2$和$λ$ -a.e. $t\in \mathbb R$。进一步证明了若$\{λ_k, k\ge 1\}$满足下列条件$$\sum_{ k\not=\ell\,,\, k'\not=\ell'\atop (k,\ell)\neq(k',\ell')}\big(1-|(λ_k-λ_\ell)-(λ_{k'}-λ_{\ell'}) |\big)_+^2 \, <\infty,$$,则级数$\sum_{k} a_k {\rm e}^{iλ_kt}$收敛于一个正Lebesgue测度集,且仅当级数$\sum_{k=1}^\infty |a_k|^2$收敛.当$\{λ_k, k\ge 1\}$是Sidon序列时,特别满足上述条件。

英文摘要:

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.

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