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arXiv 2006.03558math.DSmath.CO
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多重 Hardy 场函数下的 ergodic 平均:收敛、回归与组合应用

Multiple ergodic averages along functions from a Hardy field: convergence, recurrence and combinatorial applications

Vitaly Bergelson, Joel Moreira, Florian K. Richter

AI总结:

本文研究了从 Hardy 场函数中多重 ergodic 平均的收敛性与回归性,确认了 Frantzikinakis 的猜想,并应用组合方法扩展了 Szemeredi 定理。

摘要:

我们获得了关于从 Hardy 场函数中多重 ergodic 平均收敛性和回归性的新结果。其中,我们确认了 Frantzikinakis 在 [Fra10; Fra16] 中提出的一些猜想,并获得了包含多个已知(多项式和非多项式)Szemeredi 定理扩展的组合应用。我们结果的一个新特点是在以前的工作中没有出现,即允许多项式和非多项式函数的混合。例如,假设 $f_i(t)=a_{i,1}t^{c_{i,1}}+\cdots+a_{i,d}t^{c_{i,d}}$ 对于 $c_{i,j}>0$ 和 $a_{i,j}\in\mathbb{R}$,则 $\bullet$ 对于任何测度保持系统 $(X,\mathcal{B},μ,T)$ 和 $h_1,\dots,h_k\in L^\infty(X)$,极限 $$\lim_{N\to\infty}\frac{1}{N}\sum_{n=1}^N T^{[f_1(n)]}h_1\cdots T^{[f_k(n)]}h_k$$ 在 $L^2$ 中存在;$\bullet$ 对于任何自然数集 $E$ 有 $\overline{\mathrm{d}}(E)>0$,存在 $a,n\in\mathbb{N}$ 使得 $\{a,\, a+[f_1(n)],\ldots,a+[f_k(n)]\}\subset E$。我们还证明,如果 $f_1,\dots,f_k$ 属于 Hardy 场,具有多项式增长,并且没有线性组合是多项式,则对于任何测度保持系统 $(X,{\mathcal B},μ,T)$ 和任何 $A\in{\mathcal B}$,$$\limsup_{N\to\infty}\frac{1}{N}\sum_{n=1}^Nμ\Big(A\cap T^{-[ f_1(n) ]}A\cap\ldots\cap T^{-[f_k(n)]}A\Big)\,\geq\,μ(A)^{k+1}。

英文摘要:

We obtain new results pertaining to convergence and recurrence of multiple ergodic averages along functions from a Hardy field. Among other things, we confirm some of the conjectures posed by Frantzikinakis in [Fra10; Fra16] and obtain combinatorial applications which contain, as rather special cases, several previously known (polynomial and non-polynomial) extensions of Szemeredi's theorem on arithmetic progressions [BL96; BLL08; FW09; Fra10; BMR17]. One of the novel features of our results, which is not present in previous work, is that they allow for a mixture of polynomials and non-polynomial functions. As an illustration, assume $f_i(t)=a_{i,1}t^{c_{i,1}}+\cdots+a_{i,d}t^{c_{i,d}}$ for $c_{i,j}>0$ and $a_{i,j}\in\mathbb{R}$. Then $\bullet$ for any measure preserving system $(X,\mathcal{B},μ,T)$ and $h_1,\dots,h_k\in L^\infty(X)$, the limit $$\lim_{N\to\infty}\frac{1}{N}\sum_{n=1}^N T^{[f_1(n)]}h_1\cdots T^{[f_k(n)]}h_k$$ exists in $L^2$; $\bullet$ for any $E\subset \mathbb{N}$ with $\overline{\mathrm{d}}(E)>0$ there are $a,n\in\mathbb{N}$ such that $\{a,\, a+[f_1(n)],\ldots,a+[f_k(n)]\}\subset E$. We also show that if $f_1,\dots,f_k$ belong to a Hardy field, have polynomial growth, and are such that no linear combination of them is a polynomial, then for any measure preserving system $(X,{\mathcal B},μ,T)$ and any $A\in{\mathcal B}$, $$\limsup_{N\to\infty}\frac{1}{N}\sum_{n=1}^Nμ\Big(A\cap T^{-[ f_1(n) ]}A\cap\ldots\cap T^{-[f_k(n)]}A\Big)\,\geq\,μ(A)^{k+1}.$$

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