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arXiv 0910.3687math.DSmath.CO
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流的多重遍历平均及其应用

Multiple ergodic averages for flows and an application

Amanda Potts

摘要:

我们表明的$L^2$ -收敛的连续时间遍历平均值的产品的功能,评价在返回时间沿多项式。这些平均值是Furstenberg证明Szemerédi定理中出现的平均值的连续时间版本。对于每一个平均值,我们证明它是足够的特殊因素,主机Kra因素,它有一个nilmanifold结构的收敛。我们也给出了极限的描述。特别地,如果多项式在实数上是独立的,则极限是积分的乘积。我们进一步表明,如果多项式的集合具有“低复杂性”,则对于具有正密度的实数的每个集合$E$和对于每个$δ>0$,“$δ$ -加厚”集合$E_δ$的多项式返回时间的集合具有有界间隙。

英文摘要:

We show the $L^2$-convergence of continuous time ergodic averages of a product of functions evaluated at return times along polynomials. These averages are the continuous time version of the averages appearing in Furstenberg's proof of Szemerédi's Theorem. For each average we show that it is sufficient to prove convergence on special factors, the Host-Kra factors, which have the structure of a nilmanifold. We also give a description of the limit. In particular, if the polynomials are independent over the real numbers then the limit is the product of the integrals. We further show that if the collection of polynomials has "low complexity", then for every set $E$ of real numbers with positive density and for every $δ>0$, the set of polynomial return times for the "$δ$-thickened" set $E_δ$ has bounded gaps.

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