On the maximal ergodic theorem for certain subsets of the integers
type: Item
instance of: scholarly article
title: On the maximal ergodic theorem for certain subsets of the integers
zbMATH Open document ID: 0642.28010
DOI: 10.1007/BF02776301
published in: Israel Journal of Mathematics
publication date: 1988-01-01T00:00:00Z
review text: Giving an affirmative answer to a problem considered by \textit{A. Bellow} [Lect. Notes Math. 945, 429-431 (1982)] and \textit{H. Furstenberg} (Proc. Durham Conf., June 1982), the author proves the following deep and interesting result: Let (X,\(\mu\),T) be a dynamical system then \(\frac{1}{n}\sum_{k\leq n}T^{k^ 2}f\) converges almost surely for any \(f\in L^ 2(X,\mu)\), more generally \(k^ 2\) can be replaced by an arbitrary polynomial function p(k) with integer coefficients. The problem is reduced to the proof of an inequality \(\| M_ nf\|_ 2\leq C\| f\|_ 2,\quad f\in L^ 2(X,\mu),\) where \(M_ nf\) is the ``maximal function'' \(\sup_{j\leq n}| (\sum_{k\leq j}T^{p(k)})/card\{k: p(k)\leq j\}|.\) This inequality can be proved by showing the according inequality for the special system (\({\mathbb{Z}},\lambda,T)\), \(\lambda\) the counting measure, T the shift. This can be done by Fourier transform methods and careful estimates of exponential sums (Gauss sums if \(p(k)=k^ 2)\), estimates from \textit{A. Sárközy}'s paper [Acta Math. Acad. Sci. Hung. 31, 125- 149 (1978; Zbl 0387.10033)] are used, the case \(p(k)=k^ t\) is associated with the Waring problem. The method of major arcs is of fundamental importance, based on \textit{I. M. Vinogradov} [The method of trigonometric sums in the theory of numbers (1954; Zbl 0055.275; Russian original 1947; Zbl 0041.370)] and \textit{R. C. Vaughan} [The Hardy- Littlewood method (1981; Zbl 0455.10034)]. As consequences one obtains results on uniform distribution, e.g. \(\frac{1}{n}\sum_{k\leq n}f(x+m^ t\alpha)\to \int f(x)dx\) almost surely for \(\alpha\not\in {\mathbb{Q}}\) and \(f\in L^{\infty}({\mathbb{R}}/{\mathbb{Z}})\) or \(\frac{1}{n}\sum_{m\leq n}f(2^{mt}x)\to \int f(x)dx\) a.s. and \(f\in L^{\infty}({\mathbb{R}}/{\mathbb{Z}})\), generalizing the Riesz-Raikov result for \(t=1.\) Furthermore the author obtains according results for commuting transformations and pointwise ergodic theorems for random sets for \(f\in L^ p\), \(…
Mathematics Subject Classification ID: 28D05
Mathematics Subject Classification ID: 11L40
Mathematics Subject Classification ID: 42A05
Mathematics Subject Classification ID: 11K06
Mathematics Subject Classification ID: 42B25
zbMATH DE Number: 4046064
zbMATH Keywords: maximal ergodic theorem
zbMATH Keywords: maximal function
zbMATH Keywords: exponential sums
zbMATH Keywords: Gauss sums
zbMATH Keywords: Waring problem
zbMATH Keywords: uniform distribution
zbMATH Keywords: pointwise ergodic theorems
Wikidata QID: Q56565302
author: Jean Bourgain
MaRDI profile type: Publication
cites work: On the pointwise ergodic theorem on \(L^ p\) for arithmetic sets
cites work: On sequences of density zero in ergodic theory
cites work: On Khinchin's Conjecture about Strong Uniform Distribution
cites work: On difference sets of sequences of integers. I
cites work: Beijing Lectures in Harmonic Analysis. (AM-112)
cites work: Q3903082
cites work: Q5827024
full work available at URL: https://doi.org/10.1007/bf02776301
OpenAlex ID: W2055765757
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description: scientific article; zbMATH DE number 4046064