Journal of Modern Dynamics (JMD)

Almost-everywhere convergence and polynomials

Pages: 465 - 470, Issue 3, July 2008      doi:10.3934/jmd.2008.2.465

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Michael Boshernitzan - Department of Mathematics, Rice Unviersity, Houston, TX 77005, United States (email)
Máté Wierdl - Department of Mathematical Sciences, 373 Dunn Hall, University of Memphis, Memphis, TN 38152-3240, United States (email)

Abstract: Denote by $\Gamma$ the set of pointwise good sequences: sequences of real numbers $(a_k)$ such that for any measure--preserving flow $(U_t)_{t\in\mathbb R}$ on a probability space and for any $f\in L^\infty$, the averages $\frac{1}{n} \sum_{k=1}^{n} f(U_{a_k}x) $ converge almost everywhere.
    We prove the following two results.
1. If $f: (0,\infty)\to\mathbb R$ is continuous and if $(f(ku+v))_{k\geq 1}\in\Gamma$ for all $u, v>0$, then $f$ is a polynomial on some subinterval $J\subset (0,\infty)$ of positive length.
2. If $f: [0,\infty)\to\mathbb R$ is real analytic and if $(f(ku))_{k\geq 1}\in\Gamma$ for all $u>0$, then $f$ is a polynomial on the whole domain $[0,\infty)$.
    These results can be viewed as converses of Bourgain's polynomial ergodic theorem which claims that every polynomial sequence lies in $\Gamma$.

Keywords:  Pointwise ergodic theorems along subsequences, polynomials.
Mathematics Subject Classification:  Primary: 37A10, Secondary: 37A30.

Received: November 2007;      Revised: March 2008;      Available Online: April 2008.