## Illinois Journal of Mathematics

- Illinois J. Math.
- Volume 48, Number 2 (2004), 505-517.

### Pointwise and $L\sp 1$ mixing relative to a sub-sigma algebra

#### Abstract

We consider two natural definitions for the notion of a dynamical system being mixing relative to an invariant sub $\sigma$-algebra $\mathcal{H}$. Both concern the convergence of

\[|E(f\cdot g\circ T^n|\mathcal H)-E(f|\mathcal H)E(g\circ T^n|\mathcal H)|\to 0\]

as $|n|\to \infty$ for appropriate $f$ and $g$. The weaker condition asks for convergence in $L^1$ and the stronger for convergence a.e. We will see that these are different conditions. Our goal is to show that both these notions are robust. As is quite standard we show that one need only consider $g=f$ and $E(f|\mathcal H)=0$, and in this case $|E(f\cdot f\circ T^n|\mathcal H)|\to 0$. We will see rather easily that for $L^1$ convergence it is enough to check an $L^2$-dense family. Our major result will be to show the same is true for pointwise convergence, making this a verifiable condition. As an application we will see that if $T$ is mixing then for any ergodic $S$, $S\times T$ is relatively mixing with respect to the first coordinate sub $\sigma$-algebra in the pointwise sense.

#### Article information

**Source**

Illinois J. Math., Volume 48, Number 2 (2004), 505-517.

**Dates**

First available in Project Euclid: 13 November 2009

**Permanent link to this document**

https://projecteuclid.org/euclid.ijm/1258138395

**Digital Object Identifier**

doi:10.1215/ijm/1258138395

**Mathematical Reviews number (MathSciNet)**

MR2085423

**Zentralblatt MATH identifier**

1063.37002

**Subjects**

Primary: 37A25: Ergodicity, mixing, rates of mixing

Secondary: 37A05: Measure-preserving transformations 47A35: Ergodic theory [See also 28Dxx, 37Axx]

#### Citation

Rudolph, Daniel J. Pointwise and $L\sp 1$ mixing relative to a sub-sigma algebra. Illinois J. Math. 48 (2004), no. 2, 505--517. doi:10.1215/ijm/1258138395. https://projecteuclid.org/euclid.ijm/1258138395