## Illinois Journal of Mathematics

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

Daniel J. Rudolph

#### 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

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

Digital Object Identifier
doi:10.1215/ijm/1258138395

Mathematical Reviews number (MathSciNet)
MR2085423

Zentralblatt MATH identifier
1063.37002

#### 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