On effective Birkhoff's ergodic theorem for computable actions of amenable groups (1701.06365v1)

Abstract: We introduce computable actions of computable groups and prove the following versions of effective Birkhoff's ergodic theorem. Let $\Gamma$ be a computable amenable group, then there always exists a canonically computable tempered two-sided F{\o}lner sequence $(F_n){n \geq 1}$ in $\Gamma$. For a computable, measure-preserving, ergodic action of $\Gamma$ on a Cantor space ${0,1}^{\mathbb N}$ endowed with a computable probability measure $\mu$, it is shown that for every bounded lower semicomputable function $f$ on ${0,1}^{\mathbb N}$ and for every Martin-L\"of random $\omega \in {0,1}^{\mathbb N}$ the equality [ \lim\limits{n \to \infty} \frac{1}{|F_n|} \sum\limits_{g \in F_n} f(g \cdot \omega) = \int\limits f d \mu ] holds, where the averages are taken with respect to a canonically computable tempered two-sided F{\o}lner sequence $(F_n)_{n \geq 1}$. We also prove the same identity for all lower semicomputable $f$'s in the special case when $\Gamma$ is a computable group of polynomial growth and $F_n:=\mathrm{B}(n)$ is the F{\o}lner sequence of balls around the neutral element of $\Gamma$.