On conservative sequences and their application to ergodic multiplier problems

Abstract

The conservative sequence of a set $A$ under a transformation $T$ is the set of all $n\in \mathbb{Z}$ such that $T^{n}A\cap A\ne \varnothing$. By studying these sequences, we prove that given any countable collection of nonsingular transformations with no finite invariant measure $\{T_i\}$, there exists a rank-one transformation $S$ such that $T_i\times S$ is not ergodic for all $i$. Moreover, $S$ can be chosen to be rigid or have infinite ergodic index. We establish similar results for ${\mathbb{Z}}^d$ actions and flows. Then, we find sufficient conditions on rank-one transformations $T$ that guarantee the existence of a rank-one transformation $S$ such that $T\times S$ is ergodic, or, alternatively, conditions that guarantee that $T\times S$ is conservative but not ergodic. In particular, the infinite Chacón transformation satisfies both conditions. Finally, for a given ergodic transformation $T$, we study the Baire categories of the sets $E(T)$, $\bar{E}C(T)$ and $\bar{C}(T)$ of transformations $S$ such that $T\times S$ is ergodic, ergodic but not conservative, and conservative, respectively.

26 pages, 3 figures