On conservative sequences and their application to ergodic multiplier problems

The conservative sequence of a set $A$ under a transformation $T$ is the set of all $n in mathbb{Z}$ such that $T^n A cap A ot = varnothing$. 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 Chacon 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.