Selective and Ramsey ultrafilters on $G$-spaces

Abstract

Let $G$ be a group, $X$ be an infinite transitive $G$-space. A free ultrafilter $\UU$ on $X$ is called $G$-selective if, for any $G$-invariant partition $\PP$ of $X$, either one cell of $\PP$ is a member of $\UU$, or there is a member of $\UU$ which meets each cell of $\PP$ in at most one point. We show (Theorem 1) that in ZFC with no additional set-theoretical assumptions there exists a $G$-selective ultrafilter on $X$, describe all $G$-spaces $X$ (Theorem 2) such that each free ultrafilter on $X$ is $G$-selective, and prove (Theorem 3) that a free ultrafilter $\UU$ on $ω$ is selective if and only if $\UU$ is $G$-selective with respect to the action of any countable group $G$ of permutations of $ω$. A free ultrafilter $\UU$ on $X$ is called $G$-Ramsey if, for any $G$-invariant coloring $χ:[G]^2 \to \{0,1\}$, there is $U\in \UU$ such that $[U]^2$ is $χ$-monochrome. By Theorem 4, each $G$-Ramsey ultrafilter on $X$ is $G$-selective. Theorems 5 and 6 give us a plenty of $\mathbb{Z}$-selective ultrafilters on $\mathbb{Z}$ (as a regular $\mathbb{Z}$-space) but not $\mathbb{Z}$-Ramsey. We conjecture that each $\mathbb{Z}$-Ramsey ultrafilter is selective.