Multiplicative finite embeddability vs divisibility of ultrafilters

Abstract

We continue the exploration of various aspects of divisibility of ultrafilters, adding one more relation to the picture: multiplicative finite embeddability. We show that it lies between divisibility relations ${\mid }_M$ and $\tilde{\mid }$. The set of its minimal elements proves to be very rich, and the $\tilde{\mid }$-hierarchy is used to get a better intuition of this richness. We find the place of the set of $\tilde{\mid }$-maximal ultrafilters among some known families of ultrafilters. Finally, we introduce new notions of largeness of subsets of $\mathbb{N}$, and compare it to other such notions, important for infinite combinatorics and topological dynamics.