Divisibility classes of ultrafilters and their patterns
Boris Šobot · arXiv (Cornell University) · 2024
A divisibility relation on ultrafilters on the set $\mathbb{N}$ of natural numbers is defined as follows: ${\cal F}\hspace{1mm}\widetilde{\mid}\hspace{1mm}{\cal G}$ if and only if every set in $\cal F$ upward closed for divisibility also belongs to $\cal G$. Previously we isolated basic classes: powers of prime ultrafilters, and described the pattern of an ultrafilter, measuring the quantity of members of each basic class dividing a given ultrafilter. In this paper we define a topology on the set of basic classes which will allow us to calculate the pattern of the limit of a $\widetilde{\mid}$-increasing chain of ultrafilters. Using this we characterize which patterns can actually appear as patterns of an ultrafilter. Defining the $=_\sim$-divisibility classes by identifying mutually divisible ultrafilters, in the respective quotient order $(β\mathbb{N}/=_\sim,\widetilde{\mid})$ we identify singleton classes and consider their patterns. Finally, we give a sufficient condition for a $=_\sim$-divisibility class to have an immediate predecessor.