Past exam of the mathematics course of the University of Cambridge / 2021 / iii / Paper 130 / 4 / Solution

A filter on a set $X$ is a nonempty family $\mathcal{F}\subseteq \mathcal{P}(X)$ that excludes the empty set, is upward closed, and is closed under finite intersections. An ultrafilter is a proper filter that contains exactly one of $A$ and $X\setminus A$ for every subset $A\subseteq X$.

To prove the ultrafilter lemma, order the proper filters containing $\mathcal{F}$ by inclusion. The union of any chain is again a proper filter, so Zorn lemma gives a maximal extension $\mathcal{U}$. If neither $A$ nor $X\setminus A$ belonged to $\mathcal{U}$, adjoining either one would generate an improper filter. There would then be $U,V\in \mathcal{U}$ with $U\cap A=\varnothing$ and $V\cap (X\setminus A)=\varnothing$. But $U\cap V=\varnothing$, contradicting propriety. Hence $\mathcal{U}$ is an ultrafilter.

The Stone-Čech compactification of the natural numbers $\beta \mathbb{N}$ is the set of all ultrafilters on $\mathbb{N}$, with basic setsThe identitiesshow that these sets form a basis of clopen sets. Distinct ultrafilters disagree on some $A$; one lies in $\overline{A}$ and the other in the disjoint set $\overline{\mathbb{N}\setminus A}$. Thus $\beta \mathbb{N}$ is a Hausdorff space.

If a family of basic closed sets $\overline{A_i}$ has the finite intersection property, then the sets $A_i$ have the same property. They generate a proper filter, which extends to an ultrafilter lying in every $\overline{A_i}$. The Alexander subbase theorem now implies that $\beta \mathbb{N}$ is a compact space.