Now that you know the basics of countable elementary submodels (CESM), you might think that you are in the clear. “Mike”, you say arrogantly, “I know all the most basic properties of CESMs, without proof I remind you, what else could I possibly want?”. I gently and patiently remind you that CESMs are worthless unless you know how to apply them properly.
So let’s do that.
Here are two theorems whose proofs you might already know, but that can be proved using elementary submodels. I will show you a proof of the -system lemma (a fundamental lemma in infinitary combinatorics) and a topological theorem of Arhangel’skii. Both of these proofs are taken from Just & Weese’s book “Discovering Modern Set Theory 2”, chapter 24.
Continue reading The Delta-System Lemma
In my ongoing love affair with compactness I am constantly revisiting a particular proof of the Heine-Borel theorem, a characterization of compactness in . There are two proofs that I know of: the standard “subdivision” proof and the “creeping along” proof. I am going to focus on the creeping along proof.
Heine-Borel Theorem. A subset is compact if and only if it is closed and bounded.
To do some creeping we need to collect some useful facts.
Fact 1. A subset is bounded if and only if is contained in some closed interval
Fact 2. The set is complete (as a linear order) because every non-empty set with an upper bound has a least upper bound, called .
Fact 3. Closed subsets of compact subsets of are in fact themselves compact. With fact 1 this means that it is enough to show that closed and bounded intervals in are compact. (In general closed subsets of compact spaces are compact.)
So now let us creep:
Continue reading Creeping Along