Interactions between Measure, Category, and Descriptive Set Theory
I'm a bit of a sucker for these, really...
During my PhD I studied/collected a bunch of proofs, tricks, techniques that involve the use of measure theory, Baire category, and a bit of ergodic theory in descriptive set theory (and vice versa!). This started out as materials I needed for analysis in my philosophical projects, but it turns out that once you see enough of these, you start to prove things yourself! Who woulda thought…