Academic-turned-teacher (but still doing research as a hobby), interested in mathematical logic - specifically, computability theory and reverse mathematics, set theory, and abstract model theory. I'm also interested in other Nifty Things, in mathematics and elsewhere.
Since the SE system only allows temporary bounties, I am here informally "perma-bountying" the following ten questions, each for 500 points:
Geometry and topology:
Descriptive set theory: