Notes I’ve written from time to time.

I participate in the Cambridge Advanced Math Study Group meetup. Right now, there are three “streams”:

- Set Theory.
- General Relativity. No page for this yet.
- Lie Groups. No page for this yet, but some notes.

**Logic and Set Theory**

- Scott’s Model of the Lambda Calculus. These are based on some notes I wrote up in the early 1970s, for a seminar on an early version of Dana Scott’s model of the
*untyped*lambda calculus. These notes have nothing to say about Scott’s subsequent work on the typed lambda calculus, and the general notion of types. (Post) - Algorithmic Information Theory. I wrote these notes sometime in the early 90s, based mostly on Chaitin’s book
*Algorithmic Information Theory*. (Post) - Two items on recursion theory: Friedberg’s Enumeration without Duplication (post) and Simple Sets (post).
- John Baez and I have been having a conversation about non-standard models of Peano arithmetic (PA). I have converted the story so far into pdf documents, formatted both for a small screen and a medium-sized one. I will update these from time to time (but not
*that*frequently). - We’ve also been having a conversation about first-order categorical logic.

**Other Stuff**

- I participated in a meetup group that went through Leinster’s Basic Category Theory. I wrote up solutions to many exercises, plus supplementary material.
- Two notes on Stirling’s formula. First, easy derivations of parts of the formula, culminating in a determination of
*n*! to within a factor of √2. Second, some notes on Ahlfors’ derivation (in the 2nd edition of his*Complex Analysis*). - A post on Epstein relativity diagrams, formatted for small and medium screens.