Notes I’ve written from time to time.

**Logic and Set Theory**

- I wrote some notes for a (now defunct) Set Theory meetup.
- 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) - Three items on recursion theory: Friedberg’s Enumeration without Duplication (post) and Simple Sets (post) and Shoenfield’s computation of the complexity level of the class of recursively enumerable sets.
- 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**

- Algebraic Geometry Jottings, the story of my attempt to learn (more) algebraic geometry.
- Notes on Hatcher’s Algebraic Topology. Post.
- Notes on the wallpaper groups. Post.
- Notes on the de Smit-Lenstra article about Escher’s
*Prentententoonstelling*. Post. - Notes on Lie groups for a now defunct meetup.
- 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*). (Post) - I translated Weierstrass’s paper on the Dirichlet Principle into English from the original German. Post.
- A post on Epstein relativity diagrams, formatted for small and medium screens.
- A very short note on principal and fiber bundles (small and medium formats), and a longer one on the Laplacian on the cube. Also Three takes on the tangent bundle. Post.