Hilbert’s 10th Problem

1. Hilbert’s Tenth Problem page
2. The cows, pigs, and chickens problem

Post Correspondence Problem

1. Ling Zhao’s site at the University of Alberta

Tiling Problems

1. Wikipedia’s page, including an example of an aperiodic set of Wang tiles
2. The Geometry Junkyard, with links to many other references on tiling problems
3. Steven Dutch’s page on aperiodic tilings
4. Aperiodic tilings for texture generation

Logical Theories

1. Boolean satisfiability
2. Turing’s proof of the undecidability of the Entscheidungsproblem
3. Presburger arithmetic
   1. Wikipedia’s page

Boustrophedon Writing

1. Absoluteastronomy’s survey

Context-Free Languages