Categorification and Decategorification

This is a first part of notes from a lecture given by Alex Hoffnung on 4.02.2009 in Goettingen. You can learn from it what (do I think) a categorification and decategorification is, and see various examples of it. Next part will deal with categorification of Hecke algebras, which was the main point of Alex’ talk. However, it’s not coming until I teach myself about how to get an algebraic group out of a Dynkin diagram, because otherwise it’s “monkey see, monkey do”. Also, thanks to Peter Arndt here in Goettingen, for clarifying me a point about pullbacks of groupoids.