In the comments to this post, David MacIver provides an alternative, registration-free link to Tim Chow’s You Could Have Invented Spectral Sequences.
I poked around Tim Chow‘s site, and found two other interesting articles (in the form of old sci.math.research posts):
- Forcing for Dummies. Forcing is the technique invented by Paul Cohen to prove the independence of the Continuum Hypothesis.
- What is Class Field Theory? Class field theory describes the extensions of a field with abelian Galois group.