Disclaimer: Many of these notes were hurridly written for my students,
to give them a very rough idea of how certain theorems are proved. Thus, some
of the writeups may have minor errors.
(course notes) I thank Seva Lev for making some edits to this
note. Seva himself has a writeup of Gowers's proof of the
Balog-Szemeredi theorem, and his website can be found
here .