The previous lecture in this series is here. The next lecture in this series is here.
After discussion of Ramanujan graphs, Prof. Zhao discusses one of his favorite topics: the regularity method for sparse graphs and its application to the celebrated Green-Tao theorem that the primes contain arbitrarily long arithmetic progressions.
These videos are of a lecture course by Yufei Zhao at the Massachusetts Institute of Technology in 2019, and made available as part of its OpenCourseWare initiative. The website for the course may be found here.