The post is on time, for the first time in a few weeks.

• Alexander Kuznetsov and Alexander Perry, Categorical joins is a long-awaited preprint. The introduction is an excellent summary of the technical results, so I won't try to summarise it in 1 phrase, but let me point out that their theory can give a description of the subcategory $\langle\mathcal{O}_S\rangle^\perp$ inside the derived category of a generic Enriques surface $S$ as the complement of an exceptional object in the derived category of an Azumaya algebra on a stacky projective plane (which has been rooted twice in a cubic). That is a really interesting example application if you'd ask me!

• Evan Chen, The Napkin project is a book-in-progress which aims to explain what a certain topic in mathematics is all about, in slightly more space than what is available on your average napkin during a lunch break, but in significantly less than your average introductory book on that topic, spending around 10 pages per topic. It comes with amazing pictures, and interesting problems to think about.

• Computational Mathematics VRG is the course website for a course by Danny Krashen on teaching mathematics students how to program well, and deal with all the things that come with it. Nowadays many mathematicians use computers for their research, but not many received a formal training. I like the idea of this course, and if anyone wants me to do something similar at some point in the future, I hereby solemnly promise I will try to do it.