# New directions in additive combinatorics: day 2

I forgot to write the report last night as I got carried away with some mathematical discussions with a colleague; better late than never! First, I missed two talks today, due to forgetting the time mainly when I was talking with Simeon Ball about -arcs of projective spaces. We’ve ended up doing something, and that’s what has occupied me in the last while. Anyway, Aart Blokhuis and Simeon Ball began their mini-course on “Polynomial methods in finite geometry” yesterday, beginning with blocking sets. What I took away was that Hasse derivatives do something that the standard derivatives do not, but I’m still at a bit of loss why they are so successful in capturing the information about directions determined by a function. After morning tea, Lev spoke on the state of the art on quadratic residues and difference sets. I found this harder to follow, but there were some very interesting tables on computer output where some strange things happen. Five primes come out as solutions on a test of trillions of integers. Luckily we will have the slides posted on the webpage so I can remember exactly what these computations were about. Then Stefaan De Winter gave a beautiful talk on partial difference sets, where he and his co-authors have knocked off most of Ma’s list of parameters on at most 100 vertices. This was very impressive. There’s more on this talk over at Peter Cameron’s blog.

In the afternoon, Ben Green gave the second part of his series; this time on Rusza’s results and various improvements and advances thereof. Today (the third day) he will be giving a colloquium in the mathematics department here. As I said above, I lost track after afternoon tea and missed Ken Smith and Jim Davis’s talks; but then regained my composure and attended Oktay Olmez’s talk on directed strongly regular graphs and partial geometric designs; a great finish to another fantastic day at NUS!