PrerequisiteBasic algebraic topology and number theoryAbstractI will talk about the basics of cohomology of arithmetic groups focusing on Harder's new book. I will start from simplest cases of the group SL_2 over Z (which covers the chapter 2 and 3 of the book of Harder) and try to be as concrete as possible. The goal is to under these cohomology groups from the point of view of geometry and ar...
IntroductionThe stable motivic homotopy categories of schemes are nice ∞-categories, which admit a whole six-functor formalism. For a smooth morphism of schemes f: X→Y over a base S, the usual pullback functor f*: SH(Y)→SH(X) admits a left adjoint f_♯, the additive pushforward; they are packaged in a functor from an ∞-category of spans.Recently, Bachmann and Hoyois discovered a multiplicat...