Let L be a positive definite lattice. There are only finitely many positive definite lattices L' which are isomorphic to L modulo N for every N > 0: in fact, there is a formula for the number of such lattices, called the Siegel mass formula. In this talk, I'll review the Siegel mass formula and explain Weil's reformulation of it in terms of the volume of adelic groups.

### Rademacher Lectures

Monday, March 12, 2018 - 3:30pm

#### Jacob Lurie

Harvard University