Number/Representation Theory

Event Information Cohen-Lenstra heuristics in the Presence of Roots of Unity
14:10 on Wednesday March 22, 2017
15:00 on Wednesday March 22, 2017
BA6183, Bahen Center, 40 St. George St.
Jacob Tsimerman

University of Toronto

(joint with Lipnowski, Sawin) The class group is a natural abelian group one can associated to a number field, and it is natural to ask how it varies in families. Cohen and Lenstra famously proposed a model for families of quadratic fields based on random matrices of large rank, and this was later generalized by Cohen-Martinet to general number fields. However, their model was observed by Malle to have issues when the base field contains roots of unity. We explain that in this setting there are naturally defined additional invariants on the class group, and based on this we propose a refined model in the number field setting rooted in random matrix theory. Our conjecture is based on keeping track not only of the underlying group structure, but also certain natural pairings one can define in the presence of roots of unity. Specifically, If the base field contains roots of unity, we keep track of the class group G together with a naturally defined homomorphism G*[n]-->G from the n-torsion of the Pontryagin dual of G to G. Using methods of Ellenberg-Venkatesh-Westerland, we can prove some of our conjecture in the function field setting.