Overconvergent modular forms are highest weight vectors in the Hodge-Tate weight zero part of completed cohomology

Abstract

We construct a $({\mathfrak{gl}}_2,B({\mathbb{Q}}_p))$ and Hecke-equivariant cup product pairing between overconvergent modular forms and the local cohomology at $0$ of a sheaf on ${\mathbb{P}}^1$, landing in the compactly supported completed ${\mathbb{C}}_p$-cohomology of the modular curve. The local cohomology group is a highest-weight Verma module, and the cup product is non-trivial on a highest weight vector for any overconvergent modular form of infinitesimal weight not equal to $1$. For classical weight $k\ge 2$, the Verma has an algebraic quotient $H^1({\mathbb{P}}^1,\mathcal{O}(-k))$, and on classical forms the pairing factors through this quotient, giving a geometric description of "half" of the locally algebraic vectors in completed cohomology; the other half is described by a pairing with the roles of $H^1$ and $H^0$ reversed between the modular curve and ${\mathbb{P}}^1$. Under minor assumptions, we deduce a conjecture of Gouvea on the Hodge-Tate-Sen weights of Galois representations attached to overconvergent modular forms. Our main results are essentially a strict subset of those obtained independently by Lue Pan in arXiv:2008.07099, but the perspective here is different and the proofs are short and use simple tools: a Mayer-Vietoris cover, a cup product, and a boundary map in group cohomology.

25 pages. v3 - expanded and clarified some proofs / background. Close to final journal version