The weight reduction of mod pp Siegel modular forms for GSp4GSp_4

In this paper we investigate the (classical) weights of mod $p$ Siegel modular forms of degree 2 toward studying Serre's conjecture for $GSp_4$. We first construct various theta operators on the space of such forms a la Katz and define the theta cycles for the specific theta operators. Secondly we study the partial Hasse invariants on each Ekedahl-Oort strata and their local behaviors. This enable us to obtain a kind of weight reduction theorem for mod $p$ Siegel modular forms of degree 2 without increasing level.Comment: This is an extended version of my previous article entitled as "The weight in Serre's conjecture for $GSp_4$". Lots of revisions have mad

On the class numbers of the fields of the pnp^n-torsion points of elliptic curves over Q\mathbb{Q}

Let $E$ be an elliptic curve over $\mathbb{Q}$ which has multiplicative reduction at a fixed prime $p$. For each positive integer $n$ we put $K_n:=\mathbb{Q}(E[p^n])$. The aim of this paper is to extend the author's previous our results concerning with the order of the $p$-Sylow group of the ideal class group of $K_n$ to more general setting. We also modify the previous lower bound of the order and describe the new lower bound in terms of the Mordell-Weil rank of $E(\mathbb{Q})$ and the ramification related to $E$.Comment: 16 pages. We refined the main theore

An explicit computation of p-stabilized vectors

In this paper, we give a method to compute p-stabilized vectors in the space of parahori-fixed vectors for connected reductive groups over p-adic fields. The application to the global setting is also discussed. In particular, we give an explicit p-stabilized form of a Saito-Kurokawa lift.Comment: 21 page