We investigate possible orders of reductions of a point in the Mordell-Weil groups of certain abelian varieties and in direct products of the multiplicative group of a number field. We express the result obtained in terms of divisibility sequences.
Lafforgue has proposed a new approach to the principle of functoriality in a test case, namely, the case of automorphic induction from an idele class character of a quadratic extension. For technical reasons, he considers only the case of function fields and assumes the data is unramified. In this paper, we show that his method applies without these restrictions. The ground field is a number field or a function field and the data may be ramified.
While trying to understand the methods and the results of [3], especially in Section 2, we stumbled on an identity (*) below, which looked worth recording since we could not locate it in the literature. We would like to thank Dinesh Thakur and Dipendra Prasad for their comments.
We remark that Tate’s algorithm to determine the minimal model of an elliptic curve can be stated in a way that characterises Kodaira types from the minimum of . As an application, we deduce the behaviour of Kodaira types in tame extensions of local fields.