On the cohomology of Kobayashi's plus/minus norm groups and applications

Abstract

The plus and minus norm groups are constructed by Kobayashi as subgroups of the formal group of an elliptic curve with supersingular reduction, and they play an important role in Kobayashi's definition of the signed Selmer groups. In this paper, we study the cohomology of these plus and minus norm groups. In particular, we show that these norm groups are cohomologically trivial. As an application of our analysis, we establish certain (quasi-)projectivity properties of the non-primitive mixed signed Selmer groups of an elliptic curve with good reduction at all primes above $p$. We then build on these latter projectivity results to derive a Kida formula for the signed Selmer groups, and study the integrality property of the characteristic element attached to the signed Selmer groups.