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On the Eisenstein ideal for imaginary quadratic fields

Published online by Cambridge University Press:  01 May 2009

Tobias Berger*
Affiliation:
Department of Pure Mathematics and Mathematical Statistics, Centre for Mathematical Sciences, University of Cambridge, Cambridge CB3 0WB, UK (email: t.berger@dpmms.cam.ac.uk)
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Abstract

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For certain algebraic Hecke characters χ of an imaginary quadratic field F we define an Eisenstein ideal in a p-adic Hecke algebra acting on cuspidal automorphic forms of GL2/F. By finding congruences between Eisenstein cohomology classes (in the sense of G. Harder) and cuspidal classes we prove a lower bound for the index of the Eisenstein ideal in the Hecke algebra in terms of the special L-value L(0,χ). We further prove that its index is bounded from above by the p-valuation of the order of the Selmer group of the p-adic Galois character associated to χ−1. This uses the work of R. Taylor et al. on attaching Galois representations to cuspforms of GL2/F. Together these results imply a lower bound for the size of the Selmer group in terms of L(0,χ), coinciding with the value given by the Bloch–Kato conjecture.

Type
Research Article
Copyright
Copyright © Foundation Compositio Mathematica 2009

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