%0 Journal Article
%T Denominators of Eisenstein cohomology classes for GL_2 over imaginary quadratic fields
%A Tobias Berger
%J Mathematics
%D 2006
%I arXiv
%R 10.1007/s00229-007-0139-6
%X We study the arithmetic of Eisenstein cohomology classes (in the sense of G. Harder) for symmetric spaces associated to GL_2 over imaginary quadratic fields. We prove in many cases a lower bound on their denominator in terms of a special L-value of a Hecke character providing evidence for a conjecture of Harder that the denominator is given by this L-value. We also prove under some additional assumptions that the restriction of the classes to the boundary of the Borel-Serre compactification of the spaces is integral. Such classes are interesting for their use in congruences with cuspidal classes to prove connections between the special L-value and the size of the Selmer group of the Hecke character.
%U http://arxiv.org/abs/math/0606531v2