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Jan 6, 2011 at 15:48 vote accept David Hansen
Jan 5, 2011 at 16:40 comment added Emerton No, it is not obvious, and therein lies the difficulty.
Jan 5, 2011 at 16:24 comment added David Hansen Okay, but don't you get Eisenstein cohomology classes in this manner? Is it obvious that they're congruent modulo $\ell$-powers to cuspidal classes (which is what one would hope for in order to use pseudo-representations)? Thanks for your patience; I feel I'm missing something fundamental here.
Jan 4, 2011 at 19:32 comment added Emerton If one constructs a quasi-split $GU(2,2)$ from the quad. imag. field $K$, then the hyperbolic $3$-manifolds that arise as congruence quotients for $\mathrm{GL}_2(K)$ appear in the Borel--Serre boundary of the Shimura varieties for $GU(2,2)$.
Jan 4, 2011 at 19:22 comment added David Hansen Ah, interesting! What, precisely, is the natural way of moving to this Shimura variety? The form $\pi \otimes \pi^{\sigma}$ is conjugate self-dual, so should transfer, but it's not obvious to me that you could easily recover $\rho_{\pi}$ from a tensor product like this. Or am I on the wrong track?
Jan 3, 2011 at 4:08 history answered Emerton CC BY-SA 2.5