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Dec 22, 2022 at 16:34 vote accept Marsault Chabat
Dec 22, 2022 at 15:44 comment added David Loeffler Some people are never satisfied! "How do I know whether I should consider classical cohomology or completed cohomology" -- how do you know when you should use a screwdriver or a hammer? You use the tool that's appropriate to the problem you're trying to solve, that's how. (PS: You might want to look up the meaning of the word "cuspidal" in the context of smooth representation theory; it does not mean what you seem to think it means.)
Dec 22, 2022 at 10:53 comment added Marsault Chabat (Sorry my comments are a bit long...) If I see things correctly then these two representations are $Hom(\rho,H)$ and $Hom(\rho,\tilde{H})$ (I think the second one as been constructed by Emerton) so how do you deal with that? And what is this second representation? I think that's why I was asking for examples, because in everyday life how do I know whether I should consider classical cohomology or completed cohomology (which is, you might know, something that gets happened in my research)
Dec 22, 2022 at 10:44 comment added Marsault Chabat Thanks for this great answer David. Something I'm struggling with is local-global compatibility, in fact with your answer I know what's my problem (so thanks again for always helping on MO, it's really helpful). Here is what I am not comfortable with (I stay in the $\mathbf{Gl}_{2}$ case). Given a global Galois representation, we can associate (under assumption) a cuspidal representation. But know that there are two possibilities for the case "l=p", we can locally associate two representations, and therefore (with restricted tensor product) two global representations.
Dec 20, 2022 at 21:19 history answered David Loeffler CC BY-SA 4.0