Skip to main content
2 of 4
deleted 8 characters in body

Where am I going wrong in this interpretation of 1-dimensional geometric class field theory?

I posted this on MSE, but didn't get any responses, so I'm reposting here. I tried to write down an example of the main theorem of geometric class field theory, but I must be misunderstanding something, since my example isn't turning out right.

Main theorem. Let $K$ be a function field of a curve over a finite field. There is a bijection between unramified $\ell$-adic Galois representations of $G_K := \operatorname{Gal}(K^{sep}/K)$ and $\ell$-adic characters of $K^{\times}\backslash\mathbf{A}_K^{\times}/\mathcal{O}_K^{\times}.$ (Here I mean the ideles modulo the diagonal on the left, and on the right quotiented by the subgroup of ideles which have valuation zero at every place.)

--

I was trying to understand what this means when $K = \mathbb{F}_p(T).$ I think unramified Galois representations are the same as representations of $\operatorname{Gal}(K^{un}/K) = \operatorname{Gal}(\overline{\mathbf{F}_p}(T)/\mathbf{F}_p(T))= \hat{\mathbf{Z}}.$ So on one-side of the bijection, I have continuous $\ell$-adic characters of the profinite integers.

--

On the other side, I think that this double quotient is just $\mathbb{Z}.$ One way is by identifying it with the Picard group of $\mathbb{P}^1$, but I also can do it more directly (thanks to these two 'independent' solutions, I don't think this is the incorrect step).

--

So, I think that in this case, geometric class field theory is telling me that continuous homomorphisms $\rho : \hat{\mathbf{Z}} \to \overline{\mathbf{Q}_{\ell}}^{\times}$ are in bijection with continuous homomorphisms $\mathbf{Z}\to \overline{\mathbf{Q}_{\ell}}^{\times}.$

But this surely is incorrect -- the image of a map out of the profinite integers must be compact, but the image out of $\mathbf{Z}$ is not.

So, where have I made a mistake?