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Prime numbers, diophantine equations, diophantine approximations, analytic or algebraic number theory, arithmetic geometry, Galois theory, transcendental number theory, continued fractions

13 votes
Accepted

Is every closed subgroup of $\text{GL}_n(K[[x]])$ finitely generated?

This question has a negative answer is many respects. Firstly, there are simple constructions in the commutative case. Namely, the additive group $K[\![x]\!]$ is an infinite dimensional $\mathbb F_p …
7 votes
Accepted

Compactness of adelic quotients for unipotent groups over global fields

You don't mention if $U$ is assumed to be smooth or connected, but it doesn't matter. In general, if $H$ is any affine group scheme of finite type over a global field $K$ and if $H$ does not contain $ …
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6 votes
Accepted

Chinese Remainder Theorem backwards

Since $h_k := \prod_{i=1}^k \phi_{p^i} = (X^{p^k}-1)/(X-1)$, you are trying to compute the exponent of the cokernel of the inclusion $$\mathbf{Z}[X]/(h_k) \rightarrow \prod_{i=1}^k \mathbf{Z}[\zeta_{ …
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6 votes
Accepted

Theorem 7b of Serre's "Propriétés galoisiennes des points d'ordre fini des courbes elliptiques"

This is ultimately an application of Lang's vanishing theorem for degree-1 Galois cohomology of connected algebraic groups over finite fields (applied to tori). What follows may look complicated if y …
5 votes

Openness of finite index subgroups of $\mathrm{GL}_n(\prod O_v)$

In view of Laurent's answer, one may ask more generally for a criterion on a connected reductive $K$-group $G$ to ensure that for all compact open subgroups $U$ of $G(\mathbf{A}_K^S)$ (for a finite se …
4 votes

Absolutely irreducible p-adic representation of the absolute Galois group of Q_p

One gets loads of crystalline counterexamples using $p$-divisible groups (and more specifically from any elliptic curve with supersingular reduction). Let $k$ be a perfect field of characteristic $ …
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