Is $\Bbb Z_p^2$ a Galois group over $\Bbb Q$

extension-fieldfield-theorygalois-theoryprofinite-groups

$\newcommand{\Q}{\Bbb Q}
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\newcommand{\Z}{\Bbb Z}
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\newcommand{\ab}{\mathrm{ab}}
\newcommand{\Gal}{\mathrm{Gal}}
\newcommand{\prolim}{\varprojlim}
$

Fix a prime number $p$, and denote by $\Z_p$ the additive group of $p$-adic integers.

Why is there no Galois extension $K/\Q$ such that $\Gal(K/\Q) \cong \Z_p^2$ as topological groups?

The answer is no, according to a remark in Topics in Galois Theory (second edition, p. 16, just before §2.2), by J.-P. Serre.
By Kronecker–Weber theorem, we have $\Gal(\Q^{\ab} / \Q) \cong \widehat{\Z}^{\times}$ (since $\{\Q(\zeta_n) \mid n \geq 2\}$ is cofinal in the direct system of abelian extensions of $\Q$), so I think my question boils to prove that there is no continuous surjection $\widehat{\Z}^{\times} \to \Z_p^2$.

Notice that there does exist a continuous surjection $\widehat{\Z}^{\times} \to \Z_p$, via the projections $\widehat{\Z}^{\times} \to \Z_p^{\times} \cong (\Z / q \Z)^{\times} \times \Z_p \to \Z_p$ (where $q=p$ if $p>2$ and $q=4$ if $p=2$).

Remarks : this would show that the "infinite" inverse Galois problem is wrong. It is true that any profinite group is a Galois group over some field (Waterhouse). It is expected (Hilbert–Noether conjecture) that any finite group is a group over $\Q$ (easy for finite abelian groups, Shafarevitch theorem for solvable groups, wrong for profinite abelian groups). Fried and Kollar showed that any finite group is the automorphism group of some number field (not necessarily Galois over $\Q$).

Thank you!

Best Answer

There's indeed no continuous surjective homomorphism $\hat{\mathbf{Z}}^\times\to\mathbf{Z}_p^2$ for any prime $p$.

Indeed, we have $\hat{\mathbf{Z}}^\times\simeq\prod_p(\mathbf{Z}_p\times\mathbf{Z}/q_p\mathbf{Z})$. Any homomorphism into $\mathbf{Z}_p$ has to be trivial on $\mathbf{Z}_\ell$ for any $\ell\neq p$ and on $\mathbf{Z}/q_p\mathbf{Z}$ for all $p$. By continuity, it therefore factors through $\mathbf{Z}_p$. Since there's no surjective continuous homomorphism $\mathbf{Z}_p\to\mathbf{Z}_p^2$, we're done.

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