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$\newcommand{\Q}{\Bbb Q} \newcommand{\N}{\Bbb N} \newcommand{\R}{\Bbb R} \newcommand{\Z}{\Bbb Z} \newcommand{\C}{\Bbb C} \newcommand{\A}{\Bbb A} \newcommand{\ab}{\mathrm{ab}} \newcommand{\Gal}{\mathrm{Gal}} \newcommand{\prolim}{\varprojlim} $ It is known that every finite group is isomorphic to the Galois group of some finite extension $K / \C(T)$, using Riemann existence theorem (Remark 2.14 a) in Völklein, Groups as Galois groups, an introduction).

Is every profinite group $G$ of cardinality $2^{\aleph_0}$ isomorphic (as topological group) to the Galois group of some extension of $\C(X)$ ?

If not, what if we require these profinite groups to be abelian, or/and torsion-free?

Ideas :

Notice that not all profinite groups are Galois groups over $\C(X)$, since $\Gal(L/K) \subset L^L$ has cardinality $2^{|L|}$ (when $L$ is infinite), which is at most $2^{| \overline{\C(X)}|} = 2^{2^{\aleph_0}}$. By a work of Haran and Jarden (2000), one knows that the absolute Galois group of $\C(X)$ is the profinite completion of the (discrete) free group $F_{2^{\aleph_0}}$, which has cardinality $2^{2^{\aleph_0}}$ (I think). So I could also ask about profinite groups of such cardinality, but I'm sticking to ${2^{\aleph_0}}$ for now.

Every group of cardinality ${2^{\aleph_0}}$ is a quotient of $F_{2^{\aleph_0}}$, but I'm not sure if every profinite group of such cardinality is a topological quotient of $\widehat{F_{2^{\aleph_0}}}$ (i.e. quotient by a closed subgroup).

(An analoguous question for $\Q$ was asked here).

Watson
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  • I think the question about finding simpler / more general examples than $\frac{L(s+1/2,|\chi|)L(s-1/2,|\chi|)}{L(s+1/2,\chi)L(s-1/2,\chi)}$ of $f_n(s) = f_n(-s), F(s) = \sum_n f_n(s)$ meromorphic, $ F(s)\ne F(-s)$ would be interesting – reuns Nov 03 '18 at 22:20
  • @reuns : yes, please ask it, I would be also interested! :-) – Watson Nov 03 '18 at 22:21

1 Answers1

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The answer is yes. It is a theorem of Douady (see e.g. Theorem 3.4.8 in Szamuely's "Galois Groups and Fundamental Groups") that the absolute Galois group of $\mathbb C(X)$ is isomorphic to the free profinite group on the set $\mathbb C$. This can be deduced in purely group-theoretic way from the results you already know thanks to Riemann existence theorem, namely that for any finite set $S\subseteq\mathbb C$, the quotient parametrizing covers unramified outside $S\cup\{\infty\}$ is isomorphic to free profinite group on $S$. Analogous results hold for any algebraically closed field of characteristic zero.

Now, any profinite group with at most $2^{\aleph_0}$ elements is a quotient of a free profinite group on $2^{\aleph_0}$ generators, meaning it appears as a Galois group of an extension of $\mathbb C(X)$.

We can in fact give a precise characterization of groups which appear this way: these are those profinite groups which have a dense subset of size $2^{\aleph_0}$. I would expect not every profinite group of cardinality $2^{2^{\aleph_0}}$ to have such a subset, but I don't have a counterexample off the top of my head.

Wojowu
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  • Thanks, I essentially knew everything. The main point is to know why any profinite group with at most $2^{\aleph_0}$ elements is a quotient BY A CLOSED SUBGROUP of a free profinite group on $2^{\aleph_0}$ generators. May you expand on that? – Watson Jan 28 '22 at 08:35
  • @Watson That there is a continuous map from the free profinite group to the given group is just the universal property of free profinite groups. The kernel is then closed because it is preimage of a closed singleton. – Wojowu Jan 28 '22 at 09:33