# Gauss-Wantzel theorem, Fermat primes and solvability of S_n [closed]

Gauss-Wantzel theorem asserts that a polygon with $n$ sides is constructible if and only if $n$ is a product of a power of $2$ and distinct prime Fermat numbers, where the Fermat number of index $k$ is $F_{k}=2^{2^k}+1$. Conjecturally, the only prime Fermat numbers are those with index $k\leq 4$.

Kinda coincidentally, those values of $k$ are also those for which the symmetric group of index $k$ and order $k!$ is solvable. My question is thus: is it really a coincidence? Or can we exhibit a rather natural structural similarity between a sequence of constructible polygons the order (as number of sides) of each dividing the order of the next one and a sequence of subgroups of $S_4$ to show that there is no prime Fermat number of index at least $5$? In other words, can we reasonably expect to prove in a Galois theoretic framework that $F_{k}$ for $k$ positive is prime if and only if the general polynomial equation of degree $k$ is solvable by radicals?

Edit: As $S_{4}$ is solvable, there exists a normal series with abelian factor groups $\{e\}=G_{0}<...<G_{4}=S_{4}$. The idea is to map $G_{i}$ to a product $N_{i}$ of $n+1$ Fermat primes such that $N_{i}\mid N_{i+1}$.

## closed as unclear what you're asking by Lucia, GH from MO, paul garrett, Yemon Choi, Chris GodsilAug 16 '15 at 18:41

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• "Is it really a coincidence?" Yes. – user9072 Aug 16 '15 at 16:37
• Can you please tell me why? – Sylvain JULIEN Aug 16 '15 at 16:42
• I'm voting to close this question as off-topic because it seems speculative without any basis – Yemon Choi Aug 16 '15 at 17:57
• Other symmetric groups are not solvable by well understood reasons which have nothing common with the reasons why Fermat's numbers $F_5$, $F_6$, $\dots$, appear to be compisite. To best of my knowledge the reasons to believe that other Fermat's numbers are composite are probabilistic, not algebraic. – Fedor Petrov Aug 16 '15 at 18:34

There are five Fermat primes but only four solvable symmetric groups, so I'm afraid there is not even a coincidence here.

• But what are the chances that two small integers would differ by exactly $1$? :) – paul garrett Aug 16 '15 at 18:26
• It depends on whether count $S_0$ or not (this if exists should have $0!$ elements). – Fedor Petrov Aug 16 '15 at 18:36
• @FedorPetrov Of course it exists! It is the group of self-bijections of the empty set. – მამუკა ჯიბლაძე Aug 16 '15 at 21:24