Let $K$ be the splitting field of $x^5-3 \in \mathbb{Q}[x]$.

We can see $K = \mathbb{Q}(3^{1/5}, \zeta_5)$ where $\zeta_5 = e^{2 \pi i/5}$, and $[K: \mathbb{Q}] = 20$. It's easy to see $\sqrt{5} \in \mathbb{Q}(\zeta_5)\subseteq K$.

Let $H= \operatorname{Gal}(K/\mathbb{Q}(\sqrt{5})$). Does $H$ have to be abelian?

Edit: The Galois group of the big extension $K/\mathbb{Q}$ is the group generated by $\sigma$ (order 5 element) and $\tau$ (order 4 element) defined by $\sigma: 3^{1/5} \mapsto 3^{1/5} \zeta_5$ and $\tau: \zeta_5 \mapsto \zeta_5^2$.

I don't know what this group is, but it's definitely not abelian since if it's abelian the extension $\mathbb{Q}(3^{1/5})/\mathbb{Q}$ is Galois.

  • $\begingroup$ You should be able to find the group of $K$ over $\bf Q$, and then find $H$ as a subgroup of that group. $\endgroup$ Jul 22, 2012 at 11:33
  • $\begingroup$ @colge Have you computed the Galois group of the big extension ? $\endgroup$
    – user38268
    Jul 22, 2012 at 11:35
  • $\begingroup$ @BenjaLim I have tried this (see my edit of the quesion) but I didn't get exact representation of this group. $\endgroup$
    – colge
    Jul 22, 2012 at 12:01
  • $\begingroup$ @colge It's the Frobenius group $F_{20}$. $\endgroup$
    – Cocopuffs
    Jul 22, 2012 at 12:09
  • $\begingroup$ You should be able to find a relation involving $\sigma$ and $\tau$, decide whether there are any elements of order 10, etc. $\endgroup$ Jul 22, 2012 at 12:25

1 Answer 1


The Galois group is of order 20, generated by $x$ of order 5 any $y$ of order 4, where $yxy^{-1}=x^{3}$. Its the Frobenius group of order 20. The subgroup of order 10 is generated by $x$ and $y^2$. It is a dihedral group.

  • 1
    $\begingroup$ If the group is abelian, how do you explain the fact that $\mathbb{Q}[3^{1/5}]/\mathbb{Q}$ is not an abelian extension, as the OP mentionned? $\endgroup$
    – M Turgeon
    Jul 22, 2012 at 15:57
  • 1
    $\begingroup$ I think you've got the dicyclic group of order 20; the Frobenius group has relation $yxy^{-1}=x^3$. $\endgroup$ Jul 23, 2012 at 4:31
  • $\begingroup$ @GerryMyerson-you are right. So the subgroup of order 10 is dihedral, not abelian. $\endgroup$ Jul 28, 2012 at 17:57

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