# Galois group of a biquadratic quartic

From Hungerford, section V, chapter 4 exercise 9:

Let $x^4+ax^2+b$ in $K[x]$ (with char $K\neq$2) be irreducible with Galois group $G$.

(a) If $b$ is a square in $K$, then $G = \mathbb{Z}_2\times\mathbb{Z}_2$.
(b) If $b$ is not a square in $K$ and $b(a^2-4b)$ is a square in $K$, then $G = \mathbb{Z}_4$.
(c) If neither $b$ nor $b(a^2-4b)$ is a square in $K$, then $G = D_4$ (dihedral group).

What I've tried

With (a): Let be $u_1,u_2,u_3,u_4$ the roots of the quartic, then $b=u_1u_2u_3u_4$ is in $K$ and there exists $a\in K$ with $a^2=b$. On the other hand, $G = \mathbb{Z}_2\times\mathbb{Z}_2$ if and only if $\alpha=u_1u_2+u_3u_4$, $\beta=u_1u_3+u_2u_4$, $\gamma=u_1u_4+u_2u_3$ are in $K$, but I don't find the way to rely those results.

With (b) I know that $b=u_1u_2+u_1u_3+u_1u_4+u_2u_3+u_2u_4+u_3u_4$ but again I dont know how to continue.

Any help? Thanks.

• Not precisely the same but it has been usefull: math.stackexchange.com/questions/204709/… – Lotus Jan 24 '14 at 19:12
• Is it possible that the polynomial is meant to be a biquadratic, so it should read $$x^4+ax^2+b?$$ Then the roots come in pairs of negatives of each other. That constrains the Galois group to be a 2-group, and gives a better match with the listed alternatives. – Jyrki Lahtonen Feb 6 '14 at 22:01
• @Lotus this should help for cubics and quartics math.uconn.edu/~kconrad/blurbs/galoistheory/cubicquartic.pdf – ir7 Feb 6 '14 at 22:13
• oh sure, it was $x^4+ax^2+b$ sorry, edited – Lotus Feb 6 '14 at 22:47
• Thanks a lot you all for your comments, specially @ir7 who has post a very good link!! – Lotus Feb 6 '14 at 22:53

There are two numbers $$\alpha,\beta$$ (in some algebraic closure of $$K$$) such that the identity $$P=X^4+aX^2+b=(X^2-\alpha^2)(X^2-\beta^2)$$ holds. Then the set $$R$$ of all roots of $$P$$ is $$\lbrace \pm \alpha, \pm \beta\rbrace$$. Since $$P$$ is irreducible, $$G$$ can be identified to a transitive subgroup of $${\mathfrak S}(R)$$, the group of permutations of $$R$$.

Also, any $$\sigma\in G$$ obviously satisfies $$\sigma(-\alpha)=-\sigma(\alpha)$$ and $$\sigma(-\beta)=-\sigma(\beta)$$. The subgroup $$H$$ of permutations satisfying those two conditions consists of eight elements : in cycle notation,

$$\begin{array}{lcl} H &=\lbrace& {\sf id},(\alpha,-\alpha)(\beta,-\beta),\\ & & (\alpha,\beta)(-\alpha,-\beta),(\alpha,-\beta)(-\alpha,\beta), \\ & & (\alpha,\beta,-\alpha,-\beta), (\alpha,-\beta,-\alpha,\beta), \\ & & (\alpha,-\alpha), (\beta,-\beta) \rbrace \end{array}$$

There exactly three transitive subgroups of $$H$$, namely $$H$$ itself and

$$\begin{array}{lcl} H_1 &=&\lbrace {\sf id},(\alpha,-\alpha)(\beta,-\beta), (\alpha,\beta)(-\alpha,-\beta),(\alpha,-\beta)(-\alpha,\beta) \rbrace \\ H_2 &=& \lbrace {\sf id},(\alpha,-\alpha)(\beta,-\beta), (\alpha,\beta,-\alpha,-\beta), (\alpha,-\beta,-\alpha,\beta) \rbrace \end{array}$$

In case (a), $$(\alpha\beta)^2=b$$ is a square in $$K$$, so $$\gamma_1=\alpha\beta$$ is in $$K$$. Now if $$\tau_1=(\alpha,-\beta,-\alpha,\beta)$$, we have $$\tau_1(\gamma_1)=-\gamma_1$$, so $$\tau_1\not\in G$$ and this forces $$G=H_1$$.

In case (b), $$(\alpha\beta(\alpha^2-\beta^2))^2=b(a^2-4b)$$ is a square in $$K$$, so $$\gamma_2=\alpha\beta(\alpha^2-\beta^2)$$ is in $$K$$. Now if $$\tau_2=(\alpha,\beta)(-\alpha,-\beta)$$, we have $$\tau_2(\gamma_2)=-\gamma_2$$, so $$\tau_2\not\in G$$ and this forces $$G=H_2$$.

Finally, in case (c) we have $$\gamma_1\not\in K$$ and $$\gamma_2\not\in K$$. By the fundamental theorem of Galois theory, $$\gamma_1$$ is not fixed by all the elements of $$G$$, so $$G\neq H_2$$. Similarly $$\gamma_2$$ is not fixed by all the elements of $$G$$, so $$G\neq H_1$$. The only possibility left is then $$G=H$$.

• Observe that $\tau_1$ does not fix $\gamma_2$, so in case (b) you cannot have $G=H_2$. Instead of $\alpha \beta (\alpha-\beta)$ you probably mean $\alpha \beta (\alpha^2-\beta^2)$. Observe that $(\alpha \beta (\alpha^2-\beta^2))^2 = b(a^2-4b)$. – Martino Nov 4 '18 at 17:14
• @Martino Corrected, thanks. – Ewan Delanoy Nov 5 '18 at 5:48
• @JyrkiLahtonen Corrected, thanks. – Ewan Delanoy Nov 5 '18 at 6:27