Let $\pm \alpha, \pm \beta$ denote the roots of the polynomial $f(x)=x^4+ax^2+b\in \mathbb{Z}[x]$. Prove that $f(x)$ is irreducible over $\mathbb{Q}$ if and only if $\alpha ^2, \alpha \pm\beta$ are not elements of $\mathbb{Q}.$


Suppose $f(x)$ is irreducible. $f(x)=(x-\alpha)(x+\alpha)(x-\beta)(x+\beta)=(x^2-\alpha^2)(x^2-\beta^2)$. So, $\alpha^2$ is not in $\mathbb{Q}$. $f(x)=(x-\alpha)(x+\alpha)(x-\beta)(x+\beta)=(x^2+(\alpha-\beta)x-\alpha\beta)(x^2-(\alpha-\beta)x-\alpha\beta)$. $\alpha\beta$ is in $\mathbb{Q}$ if $b$ is a square.

How to show $\alpha\pm\beta$ are not in $\mathbb{Q}?$

  • $\begingroup$ It is an act unfriendly to aged mathematicians with failing eyesight to use both $a$ and $\alpha$ in a formula or proposition. $\endgroup$ – Lubin Apr 18 '17 at 23:03

Since your polynomial has rational coefficients you have $$2(\alpha^2+\beta^2)=x_1^2+x_2^2+x_3^2+x_4^2=(x_1+x_2+x_3+x_4)^2-2(x_1x_2+...+x_3x_4)\in \mathbb Q$$

Now, if $\alpha \pm \beta \in \mathbb Q$ then

$$(\alpha \pm \beta)^2=\alpha^2+\beta^2\pm 2\alpha \beta \in \mathbb Q$$ and hence $$\alpha \beta \in \mathbb Q$$

This shows that at least one of the factors in your factorisation is in $\mathbb Q[X]$.


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