# When $X^n-a$ is irreducible over F?

Let $$F$$ be a field, let $$\omega$$ be a primitive $$n$$th root of unity in an algebraic closure of $$F$$. If $$a \in F$$ is not an $$m$$th power in $$F(\omega)$$ for any $$m\gt 1$$ that divides $$n$$, how to show that $$x^n -a$$ is irreducible over $$F$$?

• This breaks down for $n = 4$. In ${\mathbf Q}[x]$, the polynomial $x^4 + 4$ has $a = -4$, which is not a square or 4th power in ${\mathbf Q}$, but this polynomial is reducible over ${\mathbf Q}$: it is $(x^2+2x-2)(x^2-2x-2)$. This counterexample is essentially the only kind of counterexample, in light of Bill Dubuque's answer.
– KCd
Apr 19, 2012 at 0:19
• 1. You should have: $(x^2 + 2x + 2)(x^2 - 2x + 2)$. 2. This is not a counterexample to the original question, because $-4 = (2i)^2$ and $2i \in Q(i)$ and of course $2$ divides $4$. (Anyway, I've given a proof.) Apr 19, 2012 at 19:25

Below is a classical result:

Theorem $$\$$ Suppose $$\,c\in F\,$$ a field, and $$\,0 < n\in\mathbb Z$$.

$$\ \ \ x^n - c\$$ is irreducible over $$F\! \iff c \not\in F^p\,$$ for all primes $$\,p\mid n,\,$$ and $$\ c\not\in -4F^4$$ when $$\, 4\mid n$$

A proof is in many Field Theory textbooks, e.g. Karpilovsky, Topics in Field Theory, Theorem 8.1.6.

• See also Lang's Algebra. There is a whole section about the polynomial $x^n - a$ in the chapter on Galois theory.
– KCd
Apr 19, 2012 at 0:20

I will assume "$m \geq 1$", since otherwise $a \in F(\omega)$, but $F(\omega)$ is $(n-1)$th extension and not $n$th extension, so $x^n-a$ must have been reducible.

Let $b^n=a$ (from the algebraic closure of $F$).

$x^n-a$ is irreducible even over $F(\omega)$. Otherwise $$f= \prod_{k=0}^n (x-\omega^k b) = (x^p + \cdots + \omega^o b^p)(x^{n-p} + \cdots + \omega^ó b^{n-p}),$$ so $b^p$ and $b^{n-p}$ are in $F(\omega)$. Consequenty $b^{\gcd(p,n-p)}$ is in $F(\omega)$, but $\gcd(p,n-p)$ divides $n$, so $(b^{\gcd})^\frac{n}{\gcd} = a$, a contradiction.

• I don't understand how this proves it. Wouldn't you want to show if $f$ is reducible then a is an $m$th power. It looks like you showed when $f$ is reducible that $a$ is an $n$th power. Also minor thing, the upper index of your product should be $n-1$. Jul 22, 2019 at 1:40
• @WyattKuehster note it has proved that $b^{gcd}\in F$ so that proved the claim. Nov 19, 2019 at 14:58