# When $\sigma\in S_n$ belongs to the Galois group of a separable polynomial

Let $F$ be a ﬁeld, and let $f(x) ∈ F[x]$ be a polynomial with only simple roots. Let $E$ be a splitting ﬁeld for $f(x)$. Consider the Galois group $G_f$ of the polynomial $f(x)$.

Now I need to prove $G_f$ is the set of permutations $σ ∈ S_n$ that take every $F$-algebraic relation between the roots of $f(x)$ to another $F$-algebraic relation between the roots.

That is: I need to show that:

A permutation $σ ∈ S_n$ comes from an automorphism in $Gal(E/F)$ $\Leftrightarrow$ $(\forall g(x_1,...,x_n) ∈ F[x_1,...,x_n] )(g(α_1,...,α_n) = 0\Rightarrow g(α_{σ(1)},...,α_{σ(n)}) = 0)$.

(Here $\alpha_1,...,\alpha_n$ are all the roots of $f(x)$).

I am considering the following track for one direction:

We have a unique $F$-algebra homomorphism $ψ: F[x_1,...,x_n] → E$ defined by: $x_i \mapsto α_i$.

For $σ ∈ S_n$. Let $σ'$ denote the unique automorphism of the $F$-algebra $F[x_1,...,x_n]$ defined by $x_i \mapsto x_{σ(i)}$.

Take the composition $ψ ◦ σ'$. It is a homomorphism $ϕ: F[x_1,...,x_n] → E$. Using the universal property of quotients, may be we can prove that there is an automorphism $σ_0 ∈ Gal(E/F)$ such that $ψ ◦ σ' = σ_0 ◦ ψ$ and we also have that the permutation of the roots of $f(x)$ induced by $σ_0$ is actually the same as our original permutation $σ$.

And then I am a bit confused. So may I please ask for a way if we can acutally do it? And also how to deal with the other direction? Thanks a lot!

Let $K =F(\alpha_1,\ldots,\alpha_n)$ be the splitting field of an irreducible polynomial $f \in F[x]$ whose roots are $\alpha_1,\ldots,\alpha_n$. We have an isomorphism $$\varphi : F[x_1,\ldots,x_n]/I \to K, \qquad \varphi(p(x_1,\ldots,x_n)+I)= p(\alpha_1,\ldots,\alpha_n)$$ where $I$ is the ideal $$I = \{ p \in F[x_1,\ldots,x_n], p(\alpha_1,\ldots,\alpha_n) = 0\}$$
If and only if $\sigma \in Gal(K/F)$, we have that $\sigma$ induces an automorphism of $K$ permutating the $\alpha_i$, so that $\varphi^{-1}\circ\sigma \circ\varphi$ induces an automorphism of $F[x_1,\ldots,x_n]/I$ permuting the $x_i$.
• @PropositionX Your question is about those automorphisms of $F[x_1,\ldots,x_n]$ permuting the $x_i$ and fixing $I$, ie. about the automorphisms of $F[x_1,\ldots,x_n]/I$ permuting the $x_i$. – reuns May 24 '17 at 17:06
• MayI I please ask why the map $\phi$ is an isomorphism? – PropositionX May 25 '17 at 0:02
• @PropositionX In the same way. $F[\alpha_1,\alpha_2] = F[\alpha_1](\alpha_2) = F(\alpha_1)(\alpha_2) = F(\alpha_1,\alpha_2)$. Otherwise take any $b \in F[\alpha_1,\alpha_2]$ and note there is a $F$-linear dependence between the powers of $b$, ie $b^{-1} = \sum_{m=0}^d c_m b^m \in F[\alpha_1,\alpha_2]$ – reuns May 25 '17 at 1:23