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In continuation to my last post:

In class we saw an example that says: $n=[\mathbb{F}_{p^n}:\mathbb{F}_{p}]=|\operatorname{Gal}(\mathbb{F}_{p^n}/\mathbb{F}_{p})|$ ; ($p$ is prime).

My thoughts are that if I look at any Galois extension then it is the splitting field of separable polynomials in the field I started with. I know that any automorphism of the extension field that fixes the field I started with sends each root of an irreducible factor (of one of the polynimials that the Galois expenstion is their splitting field), and also that for every such permutation I have an automorphism .

But since if an irreducable factor have $k$ roots then I can permute on them in $k!$ options I deduced that the size of Galois group is of the form $k_1!k_2!\cdots k_t!$, in particular it is either $1$ or devisable by $2$.

But this contradicts what I wrote in my first paragraph if for example we take $p=2,n=5$ since $5$ is odd.

Can someone please point out the mistake ?

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    @Belgi: Galois theory is subtle when learning it. The Dummit & Foote text has an extensive section on Galois theory (largely because Dummit was one of the authors) so you should read all the worked examples there carefully and see why each of them works.2012-05-11

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Since you don't seem to be convinced by your own example, let's look at another polynomial whose splitting field does not have the full symmetric group as its Galois group.

We take $\mathbb{Q}$ as our base field. Let $P(x) = x^3 - 3 x + 1$. For various reasons, $P$ is irreducible over $\mathbb{Q}$. Let $K$ be its splitting field over $\mathbb{Q}$, with three zeros $\alpha, \beta, \gamma$. Then, $(x - \alpha)(x - \beta)(x - \gamma) = x^3 - 3 x + 1$ so we can deduce various equations like $\alpha + \beta + \gamma = -1$. This can be done generically for any polynomial. But $P$ has a special property which gives an extra equation: $(\alpha - \beta)(\beta - \gamma)(\gamma - \alpha) = 9$ This is a polynomial equation with integer coefficients that the zeros of $P$ satisfy, so any automorphism of $K$ over $\mathbb{Q}$ must preserve this equation. But that means we cannot transpose just two of the roots, because that would invalidate the equation. So $\textrm{Gal}(K \mid \mathbb{Q})$ is the cyclic group of order 3.


Now, what is true is for any two zeros $\alpha$ and $\beta$ of a irreducible polynomial $P$, there is an automorphism $\sigma$ such that $\sigma (\alpha) = \beta$. But $\sigma$ could do anything else to any other zero – including $\alpha$. In slightly more sophisticated terms, the Galois group of the splitting field of $P$ is always a transitive subgroup of the symmetric group and acts transitively on the set of zeros of $P$. But it is not guaranteed to act freely.