I was feeling a bit rusty on my field theory, and I was reviewing out of McCarthy's excellent book, *Algebraic Extensions of Fields*. Out of Chapter 1, I was able to work out everything "left to the reader" or omitted except for one corollary, stated without proof ([see here](http://books.google.com/books?id=dC5HvUrfy44C&lpg=PP1&pg=PA19#v=onepage&q=&f=false) for the page in the book):



> 
> Let $K/k$ be a finite normal
>  extension. Then $K$ can be obtained by
>  a purely inseparable extension,
>  followed by a separable extension.
> 
> 
> 


The text immediately preceding this implies that the intermediate field that's going to make this happen is $F=\{a\in K:\sigma(a)=a$ for all $\sigma\in Gal(K/k)\}$, and I understand his argument as to why $F/k$ is purely inseparable (in fact, that's the theorem, Theorem 21, which this is a corollary to). What I don't understand is why $K/F$ is separable; I don't see how we've ruled out it being non-purely inseparable.


Note that I will be making a distinction between *non-purely inseparable* (inseparable, but not purely inseparable) and *not purely inseparable* (either separable or non-purely inseparable).


Here are some observations / my general approach:


* One big thing that seemed promising was Theorem 11 (at the bottom of [this page](http://books.google.com/books?id=dC5HvUrfy44C&lpg=PP1&pg=PA13#v=onepage&q=&f=false)), which is basically the reverse of the corollary I'm having trouble with:



> 
> Let $K$ be an arbitrary algebraic extension of $k$. Then $K$ can be obtained by separable extension followed by a purely inseparable extension.
> 
> 
> 


(the separable extension referred to is of course the separable closure of $k$ in $K$). It seems like we want to use Theorem 11 on $K/F$, and argue that there can't be "any more" pure inseparability, but I couldn't figure out a way of doing this.


* Theorem 21 is actually an "if and only if" (that is, $a\in K$ is purely inseparable over $k$ iff $\sigma(a)=a$ for all $\sigma\in Gal(K/k)$). Because this implies that any $a\in K$ with $a\notin F$ is not purely inseparable over $k$, we have that $F$ is the maximum (not just maximal) purely inseparable extension of $k$ in $K$.
* If any $a\in K$ were purely inseparable over $F$, by Theorem 8 ([see here](http://books.google.com/books?id=dC5HvUrfy44C&lpg=PP1&pg=PA10#v=onepage&q=&f=false)), there is some $e$ for which $a^{p^e}\in F$. But by the same theorem, since $F/k$ is purely inseparable, there is some $b$ for which $(a^{p^e})^{p^b}=a^{p^{e+b}}\in k$. Thus $a$ would be purely inseparable over $k$ by the converse (Corollary 1 to Theorem 9, [see here](http://books.google.com/books?id=dC5HvUrfy44C&lpg=PP1&pg=PA11#v=onepage&q=&f=false)), and hence be in $F$. Thus, $K$ (and any field between $K$ and $F$, besides $F$ itself) is not purely inseparable over $F$.


So, that's why I don't see how we've ruled out $K/F$ being *non-purely inseparable*. Sorry about making lots of references to the book - I'm just not sure what previously established results McCarthy intended to be used, and I wanted to point out what I saw as the important ones for people not familiar with the book. I'm sure I'm missing something obvious here. Does anyone see the last bit of the argument?



If K/k is a finite normal extension of fields, is there always an intermediate field F such that F/k is purely inseparable and K/F is separable?
Edit: My mistake, I misread your post. Here's the correct answer.


<http://books.google.com/books?id=FJmiSW1KRBAC&lpg=PP1&ots=k1ecm3FdbZ&dq=lang%20algebra&pg=PA251#v=onepage&q=&f=false>


Proposition 6.11


