# How to find subfields of a non-Galois field extension?

Let $K/F$ be a finite field extension.

If $K/F$ is Galois then it is well known that there is a bijection between subgroups of $Gal(K/F)$ and subfields of $K/F$.

Since finding subgroups of a finite group is always easy (at least in the meaning that we can find every subgroup by brute-force or otherwise) this gives a nice way of finding subfields and proving they are the only ones.

What can we do in the case that $K/F$ is not a Galois extension ? that is: How can I find all subfields of a non-Galois field extension ?

• An easy answer to this would be to compute the Galois closure of $K/F$ and consider the subfields of the Galois closure which are in $K$. Would that work for you? – Patrick Da Silva Oct 14 '12 at 18:54
• @PatrickDaSilva - what if the extension is not separable ? this answer seems good for the cases of characteristic $0$ and for finite fields so it's very nice on but doesn't answer all of the problem. this is just a question of intrest, I don't need an application for something so any information is good. Thank you for your answer! – Belgi Oct 14 '12 at 18:58
• Hm. I must say I assumed the extension was separable because I don't see very often non-separable extensions. =P – Patrick Da Silva Oct 14 '12 at 19:09
• @PatrickDaSilva - quickmeme.com/meme/3rc44q :P – Belgi Oct 14 '12 at 19:19
• Very nice meme, I like it – Patrick Da Silva Oct 15 '12 at 2:40