Show it's a field

[symbol0]

Let K and F be fields and R a ring such that K \subseteq R \subseteq F.
If F is algebraic over K, show R is a field.

My approach was to show that for each u \in R, u ^{-1} \in R.
Since u is algebraic over K, there is a polynomial over K with u as a root. The idea was to try to express u ^{-1} in terms of elements in R, but I couldn't make it happen.
Perhaps this was the wrong approach.
I would appreciate any suggestions.

[micromass]

You could show the following: if a is algebraic over K, then K[a] is a field.

Notation: K[a]=\{P(a)~\vert~P\in K[X]\}.

[disregardthat]

You are on to something. For u in R write u^n+k_1u^{n-1}+...+k_n = 0 where k_i are elements of K, where this polynomial is a minimal one (such that k_n \not = 0). Then you have u(u^{n-1}+k_1u^{n-2}+...+k_{n-1})=-k_n. Where does the two factors on the left live, and what does it tell you about u^{-1}?

[symbol0]

Thank you Jarle. For some reason, today the latex code is producing the wrong symbols for me.
From your equation, multiplying both sides by k_n gives us
u(....)=1 and the stuff in the parenthesis is in R since all elements are products of powers of u and k's. So the inverse is in R.

Also thank you micromass.

[disregardthat]

Thank you Jarle. For some reason, today the latex code is producing the wrong symbols for me.
From your equation, multiplying both sides by k_n gives us
u(....)=1 and the stuff in the parenthesis is in R since all elements are products of powers of u and k's. So the inverse is in R.

Also thank you micromass.

You probably mean -1/k_n and not k_n, but that's correct.

[Fredrik]

Thank you Jarle. For some reason, today the latex code is producing the wrong symbols for me.

It's been doing that every day for almost a year. You need to refresh and resend after each preview, and sometimes also after saving the changes when you edit your post.