# a problem about field extension [closed]

Let K and L are fields,L is a sub field of K,and L is isomorphic to K,whether can we get K=L?If true,how to prove? Thanks.

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## closed as too localized by Andreas Blass, Franz Lemmermeyer, Emil Jeřábek, Leonid Positselski, Andres CaicedoNov 26 '12 at 6:53

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Aakumadula and Peter Mueller already answered that the answer is no. And it is far from being yes, even if you assume that $K$ and $L$ are algebraically closed. For instance, algebraically closed fields of a given characteristic are characterized by their transcendence degree. So take an algebraically closed field $K$ with infinite transcendence degree, take a transcendence basis, remove one element and call $L$ the algebraic closure of the field generated by this smaller set. Then $L$ is a strict subfield of $K$, but is isomorphic to $K$. –  ACL Nov 23 '12 at 11:20

No. ${\mathbb C}(X^2,Y)=L$ is a subfield of $K={\mathbb C}(X,Y)$ where $X,Y$ are algebraically independent variables over $\mathbb C$. Hence $L$ is isomorphic to $K$ but not equal.
What is $Y$ good for? If $F$ is any field, and $X$ a transcendental over $F$, then $L=F(X^2)$, $K=F(X)$ is an example. –  Peter Mueller Nov 23 '12 at 10:23