Let k be a field and K/k and L/k be two field extensions that are linearly disjoint over k. If K and L are finite extensions then it easily follows that the compositum KL and the tensor product K⨂kL are isomorphic as fields.
My question, is this still true when K and L are infinite extensions?
Edit: It seems unlikely if neither K nor L is algebraic as pointed out below by Jyrki Lahtonen. So, let us assume that L is algebraic over k.
Answer
With the assumption that L/k is algebraic this is indeed correct. An outline of an argument is as follows:
Since L is an algebraic extension without loss of generality we may assume that it is finite (since an algebraic extension is a union of its finite subextensions). It is clear (at least to me) that K⨂kL is a domain (this is true even if L/k is not algebraic).
Let {l1,…,ln} be a basis of L/k then 1⊗l1,…,1⊗ln is a basis for K⨂kL over K. Therefore K⨂kL is a finitely generalted K module which is also a domain. Then it must indeed be a field.
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