Tuesday, October 30, 2018

abstract algebra - Tensor product and compositum of linearly disjoint field extensions



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 KkL 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 KkL is a domain (this is true even if L/k is not algebraic).



Let {l1,,ln} be a basis of L/k then 1l1,,1ln is a basis for KkL over K. Therefore KkL is a finitely generalted K module which is also a domain. Then it must indeed be a field.


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