Sunday, 7 June 2015

abstract algebra - Question about a proof about finite normal extensions




In my book they show that if KL is a finite normal extension, then L is the splitting field for some polynomial f(X)K[X].



They do so as follows:



Suppose a1,...,an is a basis for L as vector space over K, hence L=K(a1,,an). Now let fi be the minimal polynomial of ai. Since ai is a root of fi and since fi is irreducible, fi splits completely over L, hence f=f1fn also splits completely over L. Thus L is the splitting field of f(X).



Now my question. My definition in my book says that L is a splitting field of f(X) over K, if





  • f(x)=a(Xλ1)m1(Xλq)mq where aK,miN

  • L=K(λ1,,λq)



Now in the proof when f(X) splits into linear factors in L[x] it could have more roots than just a1,,an, hence according to the definition the splitting field would equal to K(a1,,an,λ1,,λp), where λ1,,λp are the remaining roots of f. Now, I wonder whether my reasoning is correct:



L=K(a1,,an)K(a1,,an,λ1,,λp)L



hence L is the splitting field.


Answer




The proof shows precisely that K(a1,,an)=K(a1,,an,λ1,,λp), which follows from the assumption that L/K is normal. If you read the proof carefully, that's exactly what it says: since L/K is normal, and one root of fi is in L, they are all in L.



Maybe, it's easier to parse if you assume that L=K(α1). Then, if f is the minimal polynomial of α1 and if α2,,αr are the remaining roots of f, then L being normal implies that K(α1)=K(α1,,αr).


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