In my book they show that if K⊂L 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=f1⋯fn 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(X−λ1)m1…(X−λq)mq where a∈K∗,mi∈N
- 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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