Thursday, 26 September 2013

abstract algebra - Let K and L be extensions of F. Show that KL is Galois over F if both K and L are Galois over F. Is the converse true?



Let K and L be extensions of F. Show that KL is Galois over F if
both K and L are Galois over F. Is the converse true?




I should show |Gal(KL/F)|=[KL:F] and for this I am using |Gal(K/F)|=[K:F],|Gal(L/F)|=[L:F] and [KL:F][K:F][L:F], and I get to the next but I do not know what else to do, could someone help me please? Is there a counterexample to the converse? Thank you in advance.


Answer



To prove that the converse is not true, consider the extension Q(32,ζ3) which is a Galois extension of Q, but Q(32) is not! (Edit: Take K=Q(32) and L=Q(ζ3) and F=Q) As for the direct statement, we might use splitting fields.
Now, if we want to use splitting fields, if K is a splitting field over F for f(x) and L is a splitting field of g(x) over F, then KL is a splitting field of...


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