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Let α be a complex number with α^2 = √3 − √5. Prove that Q(α)/Q is...

Let α be a complex number with α^2 = √3 − √5. Prove that Q(α)/Q is not Galois.

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Answer #1

Since Q(a)/Q is a finite (hence algebraic) extension of a field of characteristic zero, it is separable.

Here is a way to see that K=Q(a)K is not Galois. If K were Galois, then K would be invariant under any embedding K→C. However, the field K has real embeddings and complex embeddings.

In general, a Galois extension K/Q must have all of its embeddings be either real or complex, not a mix of the two. You can tell how many real embeddings K has by counting the number of real roots of the polynomial of a primitive element. In your example, the polynomial which alpha satisfies is

x8-16x4+4 has real roots and complex roots , so adjoining a root of this polynomial to Q does not give a Galois extension.

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