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Since $\varphi(p)=p-1$ is even the p'th cyclotomic field contains some quadratic field. Hecke says that in fact every quadratic field is contained by some cyclotomic field.

What is this theorem called and how is it proved?

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up vote 17 down vote accepted

This is a special case of the Kronecker-Weber theorem, which says that any abelian extension of $\mathbb{Q}$ is contained inside some cyclotomic field; any quadratic extension of $\mathbb{Q}$ is automatically abelian. I don't believe the special case of the theorem for quadratic fields has a separate name.

However, one does not need the full power of this (very advanced) theorem. The following two steps are used in exercise 8 of Chapter 2 in Marcus's Number Fields to prove precisely the case of quadratic fields:

  1. Show that $\mathbb{Q}(\zeta_p)$ contains $\sqrt{p}$ if $p\equiv 1\bmod 4$ and $\sqrt{-p}$ if $p\equiv 3\bmod 4$. (Note: This step follows from results in the preceding chapter in Marcus about the discriminant of $\mathbb{Q}(\zeta_p)$ being $$\prod_{1\leq r<s\leq p-1}(\zeta_p^r-\zeta_p^s)^2=\left(\prod_{1\leq r<s\leq p-1}(\zeta_p^r-\zeta_p^s)\right)^2=\pm p^{p-2}$$
  2. Show that $\mathbb{Q}(\zeta_8)$ contains $\sqrt{2}$.

Obviously $\mathbb{Q}(i)$ contains $\sqrt{-1}$, so to get a cyclotomic field containing $\mathbb{Q}(\sqrt{m})$, we just need to take the compositum of the cyclotomic fields corresponding to each of $m$'s prime factors (note that we can assume $m$ is squarefree).

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This result of Gauss is very well-known in algebraic number theory. The obvious web searches will easily locate proofs and much more, e.g. here's a proof from Weintraub: Galois Theory enter image description here

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many times, I read a book just "overhead"; but there are always some interesting theorems in the book, which we do not see really with beauty. You are showing such an example to me. – Groups Oct 1 '15 at 6:04

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