Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2022/ii/paper-4/18h/a/solution

By Dirichlet theorem on primes in arithmetic progressions, choose a prime with
The cyclotomic field is Galois over , and
where the last group is cyclic because the multiplicative group of a finite field is cyclic.
The cyclic group has a unique subgroup of order . By the Galois correspondence, its fixed field satisfies
Normality follows because lies in an abelian group. Thus every occurs as the Galois group of a finite extension of . This is the cyclic Galois extension of the rational numbers of every finite degree.

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