Paper 2, Section II, I

Galois Theory
Part II, 2013

For a positive integer NN, let Q(μN)\mathbb{Q}\left(\boldsymbol{\mu}_{N}\right) be the cyclotomic field obtained by adjoining all NN-th roots of unity to Q\mathbb{Q}. Let F=Q(μ24)F=\mathbb{Q}\left(\boldsymbol{\mu}_{24}\right).

(i) Determine the Galois group of FF over Q\mathbb{Q}.

(ii) Find all N>1N>1 such that Q(μN)\mathbb{Q}\left(\boldsymbol{\mu}_{N}\right) is contained in FF.

(iii) List all quadratic and quartic extensions of Q\mathbb{Q} which are contained in FF, in the form Q(α)\mathbb{Q}(\alpha) or Q(α,β)\mathbb{Q}(\alpha, \beta). Indicate which of these fields occurred in (ii).

[Standard facts on the Galois groups of cyclotomic fields and the fundamental theorem of Galois theory may be used freely without proof.]