Cyclotomic field

In mathematics, a cyclotomic field is a field which is an extension generated by roots of unity. If ζ denotes an n-th root of unity, then the n-th cyclotomic field F is the field extension $\mathbf {Q} (\zeta )$.

Ring of integers

As above, we take ζ to denote an n-th root of unity. The maximal order of F is

$O_{F}=\mathbf {Z} [\zeta ].\,$

Splitting of primes

A prime p ramifies iff p divides n. Otherwise, the splitting of p depends on the factorisation of the polynomial $X^{n}-1$ modulo p, which in turn depends on the highest common factor of p-1 and n.

Galois group

The minimal polynomial for ζ is the n-th cyclotomic polynomial $\Phi _{n}(X)$, which is a factor of $X^{n}-1$. Since the powers of ζ are the roots of the latter polynomial, F is a splitting field for $\Phi _{n}(X)$ and hence a Galois extension. The Galois group is isomorphic to the multiplicative group, $(\mathbf {Z} /n\mathbf {Z} )^{*}$ via

$a{\bmod {n}}\leftrightarrow \sigma _{a}=(\zeta \mapsto \zeta ^{a}).\,$

References

• A. Fröhlich; M.J. Taylor (1991). Algebraic number theory. Cambridge University Press. ISBN 0-521-36664-X. 
• I.N. Stewart; D.O. Tall (1979). Algebraic number theory. Chapman and Hall. ISBN 0-412-13840-9. 
• Pierre Samuel (1972). Algebraic number theory. Hermann/Kershaw.