Cyclotomic field

From Citizendium, the Citizens' Compendium

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 .

Contents 1 Ring of integers 1.1 Unit group 1.2 Class group 2 Splitting of primes 3 Galois group 4 References

Ring of integers

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

Unit group

Class group

Splitting of primes

A prime p ramifies iff p divides n. Otherwise, the splitting of p depends on the factorisation of the polynomial 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 , which is a factor of . Since the powers of ζ are the roots of the latter polynomial, F is a splitting field for and hence a Galois extension. The Galois group is isomorphic to the multiplicative group, via

References

• A. Fröhlich; M.J. Taylor (1991). Algebraic number theory. Cambridge University Press. ISBN 0-521-36664-X. 
• Serge Lang (1990). Cyclotomic Fields I and II, Combined 2nd edition. Springer-Verlag. ISBN 0-387-96671-4. 
• Pierre Samuel (1972). Algebraic number theory. Hermann/Kershaw. 
• I.N. Stewart; D.O. Tall (1979). Algebraic number theory. Chapman and Hall. ISBN 0-412-13840-9. 
• Lawrence C. Washington (1982). Introduction to Cyclotomic Fields. Springer-Verlag. ISBN 0-387-90622-3.