class number divisibility in cyclic extensions


In this entry, the class numberMathworldPlanetmathPlanetmath of a number fieldMathworldPlanetmath L is denoted by hL.

Theorem 1.

Let F/K be a cyclic Galois extensionMathworldPlanetmath of degree n. Let p be a prime such that n is not divisible by p, and assume that p does not divide hE, the class number of any intermediate field KEF. If p divides hF then pf also divides hF, where f is the multiplicative orderMathworldPlanetmath of p modulo n.

Recall that the multiplicative order of p modulo n is a number f such that pf1modn and pm is not congruentMathworldPlanetmath to 1 modulo n for any 1m<f.

Corollary 1.

Let F/K be a Galois extension such that [F:K]=q is a prime distinct from the prime p. Assume that p does not divide hK. If p divides hF then pf divides hF, where f is the multiplicative order of p modulo q.

Note that a Galois extension F/K of prime degree has no non-trivial subextensions.

Title class number divisibility in cyclic extensions
Canonical name ClassNumberDivisibilityInCyclicExtensions
Date of creation 2013-03-22 15:07:41
Last modified on 2013-03-22 15:07:41
Owner alozano (2414)
Last modified by alozano (2414)
Numerical id 4
Author alozano (2414)
Entry type Theorem
Classification msc 11R29
Classification msc 11R32
Classification msc 11R37
Related topic IdealClass
Related topic ClassNumbersAndDiscriminantsTopicsOnClassGroups