On the maximal unramified pro-2-extension of ℤ₂-extensions of certain real quadratic fields II
For the cyclotomic -extension of an imaginary quadratic field , we consider the Galois group of the maximal unramified pro--extension over . In this paper, we give some families of for which is a metabelian pro--group with the explicit presentation, and determine the case that becomes a nonabelian metacyclic pro--group. We also calculate Iwasawa theoretically the Galois groups of -class field towers of certain cyclotomic -extensions.
Given a number field Galois over the rational field , and a positive integer prime to the class number of , there exists an abelian extension (of exponent ) such that the -torsion subgroup of the Brauer group of is equal to the relative Brauer group of .
We prove lower and upper bounds for the class numbers of algebraic curves defined over finite fields. These bounds turn out to be better than most of the previously known bounds obtained using combinatorics. The methods used in the proof are essentially those from the explicit asymptotic theory of global fields. We thus provide a concrete application of effective results from the asymptotic theory of global fields and their zeta functions.