An alternative approach to Kida and Ferrero's computations of Iwasawa λ-invariants

Abstract

We prove a slight generalization of Iwasawa's ‘Riemann–Hurwitz’ formula for number fields and use it to generalize Kida and Ferrero's well-known computations of Iwasawa λ-invariants for the cyclotomic Z2-extensions of imaginary quadratic number fields. In particular, we show that if p is a Fermat prime, then similar explicit computations of Iwasawa λ-invariants hold for certain imaginary quadratic extensions of the unique subfield k⊂Q(ζp2) such that [k:Q]=p. In fact, we actually prove more by explicitly computing cohomology groups of principal ideals. The computation of lambda invariants obtained is a special case of a much more general result concerning relative lambda invariants for cyclotomic Z2-extensions of CM number fields due to Yûji Kida. However, the approach used here significantly differs from that of Kida, and the intermediate computations of cohomology groups found here do not hold in Kida's more general setting.