On 2-Groups as Galois Groups

Abstract

LetL/Kbe a finite Galois extension in characteristic ≠ 2, and consider a non-split Galois theoretical embedding problem overL/Kwith cyclic kernel of order 2. In this paper, we prove that if the Galois group ofL/Kis the direct product of two subgroups, the obstruction to solving the embedding problem can be expressed as the product of the obstructions to related embedding problems over the corresponding subextensions ofL/Kand certain quaternion algebra factors in the Brauer group ofK. In connection with this, the obstructions to realising non-abelian groups of order 8 and 16 as Galois groups over fields of characteristic ≠ 2 are calculated, and these obstructions are used to consider automatic realisations between groups of order 4, 8 and 16.