Characteristic Classes for GO(2n,C)

The complex Lie group GO(2n,C) by definition consists of all complex matrices A of size 2n, such that A times transpose(A) is a non-zero scalar. In this paper we determine explicitly the singular cohomology ring of the classifying space BGO(2n,C) with mod 2 coefficients, in terms of generators and relations. The method consists of analysing a certain derivation on the cohomology ring of BO(2n) (which is a polynomial ring in the Stiefel-Whitney classes) via a Koszul complex, and using this to `solve' the Gysin sequence for the bundle BO(2n) over BGO(2n,C).