“…Recently Yen determined their Betti numbers [139], using methods which involve knowledge of the representations of the groups (somewhat in the spirit of §12) and many additional pieces of information, in particular a conjectured formula of G. Hirsch about fibre bundles, when bundle and fibre have the homology structure of products of spheres (as Lie groups do, by Hopfs theorem). The dimensions of the primitive cocycles for the five groups are: (3,11), (3,11,15,23), (3,9,11,15,17,23), (3,11,15,19,23,27,35), (3,15,23,27,35,39,47,59). In the meantime Chevalley and Weil [26] have found a totally different approach to the whole problem, which made the computations for all groups much simpler.…”