BG = K(G,1) for discrete G; principal G-bundles classified by maps to BG. Milnor-construction E×G/G. Cohomology H*(BG,F_p) = mod-p cohomology of group. Foundation of group-cohomology + gauge-theory.
BG = K(G,1) for discrete G; principal G-bundles classified by maps to BG. Milnor-construction E×G/G. Cohomology H*(BG,F_p) = mod-p cohomology of group. Foundation of group-cohomology + gauge-theory.