The space of commuting elements in a Lie group and maps between classifying spaces

Let be a discrete group, and let be a compact-connected Lie group. Then, there is a map between the null components of the spaces of homomorphisms and based maps, which sends a homomorphism to the induced map between classifying spaces. Atiyah and Bott studied this map for a surface group, and showed that it is surjective in rational cohomology. In this paper, we prove that the map is surjective in rational cohomology for and the classical group except for, and that it is not surjective for with and with. As an application, we consider the surjectivity of the map in rational cohomology for a finitely generated nilpotent group. We also consider the dimension of the cokernel of the map in rational homotopy groups for and the classical groups except for.