Novikov-Shubin invariants for arbitrary group actions and their   positivity
  We extend the notion of Novikov-Shubin invariant for free G-CW-complexes of finite type to spaces with arbitrary G-actions and prove some statements about their positivity. In particular we apply this to classifying spaces of discrete groups. 
Wolfgang Lueck (Muenster), Holger Reich (Duesseldorf), Thomas Schick   (Goettingen)