The Space of Actions, Partition Metric and Combinatorial Rigidity
  We introduce a natural pseudometric on the space of actions of $d$-generated groups, such that the zero classes are exactly the weak equivalence classes and the metric identification with respect to this pseudometric is compact.   We analyze convergence in this space and prove that every class contains an action that properly satisfies every combinatorial type condition that it satisfies with arbitrarily small error. We also show that the class of every free non-amenable action contains an action that satisfies the measurable von Neumann problem.   The results have analogues in the realm of unitary representations as well. 
Miklos Abert and Gabor Elek
math.FA math.GR