$L^2$-Betti numbers of totally disconnected groups and their   approximation by Betti numbers of lattices
  The main result is a general approximation theorem for normalised Betti numbers for Farber sequences of lattices in totally disconnected groups. Further, we contribute some computations and complements to the general theory of $L^2$-Betti numbers of totally disconnected groups. 
Henrik Densing Petersen, Roman Sauer, and Andreas Thom
math.GR math.AT math.OA