Velocity Field Statistics and Tessellation Techniques: Unbiased   Estimators of $\Omega$
  We describe two new -- stochastic-geometrical -- methods to obtain reliable velocity field statistics from N-body simulations and from any general density and velocity fluctuation field sampled at a discrete set of locations. These methods, the Voronoi tessellation method and Delaunay tessellation method, are based on the use of the Voronoi and Delaunay tessellations of the point distribution defined by the locations at which the velocity field is sampled. Adjusting themselves automatically to the density of sampling points, they represent the optimal estimator for volume-averaged quantities. They are therefore particularly suited for checking the validity of the predictions of quasi-linear analytical density and velocity field perturbation theory through the results of N-body simulations of structure formation. We illustrate the subsequent succesfull application of the two methods to estimate the bias-independent value of $\Omega$ in the N-body simulations on the basis of the predictions of perturbation theory for the $\Omega$-dependence of the moments and PDF of the velocity divergence in gravitational instability structure formation scenarios with Gaussian initial conditions. We will also shortly discuss practical and complicating issues involved in the obvious extension of the Voronoi and Delaunay method to the analysis of observational samples of galaxy peculiar velocities. 
Rien van de Weygaert (Kapteyn Institute, University of Groningen) &   Francis Bernardeau (Service de Physique Th\'eorique, C...