The evolution of the clustering of QSOs
  The evolution of QSO clustering is investigated with a new sample of 388 QSOs with 0.3<z<2.2, B<20.5 and M_B<-23. Evidence is found for an increase of the clustering amplitude with increasing redshift. These measurements allow to further distinguish among the various physical scenarios proposed to interpret the QSO phenomenon. A single population model is inconsistent with the observations. The general properties of the QSO population would arise naturally if quasars are short-lived events connected to a characteristic halo mass ~5x10^{12} M_sun. 
F. La Franca (Universita` Roma Tre), P. Andreani and S. Cristiani   (Universita` di Padova)