Residually Finite Lattices in PU(2,1)˜ and Fundamental Groups of Smooth Projective Surfaces

This paper studies residual finiteness of lattices in the universal cover of PU(2 , 1) and applications to the existence of smooth projective varieties with fundamental group a cocompact lattice in PU(2 , 1) or a finite covering of it. First, we prove that certain lattices in the universal cover of PU(2 , 1) are residually finite. To our knowledge, these are the first such examples. We then use residually finite central extensions of torsion-free lattices in PU(2 , 1) to construct smooth projective surfaces that are not birationally equivalent to a smooth compact ball quotient but whose fundamental group is a torsion-free cocompact lattice in PU(2 , 1).