On lattice coverings of Nil space by congruent geodesic balls
  The Nil geometry, which is one of the eight 3-dimensional Thurston geometries, can be derived from {W. Heisenberg}'s famous real matrix group.   The aim of this paper to study {\it lattice coverings} in Nil space. We introduce the notion of the density of considered coverings and give upper and lower estimations to it, moreover we formulate a conjecture for the ball arrangement of the least dense lattice-like geodesic ball covering and give its covering density $\Delta\approx 1.42900615$.   The homogeneous 3-spaces have a unified interpretation in the projective 3-sphere and in our work we will use this projective model of the Nil geometry. 
Jen\H{o} Szirmai
math.MG