The Nil geometry is one of the eight 3-dimensional Thurston geometries, which can be derived from W. Heisenberg's real matrix group.
Table of n, a(n) for n=1..9.
Jeno Szirmai, On lattice coverings of Nil space by congruent geodesic balls, May 10, 2011.
Sequence in context: A084477 A046589 A072562 * A143311 A016696 A127470
Adjacent sequences: A190474 A190475 A190476 * A190478 A190479 A190480
Jonathan Vos Post, May 10 2011