The hexagonal lattice is the familiar 2-dimensional lattice in which each point has 6 neighbors. This is sometimes called the triangular lattice.
J. Blease, Series expansions for the directed-bond percolation problem, J. Phys. C 10 (1977), 917-924.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
Table of n, a(n) for n=1..8.
G. Nebe and N. J. A. Sloane, Home page for hexagonal (or triangular) lattice A2
Sequence in context: A014286 A000714 A090984 * A056823 A105544 A119917
Adjacent sequences: A006810 A006811 A006812 * A006814 A006815 A006816
N. J. A. Sloane.