The hexagonal lattice is the familiar 2-dimensional lattice in which each point has 6 neighbors. This is sometimes called the triangular lattice.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
P. G. Watson, in C. Domb and M. S. Green, Phase Transitions and Critical Phenomena, Ac. Press 1972, Vol. 2, p. 136.
Table of n, a(n) for n=1..7.
G. Nebe and N. J. A. Sloane, Home page for hexagonal (or triangular) lattice A2
Sequence in context: A197424 A306094 A018217 * A295411 A218516 A119579
Adjacent sequences: A003485 A003486 A003487 * A003489 A003490 A003491
N. J. A. Sloane.