OFFSET
1,4
COMMENTS
Also the number of polyominoes on n points, up to isomorphism of the adjacency graph.
Also the number of "grid graphs" (finite induced subgraphs of the integer lattice using only horizontal and vertical edges) on n vertices.
LINKS
P. Christopher Staecker, Some enumerations of binary digital images, arXiv 1502.06236 [math.CO], 2015.
Baoming Tang, Ehsan Khatami and Marcos Rigol, A short introduction to numerical linked-cluster expansions, Computer Physics Communications, 184 (2013), 557-564; arXiv:1207.3366 [cond-mat.stat-mech], 2012.
EXAMPLE
a(4) = 3: (1) the square tetromino has the 4-cycle as its "adjacency graph"; (2) the T-tetromino corresponds to the claw graph; (3) the straight, skew, and L-tetromino all correspond to the 4-path.
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
P. Christopher Staecker, Feb 24 2015
EXTENSIONS
a(13)-a(16) from Giovanni Resta, Apr 10 2020
a(17) from Tang et al. added by Andrey Zabolotskiy, Oct 23 2022
STATUS
approved