1,2

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

Stirling Chow and Frank Ruskey, Gray codes for column-convex polyominoes and a new class of distributive lattices, Discrete Mathematics, 309 (2009), 5284-5297. [From N. J. A. Sloane, Sep 15 2009]

A. R. Conway and A. J. Guttmann, On two-dimensional percolation, J. Phys. A: Math. Gen. 28(1995) 891-904.

S. R. Finch, Mathematical Constants, Cambridge, 2003, pp. 378-382.

J. E. Goodman and J. O'Rourke, editors, Handbook of Discrete and Computational Geometry, CRC Press, 1997, p. 229.

I. Jensen, Enumerations of lattice animals and trees, arXiv:cond-mat/0007239.

I. Jensen and A. J. Guttmann, Statistics of lattice animals (polyominoes) and polygons. J. Phys. A 33, L257-L263 (2000).

W. F. Lunnon, Counting polyominoes, pp. 347-372 of A. O. L. Atkin and B. J. Birch, editors, Computers in Number Theory. Academic Press, NY, 1971.

W. F. Lunnon, Counting hexagonal and triangular polyominoes, pp. 87-100 of R. C. Read, editor, Graph Theory and Computing. Academic Press, NY, 1972.

D. H. Redelmeier, Counting polyominoes: yet another attack, Discrete Math., 36 (1981), 191-203.

I. Jensen, Table of n, a(n) for n = 1..56

S. R. Finch, Klarner's Lattice Animal Constant

I. Jensen, Home page

I. Jensen, More terms

D. E. Knuth, Program

D. E. Knuth, First 47 terms

Tomas Oliveira e Silva, Enumeration of polyominoes

Eric Weisstein's World of Mathematics, Polyomino

Cf. A000105, A006746, A056877, A006748, A056878, A006747, A006749, A006884, A006885, A006877, A006878, A033492.

A006762 is another version.

Sequence in context: A057409 A141771 A001170 this_sequence A119255 A071969 A063030

Adjacent sequences: A001165 A001166 A001167 this_sequence A001169 A001170 A001171

nonn,nice

N. J. A. Sloane (njas(AT)research.att.com).

Extended to n=28 by Tomas Oliveira e Silva. Extended to n=46 by Iwan Jensen. Verified (and one more term found) by D. E. Knuth, Jan 09 2001.

Richard Schroeppel communicated Jensen's calculation of the first 56 terms, Feb 21 2005