%I A000228 M2682 N1072
%S A000228 1,1,3,7,22,82,333,1448,6572,30490,143552,683101,3274826,15796897,
%T A000228 76581875,372868101,1822236628,8934910362,43939164263,216651036012,
%U A000228 1070793308942
%N A000228 Number of hexagonal polyominoes (or planar polyhexes) with n cells.
%D A000228 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A000228 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 
               (includes this sequence).
%D A000228 A. T. Balaban and F. Harary, Chemical graphs V: enumeration and proposed 
               nomenclature of benzenoid cata-condensed polycyclic aromatic hydrocarbons, 
               Tetrahedron 24 (1968), 2505-2516.
%D A000228 A. T. Balaban and Paul von R. Schleyer, "Graph theoretical enumeration 
               of polymantanes", Tetrahedron, (1978), vol. 34, 3599-3609
%D A000228 M. Gardner, Polyhexes and Polyaboloes. Ch. 11 in Mathematical Magic Show. 
               New York: Vintage, pp. 146-159, 1978.
%D A000228 M. Gardner, Tiling with Polyominoes, Polyiamonds and Polyhexes. Chap. 
               14 in Time Travel and Other Mathematical Bewilderments. New York: 
               W. H. Freeman, pp. 175-187, 1988.
%D A000228 F. Harary and R. C. Read, The enumeration of tree-like polyhexes, Proc. 
               Edinb. Math. Soc. (2) 17 (1970), 1-13.
%D A000228 D. A. Klarner, Cell growth problems, Canad. J. Math., 19 (1967), 851-863.
%D A000228 J. V. Knop et al., On the total number of polyhexes, Match, No. 16 (1984), 
               119-134.
%D A000228 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 A000228 N. Trinajstich, Z. Jerievi, J. V. Knop, W. R. Muller and K. Szymanski, 
               COMPUTER GENERATION OF ISOMERIC STRUCTURES, Pure & Appl. Chem., Vol. 
               55, No. 2, pp. 379-39O, 1983.
%D A000228 Jaime Rangel-Mondragon, Polyominoes and Related Families, The Mathematica 
               Journal, 9:3 (2005), 609-640.
%H A000228 Ed Pegg, Jr., <a href="http://demonstrations.wolfram.com/PolyformExplorer/
               ">Illustrations of polyforms</a>
%H A000228 A. Clarke, <a href="http://www.geocities.com/alclarke0">Polycubes</a>
%H A000228 D. Gouyou-Beauchamps and P. Leroux, <a href="http://www.arXiv.org/abs/
               math.CO/0403168">Enumeration of symmetry classes of convex polyominoes 
               on the honeycomb lattice</a>.
%H A000228 M. Keller, <a href="http://www.solitairelaboratory.com/polyenum.html">
               Counting polyforms</a>
%H A000228 Joseph Myers, <a href="http://www.srcf.ucam.org/~jsm28/tiling/">Polyomino, 
               polyhex and polyiamond tiling</a>
%H A000228 N. J. A. Sloane, <a href="a228.gif">Illustration of initial terms</a>
%H A000228 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               Polyhex.html">Link to a section of The World of Mathematics.</a>
%Y A000228 Equals (A006535 + A030225)/2.
%Y A000228 Cf. A036359, A002216, A005963, A000228, A001998, A018190.
%Y A000228 Cf. A001207, A057973.
%Y A000228 Sequence in context: A070766 A111772 A018190 this_sequence A108070 A038147 
               A082271
%Y A000228 Adjacent sequences: A000225 A000226 A000227 this_sequence A000229 A000230 
               A000231
%K A000228 nonn,nice,hard
%O A000228 1,3
%A A000228 N. J. A. Sloane (njas(AT)research.att.com).
%E A000228 a(13) from Achim Flammenkamp (achim(AT)uni-bielefeld.de) Feb 15 1999. 
               a(14) from Brendan Owen, Dec 31, 2001
%E A000228 a(15) from Joseph Myers (jsm(AT)polyomino.org.uk), May 05 2002
%E A000228 a(16)-a(20) from Joseph Myers (jsm(AT)polyomino.org.uk), Sep 21 2002
%E A000228 a(21) from Herman Jamke (hermanjamke(AT)fastmail.fm), May 05 2007