

A006534


Number of onesided triangular polyominoes (niamonds) with n cells (turning over not allowed) (holes are allowed).
(Formerly M3287)


6



1, 1, 1, 4, 6, 19, 43, 120, 307, 866, 2336, 6588, 18373, 52119, 147700, 422016, 1207477, 3471067, 9999135, 28893560, 83665729, 242826187, 706074369, 2056870697, 6001555275, 17538335077, 51323792789, 150390053432
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OFFSET

1,4


REFERENCES

F. Harary, Graphical enumeration problems; in Graph Theory and Theoretical Physics, ed. F. Harary, Academic Press, London, 1967, pp. 141.
W. F. Lunnon, Counting hexagonal and triangular polyominoes, pp. 87100 of R. C. Read, editor, Graph Theory and Computing. Academic Press, NY, 1972.
J. Meeus, personal communication.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
P. J. Torbijn, Polyiamonds, J. Rec. Math., 2 (1969), 216227.


LINKS

Table of n, a(n) for n=1..28.
Ed Pegg, Jr., Illustrations of polyforms
R. K. Guy, O'Beirne's Hexiamond, in The Mathemagician and the Pied Puzzler  A Collection in Tribute to Martin Gardner, Ed. E. R. Berlekamp and T. Rogers, A. K. Peters, 1999, 8596.
Eric Weisstein's World of Mathematics, Polyiamond.


CROSSREFS

Cf. A000577, A006534, A030223, A030224, A001420.
Sequence in context: A026521 A222379 A202618 * A064035 A010364 A110391
Adjacent sequences: A006531 A006532 A006533 * A006535 A006536 A006537


KEYWORD

nonn,hard,nice


AUTHOR

N. J. A. Sloane.


EXTENSIONS

Corrected and extended by David W. Wilson. a(19) from Achim Flammenkamp (achim(AT)unibielefeld.de) Feb 15 1999.
a(20) to a(28) from Joseph Myers, Sep 24 2002


STATUS

approved



