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 A006534 Number of one-sided triangular polyominoes (n-iamonds) with n cells; turning over not allowed, holes are allowed. (Formerly M3287) 5
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS The figures are formed by connecting n regular triangles by edges. "Turning over not allowed" means that axial symmetric polyiamonds are counted separately, thus a(4) = 4 and a(5) = 6 while A000577(4) = 3 and A000577(5) = 4, cf. examples. - M. F. Hasler, Nov 12 2017 REFERENCES F. Harary, Graphical enumeration problems; in Graph Theory and Theoretical Physics, ed. F. Harary, Academic Press, London, 1967, pp. 1-41. W. F. Lunnon, Counting hexagonal and triangular polyominoes, pp. 87-100 of R. C. Read, editor, Graph Theory and Computing. Academic Press, NY, 1972. 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), 216-227. LINKS 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, 85-96. J. Meeus & N. J. A. Sloane, Correspondence, 1974-1975 Ed Pegg, Jr., Illustrations of polyforms Eric Weisstein's World of Mathematics, Polyiamond. EXAMPLE From M. F. Hasler, Nov 12 2017: (Start) Putting dots for the approximate center of the regular triangles (alternatively flipped up and down for neighboring dots), we have: a(4) = #{ .... , .:. , ..: , :.. } = 4, while ..: and :.. are considered equivalent and not counted twice in A000577(4) = 3. a(5) = #{ ..... , ...: , :... , ..:. , .:.. , :.: } = 6, and again the 2nd & 3rd and 4th & 5th are considered equivalent and not counted twice in A000577(5) = 4. (End) CROSSREFS Cf. A000577 (same with "turning over allowed"), 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 EXTENSIONS Corrected and extended by David W. Wilson. a(19) from Achim Flammenkamp, Feb 15 1999 a(20) to a(28) from Joseph Myers, Sep 24 2002 Edited by M. F. Hasler, Nov 12 2017 STATUS approved

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Last modified November 20 02:57 EST 2018. Contains 317371 sequences. (Running on oeis4.)