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Search: id:A121638
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| A121638 |
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Number of deco polyominoes of height n, having no 2-cell columns. A deco polyomino is a directed column-convex polyomino in which the height, measured along the diagonal, is attained only in the last column. |
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2
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| 1, 1, 2, 7, 29, 147, 889, 6252, 50163, 452356, 4529812, 49878095, 598989496, 7791393260, 109129383735, 1637539745521, 26208427321596, 445652393850867, 8023380629061127, 152470440379483009, 3049854459983511047
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OFFSET
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1,3
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COMMENT
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a(n)=A121637(n,0).
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REFERENCES
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E. Barcucci, A. Del Lungo and R. Pinzani, "Deco" polyominoes, permutations and random generation, Theoretical Computer Science, 159, 1996, 29-42.
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FORMULA
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a(n)=(n-1)a(n-1)+a(n-3) for n>=3; a(1)=1, a(2)=1, a(3)=2.
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EXAMPLE
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a(2)=1 because the deco polyominoes of height 2 are the horizontal and vertical dominoes and only the horizontal one has no 2-cell column.
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MAPLE
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a[1]:=1: a[2]:=1: a[3]:=2: for n from 4 to 23 do a[n]:=(n-1)*a[n-1]+a[n-3] od: seq(a[n], n=1..23);
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CROSSREFS
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Cf. A121637, A001053.
Sequence in context: A030849 A030973 A030805 this_sequence A030874 A030920 A030824
Adjacent sequences: A121635 A121636 A121637 this_sequence A121639 A121640 A121641
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KEYWORD
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nonn
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AUTHOR
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Emeric Deutsch (deutsch(AT)duke.poly.edu), Aug 14 2006
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