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Search: id:A121635
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| A121635 |
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Number of deco polyominoes of height n, having no 2-cell columns starting at level 0. 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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+0
2
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| 1, 2, 8, 42, 264, 1920, 15840, 146160, 1491840, 16692480, 203212800, 2674425600, 37841126400, 572885913600, 9240898867200, 158228598528000, 2866422214656000, 54775863926784000, 1101208277385216000
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OFFSET
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1,2
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COMMENT
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a(n)=A121634(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(1)=1, a(n)=(n-2)!(n^2-3n+4)/2 for n>=2. a(1)=1, a(2)=1, a(n)=(n-2)[(n-2)! + a(n-1)] for n>=3.
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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 the horizontal one has no 2-cell column starting at level 0.
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MAPLE
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a:=n->(n^2-3*n+4)*(n-2)!/2: seq(a(n), n=2..23);
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CROSSREFS
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Cf. A121634, A001710.
Adjacent sequences: A121632 A121633 A121634 this_sequence A121636 A121637 A121638
Sequence in context: A130649 A054993 A005315 this_sequence A002874 A078592 A052646
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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 13 2006
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