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Search: id:A054872
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| A054872 |
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Number of (12345, 13245, 21345, 23145, 31245, 32145)-avoiding permutations. |
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4
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| 1, 2, 6, 24, 114, 600, 3372, 19824, 120426, 749976, 4762644, 30723792, 200778612, 1326360048, 8842981848, 59425117152, 402092408346, 2737156004376, 18732169337604, 128806616999184, 889479590046108, 6165939982059600
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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Hankel transform is A083667, the number of different antisymmetric relations on n labeled points. - Paul Barry (pbarry(AT)wit.ie), Jun 26 2008
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REFERENCES
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E. Barcucci, A. Del Lungo, E. Pergola and R. Pinzani, Permutations avoiding an increasing number of length-increasing forbidden subsequences, Discrete MAthematics and Theoretical Computer Science, 4, 2000, 31-44.
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LINKS
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E. Barcucci, A. Del Lungo, E. Pergola and R. Pinzani, Permutations avoiding an increasing number of length-increasing forbidden subsequences
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FORMULA
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G.f.: 2-2*x-(1-8*x+4*x^2)^(1/2).
a(n)=2*A047891(n-1), n>=2. - Philippe DELEHAM (kolotoko(AT)wanadoo.fr), Aug 17 2007
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MAPLE
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Set j=3 in the following: f := (x, j)->1-(j+1)*x- sqrt(1-2*(j+1)*x+(j-1)^2*x^2); t := (x, j)->sum(k!*x^k, k=1..(j-1)); s := (x, j)->x^(j-2)*(j-1)!*(f(x, j))/(2)+ t(x, j);
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CROSSREFS
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Cf. A000108.
Sequence in context: A046646 A118376 A085486 this_sequence A134664 A068199 A128088
Adjacent sequences: A054869 A054870 A054871 this_sequence A054873 A054874 A054875
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KEYWORD
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nonn
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AUTHOR
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Elisa Pergola (elisa(AT)dsi.unifi.it), May 26 2000
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