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A145844
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Number of permutations of length 2n which are invariant under the reverse-complement map and have no decreasing subsequences of length 5.
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0
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1, 2, 8, 46, 332, 2784, 25888, 259382, 2749244, 30449416, 349379648
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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FORMULA
| a(n) = sum(j=0, n, A000108(j)*A000108(n-j)*C(n, j)^2 ) where
A000108(n)=Catalan(n)=(2n)!/(n!(n+1)!) and C(n, j)=n!/(k!(n-j)!)
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EXAMPLE
| a(4) = 1*1*14 + 16*1*5 + 36*2*2 + 16*5*1 + 1*14*1 = 332
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MATHEMATICA
| Table[Sum[ Binomial[n, j]^2*Binomial[2*j, j]* Binomial[2*(n - j), n - j]/((n - j + 1)*(j + 1)), {j, 0, n}], {n, 0, 20}]
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CROSSREFS
| Sequence in context: A202081 A006664 A141117 * A005840 A161881 A088791
Adjacent sequences: A145841 A145842 A145843 * A145845 A145846 A145847
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KEYWORD
| nonn
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AUTHOR
| Eric Egge (eegge(AT)carleton.edu), Oct 21 2008
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