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A145845
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Number of permutations of length 2n+1 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, 7, 34, 208, 1504, 12283, 109778, 1050820, 10614856, 111978128
(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, C(n,j)^2 * A005802(j))
= sum(j=0, n, C(n,j)^2 * (1/((j+1)^2 (j+2))) *
sum(i=0, j, C(2i,i)*C(j+1,i+i)*C(j+2,i+1)))
where C(n,j) = n!/(j!(n-j)!)
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MATHEMATICA
| Table[Sum[ Binomial[n, j]^2*(1/((j + 1)^2*(j + 2)))* Sum[Binomial[2*i, i]*Binomial[j + 1, i + 1]* Binomial[j + 2, i + 1], {i, 0, j}], {j, 0, n}], {n, 0, 20}]
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CROSSREFS
| Sequence in context: A075834 A011800 A112916 * A002720 A111539 A074059
Adjacent sequences: A145842 A145843 A145844 * A145846 A145847 A145848
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KEYWORD
| nonn
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AUTHOR
| Eric Egge (eegge(AT)carleton.edu), Oct 21 2008
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