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Search: id:A145845
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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
1
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| 1, 2, 7, 34, 208, 1504, 12283, 109778, 1050820, 10614856, 111978128
(list; graph; listen)
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OFFSET
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0,2
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FORMULA
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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
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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
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Sequence in context: A075834 A011800 A112916 this_sequence A002720 A111539 A074059
Adjacent sequences: A145842 A145843 A145844 this_sequence A145846 A145847 A145848
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KEYWORD
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nonn
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AUTHOR
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Eric Egge (eegge(AT)carleton.edu), Oct 21 2008
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