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Search: id:A145846
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| A145846 |
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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 6. |
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+0
1
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| 1, 2, 8, 47, 357, 3270, 34515, 406460, 5215829, 71677058, 1041363040, 15841778155, 250494079945, 4093630537014
(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 * A000108(n-j) * A005802(j)),
where C(n,j) = n!/(j!(n-j)!)
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MATHEMATICA
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Table[Sum[ Binomial[n, j]^2*((1/(n - j + 1))* Binomial[2*(n - j), n - j]/((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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Adjacent sequences: A145843 A145844 A145845 this_sequence A145847 A145848 A145849
Sequence in context: A096656 A102009 A135904 this_sequence A009566 A003275 A078558
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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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