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A145847
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Number of involutions of length 2n which are invariant under the reverse-complement map and have no decreasing subsequences of length 6.
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0
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1, 2, 6, 19, 67, 246, 947, 3746, 15213, 62950, 264920, 1129965, 4877215, 21262918, 93522756, 414532163
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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FORMULA
| a(n) = C(n,j)*A001006(j)*A001405(n-j),
where C(n,j) = n!/(j!(n-j)!)
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MATHEMATICA
| Table[Sum[ Binomial[n, j]*Binomial[n - j, Floor[(n - j)/2]]* Sum[Binomial[j, 2*i]*Binomial[2*i, i]/(i + 1), {i, 0, Floor[j/2]}], {j, 0, n}], {n, 0, 20}]
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CROSSREFS
| Sequence in context: A150095 A150096 A150097 * A150098 A184018 A148470
Adjacent sequences: A145844 A145845 A145846 * A145848 A145849 A145850
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KEYWORD
| nonn
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AUTHOR
| Eric Egge (eegge(AT)carleton.edu), Oct 21 2008
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