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A331883
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The number of permutations in the symmetric group S_n in which it is possible to find two disjoint increasing subsequences each with length equal to the length of the longest increasing subsequence of the permutation.
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0
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OFFSET
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1,4
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COMMENTS
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Only permutations whose longest increasing subsequence is at most n/2 need to be considered.
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LINKS
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EXAMPLE
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a(3) = 1 because the only permutation whose longest increasing subsequence is 1 is [3,2,1] and this contains two disjoint increasing subsequences of length 1.
The a(4) = 5 permutations are:
[2,1,4,3],
[2,4,1,3],
[3,1,4,2],
[3,4,1,2],
[4,3,2,1].
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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STATUS
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approved
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