A331883 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.
0, 1, 1, 5, 26, 132, 834, 6477, 56242
Offset: 1
Examples
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].
Links
- Wikipedia, Longest increasing subsequence problem
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