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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

%I

%S 0,1,1,5,26,132,834,6477,56242

%N 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.

%C Only permutations whose longest increasing subsequence is at most n/2 need to be considered.

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Longest_increasing_subsequence_problem">Longest increasing subsequence problem</a>

%e 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.

%e The a(4) = 5 permutations are:

%e [2,1,4,3],

%e [2,4,1,3],

%e [3,1,4,2],

%e [3,4,1,2],

%e [4,3,2,1].

%Y Cf. A047874, A047887, A167995.

%K nonn,more

%O 1,4

%A _Ildar Gainullin_, Jan 30 2020

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Last modified June 17 14:23 EDT 2021. Contains 345083 sequences. (Running on oeis4.)