cp's OEIS Frontend

A320600 Let w = (w_1, w_2, ..., w_n) be a permutation of the integers {1, 2, ..., n}, and let f(k, w) be the length of the longest monotone subsequence of (w_k, w_{k+1}, ..., w_n) starting with w_k. Then a(n) is the number of permutations w in S_n such that Sum_{k=1..n} f(k,w) is minimized.

%I A320600 #18 Sep 21 2019 21:57:38
%S A320600 1,2,4,4,32,156,564,1386,1764
%N A320600 Let w = (w_1, w_2, ..., w_n) be a permutation of the integers {1, 2, ..., n}, and let f(k, w) be the length of the longest monotone subsequence of (w_k, w_{k+1}, ..., w_n) starting with w_k. Then a(n) is the number of permutations w in S_n such that Sum_{k=1..n} f(k,w) is minimized.
%C A320600 a(n) is even, because if a permutation is minimal, then so is its reverse.
%C A320600 The minimal sum is given by A327672. - _Peter Kagey_, Sep 21 2019
%H A320600 Sung Soo Kim, <a href="https://doi.org/10.1080/0025570X.2018.1501259">Problems and Solutions</a>, Mathematics Magazine, 91:4 (2018), 310.
%H A320600 Michael Reid, <a href="https://www.tandfonline.com/doi/full/10.1080/0025570X.2019.1648111?af=R">Problems and Solutions</a>, Mathematics Magazine, 92:4 (2019), 314.
%e A320600 For n = 4 the a(4) = 4 permutations are
%e A320600 w_1 = (2,1,4,3),
%e A320600 w_2 = (2,4,1,3),
%e A320600 w_3 = (3,1,4,2), and
%e A320600 w_4 = (3,4,1,2).
%e A320600 In each case, f(1,w_i) + f(2,w_i) + f(3,w_i) + f(4,w_i) = A327672(4) = 7.
%K A320600 nonn,more
%O A320600 1,2
%A A320600 _Peter Kagey_, Oct 16 2018