%I #10 Dec 06 2018 07:36:00
%S 1,1,0,2,4,0,16,40,0,456,1759,0,34636,175198,0,5494621,34043062,0
%N Number of permutations of 3 indistinguishable copies of 1,...,n such that the first and second copies of j are adjacent and there are exactly j-1 numbers between the second and the third copy of j.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/LangfordsProblem.html">Langford's Problem</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Dancing_Links">Dancing Links</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Langford_pairing">Langford pairing</a>
%F a(n) = 0 for n == 2 (mod 3).
%e a(3) = 2: 111332232, 332232111.
%e a(4) = 4: 111334432242, 332232441114, 334432242111, 441114332232.
%Y Cf. A264813, A322153.
%K nonn,more
%O 0,4
%A _Alois P. Heinz_, Nov 22 2018