%I
%S 1,1,0,0,4,4,10,24,252,410,1998,7798,65188,280582,2281108,10585748,
%T 110903088
%N Number of permutations of 3 indistinguishable copies of 1,...,n such that there are exactly j numbers between the first and the third copy of j and floor(j/2) numbers between the first and the second or between the second and the third copy of j.
%C a(n) is even for n > 1.
%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>
%e a(1) = 1: 111.
%e a(4) = 4: 111224234343, 111343432422, 224234343111, 343432422111.
%Y Cf. A285527, A322153.
%K nonn,more
%O 0,5
%A _Alois P. Heinz_, Dec 06 2018
