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%I #9 Dec 12 2018 17:00:37
%S 1,1,0,0,0,0,5,12,0,0,0,0,2967,13275,0,0,0,0,38182584,222784153,0,0,0,
%T 0
%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 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(6) = 5: 111223236356454654, 111224245645635363, 223235345465461116, 223236356454654111, 224245645635363111.
%Y Cf. A321956, A320392.
%K nonn,more
%O 0,7
%A _Alois P. Heinz_, Nov 28 2018
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