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A320392 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. 2

%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

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Last modified February 25 23:44 EST 2020. Contains 332270 sequences. (Running on oeis4.)