A322153 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.

1, 1, 0, 0, 0, 0, 5, 12, 0, 0, 0, 0, 2967, 13275, 0, 0, 0, 0, 38182584, 222784153, 0, 0, 0, 0

Offset: 0

Author

Alois P. Heinz, Nov 28 2018

Keywords

• nonn
• more

Links

• Eric Weisstein's World of Mathematics, Langford's Problem
• Wikipedia, Dancing Links
• Wikipedia, Langford pairing

Crossrefs

Cf. A321956, A320392.

Formula

a(6) = 5: 111223236356454654, 111224245645635363, 223235345465461116, 223236356454654111, 224245645635363111.