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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.
2
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
0,7
LINKS
Eric Weisstein's World of Mathematics, Langford's Problem
Wikipedia, Dancing Links
Wikipedia, Langford pairing
FORMULA
a(6) = 5: 111223236356454654, 111224245645635363, 223235345465461116, 223236356454654111, 224245645635363111.
CROSSREFS
Sequence in context: A224095 A370664 A130735 * A022835 A022834 A335102
KEYWORD
nonn,more
AUTHOR
Alois P. Heinz, Nov 28 2018
STATUS
approved