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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 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,7

LINKS

Table of n, a(n) for n=0..23.

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

Cf. A321956, A320392.

Sequence in context: A174955 A224095 A130735 * A022835 A022834 A335102

Adjacent sequences: A322150 A322151 A322152 * A322154 A322155 A322156

KEYWORD

nonn,more

AUTHOR

Alois P. Heinz, Nov 28 2018

STATUS

approved

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Last modified February 2 19:01 EST 2023. Contains 360023 sequences. (Running on oeis4.)