A322153 - OEIS

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

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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: A224095 A370664 A130735 * A022835 A022834 A335102

Adjacent sequences: A322150 A322151 A322152 * A322154 A322155 A322156

KEYWORD

nonn,more

AUTHOR

Alois P. Heinz, Nov 28 2018

STATUS

approved