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A264813 Number of permutations of 3 indistinguishable copies of 1,...,n such that the first and second copies of j are adjacent and there are exactly j numbers between the second and the third copy of j. 3
1, 0, 1, 1, 0, 3, 6, 0, 53, 199, 0, 2908, 13699, 0, 369985, 2135430, 0, 87265700, 611286653, 0 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,6

COMMENTS

a(n) = 0 for n == 1 (mod 3).

LINKS

Table of n, a(n) for n=0..19.

Eric Weisstein's World of Mathematics, Langford's Problem

Wikipedia, Dancing Links

Wikipedia, Langford pairing

EXAMPLE

a(0) = 1: the empty permutation.

a(2) = 1: 221121.

a(3) = 1: 223321131.

a(5) = 3: 223325534411514, 225523344531141, 552244253341131.

a(6) = 6: 221121665544336543, 225523366534411614, 225526633544361141, 446611415563322532, 552266253344631141, 665544336543221121.

CROSSREFS

Cf. A014552, A104185, A108235, A176127, A203435, A261516, A261517, A321956.

Sequence in context: A299032 A094674 A125287 * A009014 A009036 A021281

Adjacent sequences:  A264810 A264811 A264812 * A264814 A264815 A264816

KEYWORD

nonn,more

AUTHOR

Alois P. Heinz, Nov 25 2015

STATUS

approved

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Last modified January 28 13:22 EST 2020. Contains 331321 sequences. (Running on oeis4.)