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A320392 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 first and the second or between the second and the third copy of j. 2
1, 1, 0, 0, 4, 4, 10, 24, 252, 410, 1998, 7798, 65188, 280582, 2281108, 10585748, 110903088 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

a(n) is even for n > 1.

LINKS

Table of n, a(n) for n=0..16.

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

Wikipedia, Dancing Links

Wikipedia, Langford pairing

EXAMPLE

a(1) = 1: 111.

a(4) = 4: 111224234343, 111343432422, 224234343111, 343432422111.

CROSSREFS

Cf. A285527, A322153.

Sequence in context: A219803 A320539 A145598 * A117881 A161719 A161433

Adjacent sequences:  A320389 A320390 A320391 * A320393 A320394 A320395

KEYWORD

nonn,more

AUTHOR

Alois P. Heinz, Dec 06 2018

STATUS

approved

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Last modified January 17 23:37 EST 2020. Contains 330995 sequences. (Running on oeis4.)