

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
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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



