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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
OFFSET
0,5
COMMENTS
a(n) is even for n > 1.
LINKS
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
Sequence in context: A219803 A320539 A145598 * A117881 A373101 A342989
KEYWORD
nonn,more
AUTHOR
Alois P. Heinz, Dec 06 2018
STATUS
approved