

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.


2



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: A174955 A224095 A130735 * A022835 A022834 A335102
Adjacent sequences: A322150 A322151 A322152 * A322154 A322155 A322156


KEYWORD

nonn,more


AUTHOR

Alois P. Heinz, Nov 28 2018


STATUS

approved



