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A322153
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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.
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2
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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
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0,7
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LINKS
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Table of n, a(n) for n=0..23.
Eric Weisstein's World of Mathematics, Langford's Problem
Wikipedia, Dancing Links
Wikipedia, Langford pairing
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FORMULA
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a(6) = 5: 111223236356454654, 111224245645635363, 223235345465461116, 223236356454654111, 224245645635363111.
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CROSSREFS
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Cf. A321956, A320392.
Sequence in context: A174955 A224095 A130735 * A022835 A022834 A335102
Adjacent sequences: A322150 A322151 A322152 * A322154 A322155 A322156
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KEYWORD
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nonn,more
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AUTHOR
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Alois P. Heinz, Nov 28 2018
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STATUS
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approved
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