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 A264813 Number of permutations of 3 indistinguishable copies of 1,...,n such that the first and second copies of j are adjacent and there are exactly j numbers between the second and the third copy of j. 3
 1, 0, 1, 1, 0, 3, 6, 0, 53, 199, 0, 2908, 13699, 0, 369985, 2135430, 0, 87265700, 611286653, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,6 COMMENTS a(n) = 0 for n == 1 (mod 3). LINKS Eric Weisstein's World of Mathematics, Langford's Problem Wikipedia, Dancing Links Wikipedia, Langford pairing EXAMPLE a(0) = 1: the empty permutation. a(2) = 1: 221121. a(3) = 1: 223321131. a(5) = 3: 223325534411514, 225523344531141, 552244253341131. a(6) = 6: 221121665544336543, 225523366534411614, 225526633544361141, 446611415563322532, 552266253344631141, 665544336543221121. CROSSREFS Cf. A014552, A104185, A108235, A176127, A203435, A261516, A261517, A321956. Sequence in context: A299032 A094674 A125287 * A009014 A009036 A021281 Adjacent sequences:  A264810 A264811 A264812 * A264814 A264815 A264816 KEYWORD nonn,more AUTHOR Alois P. Heinz, Nov 25 2015 STATUS approved

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Last modified January 22 02:46 EST 2022. Contains 350481 sequences. (Running on oeis4.)