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 A306558 Number of double-crossing set partitions of {1,...,n}. 0
 0, 0, 0, 0, 0, 0, 1, 14, 141, 1267, 10841, 91091, 764092 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,8 COMMENTS Two blocks of a set partitions double-cross each other if they are of the form {{...a...b...c...},{...x...y...z...}} for some a < x < b < y < c < z or x < a < y < b < z < c. LINKS Kenneth J. Dykema, Generating functions for purely crossing partitions, arXiv:1602.03469 [math.CO], 2016. EXAMPLE The a(7) = 14 double-crossing set partitions:   {{1,3,5},{2,4,6,7}}   {{1,3,6},{2,4,5,7}}   {{1,4,6},{2,3,5,7}}   {{1,2,4,6},{3,5,7}}   {{1,3,4,6},{2,5,7}}   {{1,3,5,6},{2,4,7}}   {{1,3,5,7},{2,4,6}}   {{1},{2,4,6},{3,5,7}}   {{1,3,5},{2,4,6},{7}}   {{1,3,5},{2,4,7},{6}}   {{1,3,6},{2,4,7},{5}}   {{1,3,6},{2,5,7},{4}}   {{1,4,6},{2},{3,5,7}}   {{1,4,6},{2,5,7},{3}} MATHEMATICA croXXQ[stn_]:=MatchQ[stn, {___, {___, a_, ___, b_, ___, c_, ___}, ___, {___, x_, ___, y_, ___, z_, ___}, ___}/; a

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Last modified May 18 03:03 EDT 2021. Contains 343994 sequences. (Running on oeis4.)