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 A326248 Number of crossing, nesting set partitions of {1..n}. 12
 0, 0, 0, 0, 0, 2, 28, 252, 1890, 13020, 86564 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,6 COMMENTS A set partition is crossing if it has two blocks of the form {...x,y...}, {...z,t...} where x < z < y < t or z < x < t < y, and nesting if it has two blocks of the form {...x,y...}, {...z,t...} where x < z < t < y or z < x < y < t. LINKS EXAMPLE The a(5) = 2 set partitions:   {{1,4},{2,3,5}}   {{1,3,4},{2,5}} MATHEMATICA sps[{}]:={{}}; sps[set:{i_, ___}]:=Join@@Function[s, Prepend[#, s]&/@sps[Complement[set, s]]]/@Cases[Subsets[set], {i, ___}]; croXQ[stn_]:=MatchQ[stn, {___, {___, x_, y_, ___}, ___, {___, z_, t_, ___}, ___}/; x

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Last modified October 18 07:16 EDT 2019. Contains 328146 sequences. (Running on oeis4.)