OFFSET
0,6
COMMENTS
A set partition is capturing if it has two blocks of the form {...x...y...}, {...z...t...} where x < z < t < y or z < x < y < t. This is a weaker condition than nesting, so for example {{1,3,5},{2,4}} is capturing but not nesting.
LINKS
Eric Marberg, Crossings and nestings in colored set partitions, arXiv preprint arXiv:1203.5738 [math.CO], 2012.
EXAMPLE
The a(5) = 11 capturing set partitions:
{{1,2,5},{3,4}}
{{1,3,4},{2,5}}
{{1,3,5},{2,4}}
{{1,4},{2,3,5}}
{{1,4,5},{2,3}}
{{1,5},{2,3,4}}
{{1},{2,5},{3,4}}
{{1,4},{2,3},{5}}
{{1,5},{2},{3,4}}
{{1,5},{2,3},{4}}
{{1,5},{2,4},{3}}
MATHEMATICA
sps[{}]:={{}}; sps[set:{i_, ___}]:=Join@@Function[s, Prepend[#, s]&/@sps[Complement[set, s]]]/@Cases[Subsets[set], {i, ___}];
capXQ[stn_]:=MatchQ[stn, {___, {___, x_, ___, y_, ___}, ___, {___, z_, ___, t_, ___}, ___}/; x<z&&y>t||x>z&&y<t];
Table[Length[Select[sps[Range[n]], capXQ[#]&]], {n, 0, 8}]
CROSSREFS
KEYWORD
nonn,changed
AUTHOR
Gus Wiseman, Jun 19 2019
EXTENSIONS
a(12) and beyond from Christian Sievers, Aug 23 2024
STATUS
approved