OFFSET
0,4
COMMENTS
Covering means there are no isolated vertices. Two edges {a,b}, {c,d} are crossing if a < c < b < d or c < a < d < b, and nesting if a < c < d < b or c < a < b < d.
Is this (apart from offsets) the same as A073717? - R. J. Mathar, Jul 04 2019
LINKS
Eric Marberg, Crossings and nestings in colored set partitions, arXiv preprint arXiv:1203.5738 [math.CO], 2012.
MATHEMATICA
Table[Length[Select[Subsets[Subsets[Range[n], {2}]], Union@@#==Range[n]&&!MatchQ[#, {___, {x_, y_}, ___, {z_, t_}, ___}/; x<z<y<t||z<x<t<y||x<z<t<y||z<x<y<t]&]], {n, 0, 5}]
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Jun 27 2019
STATUS
approved