OFFSET
0,2
FORMULA
Binomial transform of A368597.
EXAMPLE
The a(0) = 1 through a(3) = 30 loop-graphs (loops shown as singletons):
{} {} {} {}
{{1}} {{1}} {{1}}
{{2}} {{2}}
{{1},{2}} {{3}}
{{1},{1,2}} {{1},{2}}
{{2},{1,2}} {{1},{3}}
{{2},{3}}
{{1},{1,2}}
{{1},{1,3}}
{{2},{1,2}}
{{2},{2,3}}
{{3},{1,3}}
{{3},{2,3}}
{{1},{2},{3}}
{{1},{2},{1,3}}
{{1},{2},{2,3}}
{{1},{3},{1,2}}
{{1},{3},{2,3}}
{{2},{3},{1,2}}
{{2},{3},{1,3}}
{{1},{1,2},{1,3}}
{{1},{1,2},{2,3}}
{{1},{1,3},{2,3}}
{{2},{1,2},{1,3}}
{{2},{1,2},{2,3}}
{{2},{1,3},{2,3}}
{{3},{1,2},{1,3}}
{{3},{1,2},{2,3}}
{{3},{1,3},{2,3}}
{{1,2},{1,3},{2,3}}
MATHEMATICA
Table[Length[Select[Subsets[Subsets[Range[n], {1, 2}]], Length[#]==Length[Union@@#]&]], {n, 0, 5}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Jan 18 2024
STATUS
approved