OFFSET
0,4
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..50
Gus Wiseman, The a(3) = 7 unlabeled non-choosable loop-graphs.
FORMULA
First differences of A369146.
EXAMPLE
The a(0) = 0 through a(3) = 7 loop-graphs (loops shown as singletons):
. . {{1},{2},{1,2}} {{1},{2},{3},{1,2}}
{{1},{2},{1,2},{1,3}}
{{1},{2},{1,3},{2,3}}
{{1},{1,2},{1,3},{2,3}}
{{1},{2},{3},{1,2},{1,3}}
{{1},{2},{1,2},{1,3},{2,3}}
{{1},{2},{3},{1,2},{1,3},{2,3}}
MATHEMATICA
brute[m_]:=First[Sort[Table[Sort[Sort /@ (m/.Rule@@@Table[{(Union@@m)[[i]], p[[i]]}, {i, Length[p]}])], {p, Permutations[Range[Length[Union@@m]]]}]]];
Table[Length[Union[brute /@ Select[Subsets[Subsets[Range[n], {1, 2}]], Union@@#==Range[n] && Length[Select[Tuples[#], UnsameQ@@#&]]==0&]]], {n, 0, 4}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Jan 23 2024
EXTENSIONS
a(6) onwards from Andrew Howroyd, Feb 02 2024
STATUS
approved