OFFSET
0,4
EXAMPLE
The a(4) = 42 graphs:
{12} {12,13} {12,13,24} {12,13,14,23}
{13} {12,14} {12,13,34} {12,13,14,24}
{14} {12,23} {12,14,23} {12,13,14,34}
{23} {12,24} {12,14,34} {12,13,23,24}
{24} {13,14} {12,23,34} {12,13,23,34}
{34} {13,23} {12,24,34} {12,14,23,24}
{13,34} {13,14,23} {12,14,24,34}
{14,24} {13,14,24} {12,23,24,34}
{14,34} {13,23,24} {13,14,23,34}
{23,24} {13,24,34} {13,14,24,34}
{23,34} {14,23,24} {13,23,24,34}
{24,34} {14,23,34} {14,23,24,34}
MATHEMATICA
graprms[m_]:=Union[Table[Sort[Sort/@(m/.Rule@@@Table[{p[[i]], i}, {i, Length[p]}])], {p, Permutations[Union@@m]}]];
Table[Length[Select[Subsets[Subsets[Range[n], {2}]], Length[graprms[#]]==Length[Union@@#]!/2&]], {n, 0, 4}]
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Dec 12 2019
STATUS
approved