OFFSET
0,4
EXAMPLE
Non-isomorphic representatives of the a(3) = 6 set-systems:
{123}
{12}{13}
{12}{123}
{12}{13}{23}
{12}{13}{123}
{12}{13}{23}{123}
MATHEMATICA
sysnorm[{}]:={}; sysnorm[m_]:=If[Union@@m!=Range[Max@@Flatten[m]], sysnorm[m/.Rule@@@Table[{(Union@@m)[[i]], i}, {i, Length[Union@@m]}]], First[Sort[sysnorm[m, 1]]]]; sysnorm[m_, aft_]:=If[Length[Union@@m]<=aft, {m}, With[{mx=Table[Count[m, i, {2}], {i, Select[Union@@m, #>=aft&]}]}, Union@@(sysnorm[#, aft+1]&/@Union[Table[Map[Sort, m/.{par+aft-1->aft, aft->par+aft-1}, {0, 1}], {par, First/@Position[mx, Max[mx]]}]])]];
Table[Length[Union[sysnorm/@Select[Subsets[Select[Subsets[Range[n]], Length[#]>1&]], Union@@#==Range[n]&]]], {n, 4}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 07 2018
EXTENSIONS
More terms from Gus Wiseman, Dec 13 2018
STATUS
approved