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 A317794 Number of non-isomorphic set-systems on n vertices with no singletons. 15
 1, 1, 2, 8, 180, 612032, 200253854316544, 263735716028826427534807159537664, 5609038300883759793482640992086670066760184863720423808367168537493504 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS EXAMPLE Non-isomorphic representatives of the a(3) = 8 set-systems:   0,   {12}, {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&]], Or[Length[#]==0, Union@@#==Range[Max@@Union@@#]]&]]], {n, 4}] (* second program *) Table[Sum[2^PermutationCycles[Ordering[Map[Sort, Subsets[Range[n], {2, n}]/.Rule@@@Table[{i, prm[[i]]}, {i, n}], {1}]], Length]/n!, {prm, Permutations[Range[n]]}], {n, 6}] (* Gus Wiseman, Dec 12 2018 *) CROSSREFS The spanning case is A317795. Cf. A000088, A000612, A003180, A007716, A055621, A283877, A300913, A306005, A317533, A317757, A319876. Sequence in context: A009713 A265597 A213200 * A009377 A181234 A156526 Adjacent sequences:  A317791 A317792 A317793 * A317795 A317796 A317797 KEYWORD nonn AUTHOR Gus Wiseman, Aug 07 2018 EXTENSIONS More terms from Gus Wiseman, Dec 12 2018 STATUS approved

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Last modified December 1 10:49 EST 2021. Contains 349428 sequences. (Running on oeis4.)