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 A326948 Number of connected T_0 set-systems on n vertices. 3
 1, 1, 3, 86, 31302, 2146841520, 9223371978880250448, 170141183460469231408869283342774399392, 57896044618658097711785492504343953919148780260559635830120038252613826101856 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS The dual of a set-system has, for each vertex, one edge consisting of the indices (or positions) of the edges containing that vertex. For example, the dual of {{1,2},{2,3}} is {{1},{1,2},{2}}. The T_0 condition means that the dual is strict (no repeated edges). LINKS FORMULA Logarithmic transform of A059201. EXAMPLE The a(3) = 86 set-systems:   {12}{13}         {1}{2}{13}{123}     {1}{2}{3}{13}{23}   {12}{23}         {1}{2}{23}{123}     {1}{2}{3}{13}{123}   {13}{23}         {1}{3}{12}{13}      {1}{2}{3}{23}{123}   {1}{2}{123}      {1}{3}{12}{23}      {1}{2}{12}{13}{23}   {1}{3}{123}      {1}{3}{12}{123}     {1}{2}{12}{13}{123}   {1}{12}{13}      {1}{3}{13}{23}      {1}{2}{12}{23}{123}   {1}{12}{23}      {1}{3}{13}{123}     {1}{2}{13}{23}{123}   {1}{12}{123}     {1}{3}{23}{123}     {1}{3}{12}{13}{23}   {1}{13}{23}      {1}{12}{13}{23}     {1}{3}{12}{13}{123}   {1}{13}{123}     {1}{12}{13}{123}    {1}{3}{12}{23}{123}   {2}{3}{123}      {1}{12}{23}{123}    {1}{3}{13}{23}{123}   {2}{12}{13}      {1}{13}{23}{123}    {1}{12}{13}{23}{123}   {2}{12}{23}      {2}{3}{12}{13}      {2}{3}{12}{13}{23}   {2}{12}{123}     {2}{3}{12}{23}      {2}{3}{12}{13}{123}   {2}{13}{23}      {2}{3}{12}{123}     {2}{3}{12}{23}{123}   {2}{23}{123}     {2}{3}{13}{23}      {2}{3}{13}{23}{123}   {3}{12}{13}      {2}{3}{13}{123}     {2}{12}{13}{23}{123}   {3}{12}{23}      {2}{3}{23}{123}     {3}{12}{13}{23}{123}   {3}{13}{23}      {2}{12}{13}{23}     {1}{2}{3}{12}{13}{23}   {3}{13}{123}     {2}{12}{13}{123}    {1}{2}{3}{12}{13}{123}   {3}{23}{123}     {2}{12}{23}{123}    {1}{2}{3}{12}{23}{123}   {12}{13}{23}     {2}{13}{23}{123}    {1}{2}{3}{13}{23}{123}   {12}{13}{123}    {3}{12}{13}{23}     {1}{2}{12}{13}{23}{123}   {12}{23}{123}    {3}{12}{13}{123}    {1}{3}{12}{13}{23}{123}   {13}{23}{123}    {3}{12}{23}{123}    {2}{3}{12}{13}{23}{123}   {1}{2}{3}{123}   {3}{13}{23}{123}    {1}{2}{3}{12}{13}{23}{123}   {1}{2}{12}{13}   {12}{13}{23}{123}   {1}{2}{12}{23}   {1}{2}{3}{12}{13}   {1}{2}{12}{123}  {1}{2}{3}{12}{23}   {1}{2}{13}{23}   {1}{2}{3}{12}{123} MATHEMATICA dual[eds_]:=Table[First/@Position[eds, x], {x, Union@@eds}]; csm[s_]:=With[{c=Select[Tuples[Range[Length[s]], 2], And[OrderedQ[#], UnsameQ@@#, Length[Intersection@@s[[#]]]>0]&]}, If[c=={}, s, csm[Sort[Append[Delete[s, List/@c[[1]]], Union@@s[[c[[1]]]]]]]]]; Table[Length[Select[Subsets[Subsets[Range[n], {1, n}]], Union@@#==Range[n]&&Length[csm[#]]<=1&&UnsameQ@@dual[#]&]], {n, 0, 3}] CROSSREFS The same with covering instead of connected is A059201, with unlabeled version A319637. The non-T_0 version is A323818 (covering) or A326951 (not-covering). The non-connected version is A326940, with unlabeled version A326946. Cf. A000371, A003465, A245567, A316978, A319559, A319564, A326939, A326941, A326947. Sequence in context: A185142 A279020 A302947 * A159053 A172494 A279131 Adjacent sequences:  A326945 A326946 A326947 * A326949 A326950 A326951 KEYWORD nonn,more AUTHOR Gus Wiseman, Aug 08 2019 STATUS approved

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Last modified April 7 15:44 EDT 2020. Contains 333306 sequences. (Running on oeis4.)