A326336 Number of set partitions of {1..n} whose capturing blocks are connected.

1, 1, 1, 1, 2, 7, 24, 100, 458, 2279, 12270

Offset: 0

Author

Gus Wiseman, Jun 28 2019

Keywords

• nonn
• more

Comments

Two blocks are capturing if they are of the form {...x...y...}, {...z...t...} where x < z < t < y or z < x < y < t. A set partition has its capturing blocks connected if the graph whose vertices are the blocks and whose edges are capturing pairs of blocks is connected.

Examples

			The a(0) = 1 through a(6) = 24 set partitions:
  {}  {1}  {12}  {123}  {1234}    {12345}    {123456}
                        {14}{23}  {125}{34}  {1236}{45}
                                  {134}{25}  {1245}{36}
                                  {135}{24}  {1246}{35}
                                  {14}{235}  {125}{346}
                                  {145}{23}  {1256}{34}
                                  {15}{234}  {126}{345}
                                             {134}{256}
                                             {1345}{26}
                                             {1346}{25}
                                             {135}{246}
                                             {1356}{24}
                                             {136}{245}
                                             {14}{2356}
                                             {145}{236}
                                             {1456}{23}
                                             {146}{235}
                                             {15}{2346}
                                             {156}{234}
                                             {16}{2345}
                                             {15}{26}{34}
                                             {16}{23}{45}
                                             {16}{24}{35}
                                             {16}{25}{34}
		

Crossrefs

Simple graphs whose capturing blocks are connected are A326330.

Set partitions whose crossing blocks are connected are A099947.

Set partitions whose nesting blocks are connected are A326335.

Cf. A000110, A001519, A016098, A122880, A324173, A326243, A326248, A326293, A326331, A326337.

Programs

Mathematica

capXQ[stn_]:=MatchQ[stn,{_,{_,x_,_,y_,_},_,{_,z_,_,t_,_},_}/;x0]&]},If[c=={},s,csm[Sort[Append[Delete[s,List/@c[[1]]],Union@@s[[c[[1]]]]]]]]];
captcmpts[stn_]:=csm[Union[List/@stn,Select[Subsets[stn,{2}],capXQ]]];
sps[{}]:={{}};sps[set:{i_,_}]:=Join@@Function[s,Prepend[#,s]&/@sps[Complement[set,s]]]/@Cases[Subsets[set],{i,_}];
Table[Length[Select[sps[Range[n]],Length[captcmpts[#]]<=1&]],{n,0,6}]