A327113 Number of set-systems covering n vertices with cut-connectivity 2.

0, 0, 4, 0, 4752

Offset

0,3

Comments

A set-system is a finite set of finite nonempty sets. Elements of a set-system are sometimes called edges. The cut-connectivity of a set-system is the minimum number of vertices that must be removed (along with any empty or duplicate edges) to obtain a disconnected or empty set-system. Except for cointersecting set-systems (A327040), this is the same as vertex-connectivity (A327334, A327051).

Links

Table of n, a(n) for n=0..4.

Example

The a(2) = 4 set-systems:
  {{1,2}}
  {{1},{1,2}}
  {{2},{1,2}}
  {{1},{2},{1,2}}

Mathematica

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]]]]]]]]];
vConn[sys_]:=If[Length[csm[sys]]!=1, 0, Min@@Length/@Select[Subsets[Union@@sys], Function[del, Length[csm[DeleteCases[DeleteCases[sys, Alternatives@@del, {2}], {}]]]!=1]]];
Table[Length[Select[Subsets[Subsets[Range[n], {1, n}]], Union@@#==Range[n]&&vConn[#]==2&]], {n, 0, 3}]

Crossrefs

Covering graphs with cut-connectivity >= 2 are A013922, if we assume A013922(2) = 1.

Covering antichains (blobs) with cut-connectivity >= 2 are A275307, if we assume A275307(1) = 0.

2-vertex-connected integer partitions are A322387.

Connected covering set-systems are A323818.

Covering set-systems with cut-connectivity >= 2 are A327112.

The cut-connectivity of the set-system with BII-number n is A326786(n).

BII-numbers of set-systems with cut-connectivity 2 are A327082.

Cf. A002218, A003465, A048143, A259862, A322389, A327101, A327113, A327126, A327128, A327130.

Sequence in context: A331582 A306819 A336327 * A051390 A124120 A365952

Adjacent sequences: A327110 A327111 A327112 * A327114 A327115 A327116

Keyword

nonn,more

Author

Gus Wiseman, Aug 24 2019

Status

approved