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A327112 Number of set-systems covering n vertices with cut-connectivity >= 2, or 2-cut-connected set-systems. 9
0, 0, 4, 72, 29856 (list; graph; refs; listen; history; text; internal format)
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

Non-isomorphic representatives of the a(3) = 72 set-systems:

  {{123}}

  {{3}{123}}

  {{23}{123}}

  {{2}{3}{123}}

  {{1}{23}{123}}

  {{3}{23}{123}}

  {{12}{13}{23}}

  {{13}{23}{123}}

  {{1}{2}{3}{123}}

  {{1}{3}{23}{123}}

  {{2}{3}{23}{123}}

  {{3}{12}{13}{23}}

  {{2}{13}{23}{123}}

  {{3}{13}{23}{123}}

  {{12}{13}{23}{123}}

  {{1}{2}{3}{23}{123}}

  {{2}{3}{12}{13}{23}}

  {{1}{2}{13}{23}{123}}

  {{2}{3}{13}{23}{123}}

  {{3}{12}{13}{23}{123}}

  {{1}{2}{3}{12}{13}{23}}

  {{1}{2}{3}{13}{23}{123}}

  {{2}{3}{12}{13}{23}{123}}

  {{1}{2}{3}{12}{13}{23}{123}}

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 2-cut-connected graphs are A013922, if we assume A013922(2) = 1.

Covering 1-cut-connected antichains (clutters) are A048143, if we assume A048143(0) = A048143(1) =0.

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

Covering set-systems with cut-connectivity 2 are A327113.

2-vertex-connected integer partitions are A322387.

BII-numbers of set-systems with cut-connectivity >= 2 are A327101.

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

Cf. A002218, A003465, A013922, A259862, A323818, A327082, A327126, A327130.

Sequence in context: A087315 A081460 A327040 * A284673 A055556 A168299

Adjacent sequences:  A327109 A327110 A327111 * A327113 A327114 A327115

KEYWORD

nonn,more

AUTHOR

Gus Wiseman, Aug 24 2019

STATUS

approved

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Last modified April 13 09:12 EDT 2021. Contains 342935 sequences. (Running on oeis4.)