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A326965 Number of set-systems on n vertices where every covered vertex is the unique common element of some subset of the edges. 20
1, 2, 5, 46, 19181, 2010327182, 9219217424630040409, 170141181796805106025395618012972506978, 57896044618658097536026644159052312978532934306727333157337631572314050272137 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,2
COMMENTS
A set-system is a finite set of finite nonempty sets. 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}}. An antichain is a set-system where no edge is a subset of any other. This sequence counts set-systems whose dual is a (strict) antichain, also called T_1 set-systems.
LINKS
FORMULA
Binomial transform of A326961.
a(n) = A326967(n)/2.
EXAMPLE
The a(0) = 1 through a(2) = 5 set-systems:
{} {} {}
{{1}} {{1}}
{{2}}
{{1},{2}}
{{1},{2},{1,2}}
MATHEMATICA
tmQ[eds_]:=Union@@Select[Intersection@@@Rest[Subsets[eds]], Length[#]==1&]==Union@@eds;
Table[Length[Select[Subsets[Subsets[Range[n], {1, n}]], tmQ]], {n, 0, 3}]
CROSSREFS
Set-systems are A058891.
T_0 set-systems are A326940.
The covering case is A326961.
The version with empty edges allowed is A326967.
Set-systems whose dual is a weak antichain are A326968.
The unlabeled version is A326972.
The BII_numbers of these set-systems are A326979.
Sequence in context: A121621 A225147 A119715 * A023273 A041729 A078665
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 10 2019
STATUS
approved

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Last modified March 28 20:05 EDT 2024. Contains 371254 sequences. (Running on oeis4.)