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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. Cf. A059052, A326951, A326966, A326970, A326971, A326976, A326977. Sequence in context: A121621 A225147 A119715 * A023273 A041729 A078665 Adjacent sequences:  A326962 A326963 A326964 * A326966 A326967 A326968 KEYWORD nonn AUTHOR Gus Wiseman, Aug 10 2019 STATUS approved

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Last modified September 23 18:51 EDT 2020. Contains 337315 sequences. (Running on oeis4.)