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A326961 Number of set-systems covering n vertices where every vertex is the unique common element of some subset of the edges, also called covering T_1 set-systems. 14
1, 1, 2, 36, 19020, 2010231696, 9219217412568364176, 170141181796805105960861096082778425120, 57896044618658097536026644159052312977171804852352892309392604715987334365792 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Same as A059523 except with a(1) = 1 instead of 2.

Alternatively, these are set-systems covering n vertices whose dual is a (strict) antichain. 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. An antichain is a set of sets, none of which is a subset of any other.

LINKS

Table of n, a(n) for n=0..8.

FORMULA

Inverse binomial transform of A326965.

EXAMPLE

The a(3) = 36 set-systems:

  {{1}{2}{3}}        {{12}{13}{23}{123}}     {{2}{3}{12}{13}{23}}

  {{12}{13}{23}}     {{1}{2}{3}{12}{13}}     {{2}{3}{12}{13}{123}}

  {{1}{2}{3}{12}}    {{1}{2}{3}{12}{23}}     {{2}{12}{13}{23}{123}}

  {{1}{2}{3}{13}}    {{1}{2}{3}{13}{23}}     {{3}{12}{13}{23}{123}}

  {{1}{2}{3}{23}}    {{1}{2}{12}{13}{23}}    {{1}{2}{3}{12}{13}{23}}

  {{1}{2}{13}{23}}   {{1}{2}{3}{12}{123}}    {{1}{2}{3}{12}{13}{123}}

  {{1}{2}{3}{123}}   {{1}{2}{3}{13}{123}}    {{1}{2}{3}{12}{23}{123}}

  {{1}{3}{12}{23}}   {{1}{2}{3}{23}{123}}    {{1}{2}{3}{13}{23}{123}}

  {{2}{3}{12}{13}}   {{1}{3}{12}{13}{23}}    {{1}{2}{12}{13}{23}{123}}

  {{1}{12}{13}{23}}  {{1}{2}{13}{23}{123}}   {{1}{3}{12}{13}{23}{123}}

  {{2}{12}{13}{23}}  {{1}{3}{12}{23}{123}}   {{2}{3}{12}{13}{23}{123}}

  {{3}{12}{13}{23}}  {{1}{12}{13}{23}{123}}  {{1}{2}{3}{12}{13}{23}{123}}

MATHEMATICA

tmQ[eds_]:=Union@@Select[Intersection@@@Rest[Subsets[eds]], Length[#]==1&]==Union@@eds;

Table[Length[Select[Subsets[Subsets[Range[n], {1, n}]], Union@@#==Range[n]&&tmQ[#]&]], {n, 0, 3}]

CROSSREFS

Covering set-systems are A003465.

Covering T_0 set-systems are A059201.

The version with empty edges allowed is A326960.

The non-covering version is A326965.

Covering set-systems whose dual is a weak antichain are A326970.

The unlabeled version is A326974.

The BII-numbers of T_1 set-systems are A326979.

Cf. A058891, A059052, A059523, A323818, A326972, A326973, A326976, A326977.

Sequence in context: A134366 A265944 A127234 * A181555 A306644 A283261

Adjacent sequences:  A326958 A326959 A326960 * A326962 A326963 A326964

KEYWORD

nonn

AUTHOR

Gus Wiseman, Aug 12 2019

STATUS

approved

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Last modified February 19 19:06 EST 2020. Contains 332047 sequences. (Running on oeis4.)