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A326949 Number of unlabeled T_0 sets of subsets of {1..n}. 7
2, 4, 10, 68, 3838, 37320356 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,1
COMMENTS
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}}. The T_0 condition means that the dual is strict (no repeated edges).
LINKS
FORMULA
a(n) = 2 * A326946(n).
EXAMPLE
Non-isomorphic representatives of the a(0) = 2 through a(2) = 10 sets of sets:
{} {} {}
{{}} {{}} {{}}
{{1}} {{1}}
{{},{1}} {{},{1}}
{{1},{2}}
{{2},{1,2}}
{{},{1},{2}}
{{},{2},{1,2}}
{{1},{2},{1,2}}
{{},{1},{2},{1,2}}
CROSSREFS
The non-T_0 version is A003180.
The labeled version is A326941.
The covering case is A326942 (first differences).
The case without empty edges is A326946.
Sequence in context: A326325 A080090 A125263 * A215439 A120402 A182238
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Aug 08 2019
EXTENSIONS
a(5) from Max Alekseyev, Oct 11 2023
STATUS
approved

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Last modified February 23 18:28 EST 2024. Contains 370283 sequences. (Running on oeis4.)