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).
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
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Aug 08 2019
EXTENSIONS
a(5) from Max Alekseyev, Oct 11 2023
STATUS
approved