OFFSET
0,3
COMMENTS
A set-system is a finite set of finite nonempty sets. Elements of a set-system are sometimes called edges. A leaf is an edge containing a vertex that does not belong to any other edge, while an endpoint is a vertex belonging to only one edge.
Also set-systems with minimum covered vertex-degree 1.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..12
FORMULA
Binomial transform of A327229.
EXAMPLE
The a(2) = 6 set-systems:
{{1}}
{{2}}
{{1,2}}
{{1},{2}}
{{1},{1,2}}
{{2},{1,2}}
MATHEMATICA
Table[Length[Select[Subsets[Subsets[Range[n], {1, n}]], Min@@Length/@Split[Sort[Join@@#]]==1&]], {n, 0, 4}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Sep 01 2019
EXTENSIONS
Terms a(5) and beyond from Andrew Howroyd, Jan 21 2023
STATUS
approved