OFFSET
0,1
COMMENTS
Differs from A326878 in having a(0) = 2 instead of 1.
FORMULA
a(n) = A000798(n) + A006058(n). - Jean-François Alcover, Dec 30 2019, after Gus Wiseman's comment in A006058.
EXAMPLE
The a(0) = 2 through a(2) = 7 sets of subsets:
{} {{1}} {{1,2}}
{{}} {{},{1}} {{},{1,2}}
{{1},{1,2}}
{{2},{1,2}}
{{},{1},{1,2}}
{{},{2},{1,2}}
{{},{1},{2},{1,2}}
MATHEMATICA
Table[Length[Select[Subsets[Subsets[Range[n]]], Union@@#==Range[n]&&SubsetQ[#, Union[Union@@@Tuples[#, 2], Intersection@@@Tuples[#, 2]]]&]], {n, 0, 3}]
(* Second program: *)
a /@ Range[0, 18] (* Jean-François Alcover, Dec 30 2019 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Aug 04 2019
EXTENSIONS
STATUS
approved