OFFSET
0,2
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..50
FORMULA
Binomial transform of A054780.
EXAMPLE
The a(0) = 1 through a(2) = 6 set-systems:
{} {} {}
{{1}} {{1}}
{{2}}
{{1},{2}}
{{1},{1,2}}
{{2},{1,2}}
MATHEMATICA
Table[Length[Select[Subsets[Rest[Subsets[Range[n]]]], Length[Union@@#]==Length[#]&]], {n, 0, 3}]
PROG
(PARI) \\ Here b(n) is A054780(n).
b(n) = sum(k=0, n, (-1)^(n-k) * binomial(n, k) * binomial(2^k-1, n))
a(n) = sum(k=0, n, binomial(n, k) * b(k)) \\ Andrew Howroyd, Dec 29 2023
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Dec 08 2023
STATUS
approved