OFFSET
0,4
MAPLE
b:= proc(n) option remember; `if`(n=0, 1,
2^binomial(n, 2)-add(b(k)*binomial(n, k), k=0..n-1))
end:
a:= n-> 2^(n*(n-1)/2)-b(n):
seq(a(n), n=0..17); # Alois P. Heinz, Sep 04 2019
MATHEMATICA
Table[Length[Select[Subsets[Subsets[Range[n], {2}]], Union@@#!=Range[n]&]], {n, 0, 5}]
PROG
(PARI) b(n) = sum(k=0, n, (-1)^(n-k)*binomial(n, k)*2^binomial(k, 2)); \\ A006129
a(n) = 2^(n*(n-1)/2) - b(n); \\ Michel Marcus, Sep 05 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Sep 04 2019
STATUS
approved