OFFSET
1,2
LINKS
Bertran Steinsky, Enumeration of labelled chain graphs and labeled essential directed acyclic graphs, Discrete Mathematics, 270 (2003), 267-278.
FORMULA
a(n) = -Sum_{k=1..n} binomial(n, k) * b(k) * a(n - k) * 2^(k*(n-k)) with a(0) = b(0) = 1 and b(n) = -Sum_{k=0..n-1} binomial(n, k) * b(k) * 2^(binomial(n-r, 2)) [from Steinsky]. - Sean A. Irvine, Oct 15 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Detlef Pauly (dettodet(AT)yahoo.de), Aug 22 2003
STATUS
approved