OFFSET
0,3
COMMENTS
A set-system is a finite set of finite nonempty sets of positive integers.
As an alternative description, a(n) is the number of non-isomorphic sets of sets of sets with n leaves where the inner sets of sets all have different multiset unions.
EXAMPLE
Non-isomorphic representatives of the a(1) = 1 through a(3) = 9 sets:
{} {{{1}}} {{{1,2}}} {{{1,2,3}}}
{{{1},{2}}} {{{1},{1,2}}}
{{{1}},{{2}}} {{{1},{2,3}}}
{{{1}},{{1,2}}}
{{{1}},{{2,3}}}
{{{1},{2},{3}}}
{{{1}},{{1},{2}}}
{{{1}},{{2},{3}}}
{{{1}},{{2}},{{3}}}
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Dec 26 2019
STATUS
approved