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A330124
Number of unlabeled set-systems with n vertices and no endpoints.
2
1, 1, 2, 22, 1776
OFFSET
0,3
COMMENTS
A set-system is a finite set of finite nonempty sets. An endpoint is a vertex appearing only once (degree 1).
EXAMPLE
Non-isomorphic representatives of the a(3) = 22 set-systems:
0
{1}{2}{12}
{12}{13}{23}
{1}{23}{123}
{12}{13}{123}
{1}{2}{13}{23}
{1}{2}{3}{123}
{1}{12}{13}{23}
{1}{2}{13}{123}
{1}{12}{13}{123}
{1}{12}{23}{123}
{12}{13}{23}{123}
{1}{2}{3}{12}{13}
{1}{2}{12}{13}{23}
{1}{2}{3}{12}{123}
{1}{2}{12}{13}{123}
{1}{2}{13}{23}{123}
{1}{12}{13}{23}{123}
{1}{2}{3}{12}{13}{23}
{1}{2}{3}{12}{13}{123}
{1}{2}{12}{13}{23}{123}
{1}{2}{3}{12}{13}{23}{123}
CROSSREFS
Partial sums of the covering case A330196.
The labeled version is A330059.
The "multi" version is A302545.
Unlabeled set-systems with no endpoints counted by weight are A330054.
Unlabeled set-systems with no singletons are A317794.
Unlabeled set-systems counted by vertices are A000612.
Unlabeled set-systems counted by weight are A283877.
The case with no singletons is A320665.
Sequence in context: A193486 A337577 A054948 * A104149 A113761 A319620
KEYWORD
nonn,more
AUTHOR
Gus Wiseman, Dec 05 2019
STATUS
approved