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%I #5 Dec 06 2019 09:36:37
%S 1,0,1,20,1754
%N Number of unlabeled set-systems covering n vertices with no endpoints.
%C A set-system is a finite set of finite nonempty sets. An endpoint is a vertex appearing only once (degree 1).
%e Non-isomorphic representatives of the a(3) = 20 set-systems:
%e {12}{13}{23}
%e {1}{23}{123}
%e {12}{13}{123}
%e {1}{2}{13}{23}
%e {1}{2}{3}{123}
%e {1}{12}{13}{23}
%e {1}{2}{13}{123}
%e {1}{12}{13}{123}
%e {1}{12}{23}{123}
%e {12}{13}{23}{123}
%e {1}{2}{3}{12}{13}
%e {1}{2}{12}{13}{23}
%e {1}{2}{3}{12}{123}
%e {1}{2}{12}{13}{123}
%e {1}{2}{13}{23}{123}
%e {1}{12}{13}{23}{123}
%e {1}{2}{3}{12}{13}{23}
%e {1}{2}{3}{12}{13}{123}
%e {1}{2}{12}{13}{23}{123}
%e {1}{2}{3}{12}{13}{23}{123}
%Y First differences of the non-covering version A330124.
%Y The "multi" version is A302545.
%Y Unlabeled set-systems with no endpoints counted by vertices are A317794.
%Y Unlabeled set-systems with no endpoints counted by weight are A330054.
%Y Unlabeled set-systems counted by vertices are A000612.
%Y Unlabeled set-systems counted by weight are A283877.
%Y Cf. A007716, A016031, A306005, A317795, A320665, A330052, A330053, A330055.
%K nonn,more
%O 0,4
%A _Gus Wiseman_, Dec 05 2019
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