cp's OEIS Frontend

A326904 Number of unlabeled set-systems (without {}) on n vertices that are closed under intersection.

%I A326904 #10 Aug 09 2019 11:23:57
%S A326904 1,2,4,10,38,368,29328,216591692,5592326399531792
%N A326904 Number of unlabeled set-systems (without {}) on n vertices that are closed under intersection.
%C A326904 A set-system is a finite set of finite nonempty sets, so no two edges of such a set-system can be disjoint.
%C A326904 Apart from the offset the same as A193675. - _R. J. Mathar_, Aug 09 2019
%F A326904 a(n > 0) = 2 * A193674(n - 1).
%e A326904 Non-isomorphic representatives of the a(0) = 1 through a(3) = 10 set-systems:
%e A326904   {}  {}     {}           {}
%e A326904       {{1}}  {{1}}        {{1}}
%e A326904              {{1,2}}      {{1,2}}
%e A326904              {{2},{1,2}}  {{1,2,3}}
%e A326904                           {{2},{1,2}}
%e A326904                           {{3},{1,2,3}}
%e A326904                           {{2,3},{1,2,3}}
%e A326904                           {{3},{1,3},{2,3}}
%e A326904                           {{3},{2,3},{1,2,3}}
%e A326904                           {{3},{1,3},{2,3},{1,2,3}}
%Y A326904 The covering case is A108800(n - 1).
%Y A326904 The case with an edge containing all of the vertices is A193674(n - 1).
%Y A326904 The case with union instead of intersection is A193674.
%Y A326904 The labeled version is A326901.
%Y A326904 Cf. A000798, A001930, A006058, A102895, A102898, A326876, A326866, A326878, A326882, A326903, A326906.
%K A326904 nonn,more
%O A326904 0,2
%A A326904 _Gus Wiseman_, Aug 04 2019