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 A108798 Number of nonisomorphic systems enumerated by A102894; that is, the number of inequivalent closure operators in which the empty set is closed. Also, the number of union-closed sets with n elements that contain the universe and the empty set. 15
 1, 1, 3, 14, 165, 14480, 108281182, 2796163091470050 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Also the number of unlabeled finite sets of subsets of {1..n} that contain {} and {1..n} and are closed under intersection. - Gus Wiseman, Aug 02 2019 LINKS Bonacina, Maria Paola; Dershowitz, Nachum Canonical ground Horn theories, Lecture Notes in Computer Science 7797, 35-71 (2013). G. Brinkmann and R. Deklerck, Generation of Union-Closed Sets and Moore Families, Journal of Integer Sequences, Vol.21 (2018), Article 18.1.7. G. Brinkmann and R. Deklerck, Generation of Union-Closed Sets and Moore Families, arXiv:1701.03751 [math.CO], 2017 FORMULA a(n) = A108800(n)/2. EXAMPLE From Gus Wiseman, Aug 02 2019: (Start) Non-isomorphic representatives of the a(0) = 1 through a(3) = 14 union-closed sets of sets: {} {}{1} {}{12} {}{123} {}{2}{12} {}{3}{123} {}{1}{2}{12} {}{23}{123} {}{1}{23}{123} {}{3}{23}{123} {}{13}{23}{123} {}{2}{3}{23}{123} {}{2}{13}{23}{123} {}{3}{13}{23}{123} {}{12}{13}{23}{123} {}{2}{3}{13}{23}{123} {}{3}{12}{13}{23}{123} {}{2}{3}{12}{13}{23}{123} {}{1}{2}{3}{12}{13}{23}{123} (End) CROSSREFS The labeled version is A102894. Cf. A000612, A001930, A003180, A102895, A102897, A108800, A193674, A193675, A326867, A326869, A326883. Sequence in context: A090897 A120459 A272368 * A260995 A220438 A250001 Adjacent sequences: A108795 A108796 A108797 * A108799 A108800 A108801 KEYWORD nonn,more AUTHOR Don Knuth, Jul 01 2005 EXTENSIONS a(6) added (using A193674) by N. J. A. Sloane, Aug 02 2011 Added a(7), and reference to union-closed sets. - Gunnar Brinkmann, Feb 05 2018 STATUS approved

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Last modified December 7 04:29 EST 2022. Contains 358649 sequences. (Running on oeis4.)