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A108798
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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.
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15
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OFFSET
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0,3
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COMMENTS
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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
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LINKS
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FORMULA
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EXAMPLE
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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)
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CROSSREFS
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Cf. A000612, A001930, A003180, A102895, A102897, A108800, A193674, A193675, A326867, A326869, A326883.
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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