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Search: id:A055621
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| A055621 |
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Number of covers of an unlabeled n-set. |
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+0
4
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| 1, 4, 34, 1952, 18664632, 12813206150470528, 33758171486592987151274638874693632, 1435913805026242504952006868879460423801146743462225386100617731367239680
(list; graph; listen)
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OFFSET
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1,2
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REFERENCES
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F. Bergeron, G. Labelle and P. Leroux, Combinatorial Species and Tree-Like Structures, Cambridge, 1998, pg 78 (2.3.39)
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LINKS
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Eric Weisstein's World of Mathematics, See also
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FORMULA
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a(n)=(A003180(n)-A003180(n-1))/2.
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EXAMPLE
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There are 4 nonisomorphic covers of {1,2}, namely {{1},{2}}, {{1,2}}, {{1},{1,2}}, and {{1},{2},{1,2}}.
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CROSSREFS
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Cf. A003465 (labeled case).
Cf. A003465 (labeled case). Differences of A000612.
Sequence in context: A030243 A088077 A113231 this_sequence A000860 A027681 A097382
Adjacent sequences: A055618 A055619 A055620 this_sequence A055622 A055623 A055624
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KEYWORD
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easy,nonn
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AUTHOR
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Vladeta Jovovic (vladeta(AT)Eunet.yu), Jun 04 2000
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EXTENSIONS
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More terms from David Moews (dmoews(AT)xraysgi.ims.uconn.edu) Jul 04 2002
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