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A055621
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Number of covers of an unlabeled n-set.
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4
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1, 4, 34, 1952, 18664632, 12813206150470528, 33758171486592987151274638874693632, 1435913805026242504952006868879460423801146743462225386100617731367239680
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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REFERENCES
| 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
| Eric Weisstein's World of Mathematics, Cover
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FORMULA
| a(n)=(A003180(n)-A003180(n-1))/2.
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EXAMPLE
| 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
| Cf. A003465 (labeled case). Differences of A000612.
Sequence in context: A088077 A162079 A113231 * A000860 A027681 A193994
Adjacent sequences: A055618 A055619 A055620 * A055622 A055623 A055624
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KEYWORD
| easy,nonn
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AUTHOR
| Vladeta Jovovic (vladeta(AT)eunet.rs), Jun 04 2000
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EXTENSIONS
| More terms from David Moews (dmoews(AT)xraysgi.ims.uconn.edu) Jul 04 2002
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