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Search: id:A087729
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| A087729 |
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Let X be the poset of finite subsets of the positive integers. The sequence is the number of downsets in X of cardinality n modulo equivalence by permutations of the positive integers. |
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+0
1
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| 1, 1, 1, 2, 2, 3, 5, 8, 11, 18, 29, 49, 83, 148, 267, 507, 977, 1958, 4041, 8626, 18942
(list; graph; listen)
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OFFSET
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1,4
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COMMENT
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a(21) took about 78 hours to compute on a fast DEC alpha (using MAGMA).
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EXAMPLE
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a(5) = 2 because the only example up to permutation are {{},{1},{2},{1,2},{3}} {{},{1},{2},{3},{4}}.
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CROSSREFS
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Sequence in context: A111181 A076777 A111123 this_sequence A039890 A018136 A022863
Adjacent sequences: A087726 A087727 A087728 this_sequence A087730 A087731 A087732
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KEYWORD
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hard,more,nonn,nice
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AUTHOR
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Victor S. Miller (victor(AT)idaccr.org), Sep 30 2003
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