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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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1, 1, 1, 2, 2, 3, 5, 8, 11, 18, 29, 49, 83, 148, 267, 507, 977, 1958, 4041, 8626, 18942
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,4
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COMMENTS
| a(21) took about 78 hours to compute on a fast DEC alpha (using MAGMA).
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EXAMPLE
| 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
| Sequence in context: A076777 A111123 A179523 * A039890 A152948 A018136
Adjacent sequences: A087726 A087727 A087728 * A087730 A087731 A087732
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KEYWORD
| hard,more,nonn,nice
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AUTHOR
| Victor S. Miller (victor(AT)idaccr.org), Sep 30 2003
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