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A087729 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. 0
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; text; internal format)
OFFSET

1,4

COMMENTS

a(21) took about 78 hours to compute on a fast DEC alpha (using MAGMA).

LINKS

Table of n, a(n) for n=1..21.

EXAMPLE

a(5) = 2 because the only example up to permutation are {{},{1},{2},{1,2},{3}} {{},{1},{2},{3},{4}}.

CROSSREFS

Sequence in context: A111123 A261091 A179523 * A039890 A152948 A018136

Adjacent sequences:  A087726 A087727 A087728 * A087730 A087731 A087732

KEYWORD

hard,more,nonn,nice

AUTHOR

Victor S. Miller, Sep 30 2003

STATUS

approved

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Last modified February 27 21:59 EST 2020. Contains 332312 sequences. (Running on oeis4.)