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.

1, 1, 1, 2, 2, 3, 5, 8, 11, 18, 29, 49, 83, 148, 267, 507, 977, 1958, 4041, 8626, 18942

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 A347546 A152948

Adjacent sequences: A087726 A087727 A087728 * A087730 A087731 A087732

Keyword

hard,more,nonn,nice

Author

Victor S. Miller, Sep 30 2003

Status

approved