

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



