OFFSET
1,4
COMMENTS
a(21) took about 78 hours to compute on a fast DEC alpha (using Magma).
EXAMPLE
a(5) = 2 because the only example up to permutation are {{},{1},{2},{1,2},{3}} {{},{1},{2},{3},{4}}.
CROSSREFS
KEYWORD
hard,more,nonn,nice
AUTHOR
Victor S. Miller, Sep 30 2003
STATUS
approved