a(0)=1到a(3)=19个集合系统的非同构表示在并集下闭合:
{} {} {} {}
{{1}} {{1}} {{1}}
{{1,2}} {{1,2}}
{{2},{1,2}} {{1,2,3}}
{{1},{2},{1,2}} {{2},{1,2}}
{{3},{1,2,3}}
{{1},{2},{1,2}}
{{2,3},{1,2,3}}
{{1},{2,3},{1,2,3}}
{{3},{2,3},{1,2,3}}
{{1,3},{2,3},{1,2,3}}
{{2},{3},{2,3},{1,2,3}}
{{2},{1,3},{2,3},{1,2,3}}
{{3},{1,3},{2,3},{1,2,3}}
{{1,2},{1,3},{2,3},{1,2,3}}
{{2},{3},{1,3},{2,3},{1,2,3}}
{{3},{1,2},{1,3},{2,3},{1,2,3}}
{{2},{3},{1,2},{1,3},{2,3},{1,2,3}}
{{1},{2},{3},{1,2},{1,3},{2,3},{1,2,3}}
(结束)