4=(4)=(3)+(1)=(3+1)=(2+1)+(1).
a(n)是其多集合并是n的整数分区的集合系统(集合集)的数量。例如,a(1)=1到a(6)=12个集合系统是:
{{1}} {{2}} {{3}} {{4}} {{5}} {{6}}
{{1,2}} {{1,3}} {{1,4}} {{1,5}}
{{1},{2}} {{1},{3}} {{2,3}} {{2,4}}
{{1},{1,2}} {{1},{4}} {{1,2,3}}
{{2},{3}} {{1},{5}}
{{1},{1,3}} {{2},{4}}
{{2},{1,2}} {{1},{1,4}}
{{1},{2,3}}
{{2},{1,3}}
{{3},{1,2}}
{{1},{2},{3}}
{{1},{2},{1,2}}
(结束)