Partitions of a set P=[A1,A2,...,Ak}is a partition of S: A,卡0 A∩A=0 A1UA2U..UAk=S of k-partitions of an n-set "Stirling number of the second kind" B.-∑{} of partitions of an n-set k=1 “Bell number'Partitions of a set # of k-partitions of an n-set “Stirling number of the second kind” # of partitions of an n-set “Bell number” ￾n k ￾ Bn = ￾ n k=1 ￾n k ￾ P = {A1, A2,...,Ak} is a partition of S: Ai ￾= ￾ Ai ￾ Aj = ￾ A1 ￾ A2 ￾ ··· ￾ Ak = S