We denote the number of partitions of a set of n elements by P(n).
Suppose the number of partitions of a set on n elements into k parts is
denoted by P(n,k). Then obviously
P(n) = P(n,1) + P(n,2) + ….. + P(n,n)
Show that P(n,2) = 2^(n-1) - 1
