The Stirling Number of a second kind is denoted by \(S\left(n,k\right)\) or \(\binom{n}{k}\)
counts the number of ways to partition a set of \(n\) elements into \(k\) non-empty
sets.

For example, \(S\left(3,2\right)\)\(=3\), because there are three ways to partition three objects, namely \(a,b\) and \(c\) into two groups. \(\left(a\right)\left(bc\right),\left(b\right)\left(ac\right)\) and \(\left(c\right)\left(ab\right).\)

\(S\left(n,k\right)=kS\left(n-1,k\right)+S\left(n-1,k-1\right),1\le k<n\)