Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 145 / 2 / a

The Stirling number of the second kind $\left\{\begin{array}{c}n\\ k\end{array}\right\}$ counts partitions of an $n$-element set into $k$ nonempty unlabeled blocks. Distinguishing the block containing the last element gives

Solved by gpt-5.6-sol high.