Throughout, we let denote the cardinality of a finite set .
Recall that the Stirling numbers of the second kind can be defined as the coefficients that express powers of an indeterminate as linear combinations of falling powers of :
where and .
Let be the number of surjections from an -element set to a -element set. Then .
The assertion is the polynomial identity
but this is clear since it holds whenever is an integer , by a bijective proof: counts the number of functions from an -element set to a -element set, and each such function factors as a surjection onto some -element subset (of the -element set), followed by the subset inclusion. There are \binom(j}{k}
such subset inclusions.
It is well-known (and can be proven by any number of methods, e.g., generating functions) that