grandes-ecoles 2016 QII.B.1

grandes-ecoles · France · centrale-maths2__pc Combinations & Selection Counting Functions or Mappings with Constraints
For every pair $(p, k)$ of nonzero natural numbers, we denote by $S(p, k)$ the number of surjections from $\llbracket 1, p \rrbracket$ to $\llbracket 1, k \rrbracket$. Consistently, for every $p \in \mathbb{N}^*$, we set $S(p, 0) = 0$.
How many applications are there from $\llbracket 1, p \rrbracket$ to $\llbracket 1, n \rrbracket$?
For every pair $(p, k)$ of nonzero natural numbers, we denote by $S(p, k)$ the number of surjections from $\llbracket 1, p \rrbracket$ to $\llbracket 1, k \rrbracket$. Consistently, for every $p \in \mathbb{N}^*$, we set $S(p, 0) = 0$.

How many applications are there from $\llbracket 1, p \rrbracket$ to $\llbracket 1, n \rrbracket$?