(Sterling Numbers) The Sterling number $S(n, r)$ counts the number of ways $n$ people can be divided in $r$ non empty teams.
(Equivalently, it is the number of ways to partition a set of size $n$ into $k$ nonempty subsets.) The teams should be considered non
specific. This means that the teaming {\{a\}, \{b\}, \{c,d\}} is same as {\{b\}, \{a\}, \{c,d\}}.
1. What are the values of $S(n, n)$ and $S(n, 1)$? (1-point)
2. List all the ways one can form 3 teams from a set of 5 people. Use this to determine the value of $S(5, 3)$. (2-points)
3. Show (using a combinatorial proof or otherwise) that for any natural numbers $n$ and $r$,
$S(n+1,r) = S(n, r-1) +r S(n,r)$