Question 49
We are given the set $M_n = \{0, 1, 2, ..., n\}$ of all non-negative integers less than or equal to $n$. We call a subset $S$ of $M_n$ *outstanding* if it is not empty and a $k$-element subset of $S$ exists for all $k \in S$. Determine the number of outstanding subsets of $M_n$.