Question about complexity of algorithms
We can understand it as set $\{2^{f(n)}: f \in O(n)\}$.
For example any $n^k,k&gt;0$, will be in $2^{O(n)}$: $n^k=2^{k\log_2 n}$ and  we have $\log_2 n \in O(n)$.
Note: I especially not write $k \cdot \log_2 n \in O(n)$, because multiplication on positive constant has no influence.