I want to solve the following exercise:
Does there exist a nonzero function $f: \mathbb{R} \rightarrow \mathbb{R}$ such that $f^{(iv)}(t) + f(t) = 0$ for all $t \in \mathbb{R}$ and $\lim_{t \rightarrow + \infty} f(t) = \lim_{t \rightarrow + \infty}f'(t) = 0$?
but I don't understand what is meant with $f^{(iv)}$ here. They don't say what $i$ or $v$ are anywhere.