List in increasing order all positive integers $n \leq 40$ such that $n$ cannot be written in the form $a^{2} - b^{2}$, where $a$ and $b$ are positive integers.
List in increasing order all positive integers $n \leq 40$ such that $n$ cannot be written in the form $a^{2} - b^{2}$, where $a$ and $b$ are positive integers.