20. Let $n \in N, n \geqslant 3$, and let $f(n)$ denote the smallest natural number that is not a divisor of $n$, for example $f(12)=5$. If $f(n) \geqslant 3$, then $f(f(n))$ and so on can be defined. If $\underbrace{f(f(\cdots f}_{k \uparrow f}(n) \cdots))=2$, then $k$ is called the length of $n$. For all $n \in N, n \geqslant 3$, the set of lengths of $n$ expressed by enumeration is $\qquad$
20. $\{1,2,3\}$