Does there exist a function
![f : \mathbb N \to \mathbb N](/media/m/d/2/7/d27759f0adbec42c0087891b61c15eb3.png)
, such that
![f(f(n)) =n + 1987](/media/m/6/1/1/611937126cb79d7a984e5cd1f2530d83.png)
for every natural number
![n](/media/m/a/e/5/ae594d7d1e46f4b979494cf8a815232b.png)
? (IMO Problem 4)
Proposed by Vietnam.
%V0
Does there exist a function $f : \mathbb N \to \mathbb N$, such that $f(f(n)) =n + 1987$ for every natural number $n$? (IMO Problem 4)
Proposed by Vietnam.