It is known that the function f (x) is a monotone increasing function on (0, positive infinity). When n belongs to a positive integer, f (n) belongs to a positive integer, and f (f (n)) = 3N, then f (5) =

It is known that the function f (x) is a monotone increasing function on (0, positive infinity). When n belongs to a positive integer, f (n) belongs to a positive integer, and f (f (n)) = 3N, then f (5) =

F (f (1)) = 3, because f (1) is a positive integer, f (1) > = 1, when f (1) = 1, f (f (1)) = 3 does not hold, so f (1) > 1, so f (f (1)) > F (1), because f (f (1)) = 3, so f (1) can only be 2, so f (2) = 3
f(f(2))=6 => f(3)=6
f(f(3))=9 => f(6)=9
Because f (3) = 6, f (6) = 9, the function increases monotonically, and the function value of positive integer is positive integer, so there must be f (4) = 7, f (5) = 8