Here is the question again : Give an example of two increasing functions f (n) and g(n) from the set of positive integers to the set of positive integers such that neither f (n) is O(g(n)) nor g(n) is O(f (n)).

I tried with several functions such as sine and cosine, but they are not increasing functions but Oscillatory. Then I tried x + sinx and x + cosx. However for sufficiently large coefficients c and n0, they also fail two satisfy the criteria. I am starting to doubt if such pairs of functions even exist !?