Quote:
Originally Posted by magdon
The growth function cannot be any polynomial function of N. A valid growth function must satisfy the following theorem:
Theorem: If for some (any) , then for all N, .
In your example growth function below, try to show that the precondition of the theorem is satisfied with and hence deduce a linear bound on m(N). This contradicts the growth function being cubic. More generally, a cubic growth function cannot have a break point less than 4.

To show that a growth function is invalid, is it sufficient to do this?
 Determine the smallest where , such that would be our if the growth function were valid;
 Find any concrete value of where one of the inequalities vs is violated.