We know that there exist some functions f(x) such that their derivative f′(x) is strictly greater than the function itself. for example the function 5x has a derivative 5xln(5) which is greater than 5x. Exponential functions in general are known to be proportional to their derivatives.

The question I have is whether it is possible for a function to grow “even faster” than this. To be more precise let’s take the ratio f′(x)/f(x) for exponential functions this ratio is a constant. For most elementary functions we care about, this ratio usually tends to 0. But are there functions for which this ratio grows arbitrarily large? If so, is there an upper limit for how large the ratio f′(x)/f(x) can grow? I also ask a similar question for integrals.

**Answer**

Consider the differential equation

f′f=g

where g is the fast-growing function you want. For instance, for g(x)=ex (and say the initial condition f(0)=1) you get

f(x)=eex−1

The ratio f′/f grows arbitrarily large.

**Attribution***Source : Link , Question Author : Ryan Cole , Answer Author : Clement C.*