# A suggestion on functional equations [closed]

The Help I need I am preparing for a competitive math exam. I need to learn methods of solving questions like this

If f$\left(\frac{x+y}{3}\right)= \frac{2+f(x)+f(y)}{3}$ for all real $x$ and $y$ and $f'(2)=2$, then determine y=f(x)

My Approach I can not approach because i haven't studied enough .That's why i need book suggestions I don't want to study functional equations deeply right now. I want study just enough to solve these kind of (elementary level)problems as given above.

Please Suggest me some appropiate books

## closed as off-topic by user21820, B. Mehta, Henrik, Jack, NamasteNov 30 '17 at 18:45

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "This question is not about mathematics, within the scope defined in the help center." – user21820, B. Mehta
If this question can be reworded to fit the rules in the help center, please edit the question.

Roughly speaking, there are no methods. Functional equations are mostly a dead end. The lucky few that can be solved are usually dealt with by repeated plugging of various suitable values as arguments.

To begin with, let's get rid of 2, for I feel it would look a little simpler that way: $$f(x)=g(x)+2\text{, then}\\ g\left({x+y\over3}\right)={g(x)+g(y)\over3}$$ Now plug $0$ for both arguments, and you'll find out that $g(0)=0$.

Now plug $x$ and $2x$, and you'll see that $g(2x)=2g(x)$.

At this point we might start to suspect that our equation is in fact equivalent to Cauchy's functional equation. We may continue in this manner until our domain is shredded according to Hamel basis. If you don't like the taste of it, then skip this step. Anyway, one particular family of solutions was already obvious before it: $g(x)=kx$, or $\color{red}{f(x)=kx+2}$. (Plug it into your original equation to make sure it fits!)

Now, if you want your derivative anywhere to be 2, then $k=2$ and we are done.

So it goes.

• Please suggest me a book according to my purpose – Kislay Tripathi Oct 26 '17 at 12:57