# Functional equation with strange property about irrational numbers

Let $f:\mathbb R \to \mathbb R$ be a function such that for any irrational number r, and any real number x we have $f(x)=f(x+r)$. Show that f is a constant function.

It's easy to see any constant function satisfies the original properti... But I don't see how to show this is the only solution.

If $y$ is irrational, the set $x = 0$, $r = y$ to get $f(y) = f(0)$.

If $y$ is rational, then set $x = y$, $r = \pi-y$ to get $f(y) = f(\pi)$

But by the first statement, $f(\pi) = f(0)$. Hence, $f$ is constant.

• I think that in the last line, $\pi$ has to be $y$. – The Great Seo Aug 3 '14 at 4:20
• The first statement is that $f(y) = f(0)$ for any irrational number $y$. In particular, $y = \pi$ is irrational, so $f(\pi) = f(0)$. – JimmyK4542 Aug 3 '14 at 4:22
• If you have $f(\pi)=f(0)$, it doesn't imply that for all $q\in\Bbb Q\quad f(q)=f(0)$, which is necessary to prove that $f$ is constant. – The Great Seo Aug 3 '14 at 4:23
• Oh, I mistaked. Your answer just omitted that. Sorry for that. – The Great Seo Aug 3 '14 at 4:27

Wrong question.

The general solution of $f(x)=f(x+r)$ is $f(x)=\theta(x)$ , where $\theta(x)$ is an arbitrary periodic functions with period $r$ .

Adding the condition $f:\mathbb{R}\to\mathbb{R}$ the general solution will be modified to $f(x)=\theta(x)$ , where $\theta(x)$ is an arbitrary periodic $\mathbb{R}\to\mathbb{R}$ functions with period $r$ .

For example a particular solution $f(x)=\sin\dfrac{2\pi x}{r}$ , it is a $\mathbb{R}\to\mathbb{R}$ function, a periodic function with period $r$ and satisfies $f(x)=f(x+r)$ .

• $r$ is an arbitrary irrational number; this function is periodic with every irrational period. – Ian Aug 3 '14 at 4:21