What are the properties of continuous functions satisfying $f(x)=f(x^2+c)$?

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In summary, the properties of continuous functions satisfying $f(x)=f(x^2+c)$ are as follows: the domain of these functions is the set of all real numbers, they can have infinite solutions, some examples include $f(x)=\sin(x)$, $f(x)=e^x$, and $f(x)=\frac{1}{x+1}$, they are not one-to-one, and they are important in mathematics for their non-trivial solutions and various applications in fields such as number theory and dynamical systems.
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Ackbach
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Here is this week's POTW:

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Let $c>0$ be a constant. Give a complete description, with proof, of the set of all continuous functions $f:\mathbb{R}\to\mathbb{R}$ such that $f(x)=f(x^2+c)$ for all $x\in\mathbb{R}$.

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Remember to read the http://www.mathhelpboards.com/showthread.php?772-Problem-of-the-Week-%28POTW%29-Procedure-and-Guidelines to find out how to http://www.mathhelpboards.com/forms.php?do=form&fid=2!
 
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No one answered this week's POTW. This was a Putnam problem from 1996. I post an Internet solution below:

We first consider the case $c<1/4$; we shall show in this case that $f$ must be constant. The relation
$$f(x)=f(x^2+c)=f((-x)^2+c)=f(-x)$$
proves that $f$ is an even function. Let $r_1\le r_2$ be the roots of $x^2+c-x,$ both of which are real. If $x>r_2,$ define $x_0=x$ and $x_{n+1}=\sqrt{x_n-c}$ for each positive integer $x$. By induction on $n$, $r_2<x_{n+1}<x_n$ for all $n$, so the sequence $\{x_n\}$ tends to a limit $L$ which is a root of $x^2+c=x$ not less than $r_2$. Of course this means $L=r_2$. Since $f(x)=f(x_n)$ for all $n$ and $x_n\to r_2$, we conclude $f(x)=f(r_2)$, so $f$ is constant on $x\ge r_2$.

If $r_1<x<r_2$ and $x_n$ is defined as before, then by induction, $x_n<x_{n+1}<r_2$. Note that the sequence can be defined because $r_1>c$; the latter follows by noting that the polynomial $x^2-x+c$ is positive at $x=c$ and has its minimum at $1/2>c$, so both roots are greater than $c$. In any case, we deduce that $f(x)$ is also constant on $r_1\le x\le r_2$.

Finally, suppose $x<r_1$. Now define $x_0=x, \; x_{n+1}=x_n^2+c$. Given that $x_n<r_1$, we have $x_{n+1}>x_n$. Thus if we had $x_n<r_1$ for all $n$, by the same argument as in the first case we deduce $x_n\to r_1$ and so $f(x)=f(r_1)$. Actually, this doesn't happen; eventually we have $x_n>r_1$, in which case $f(x)=f(x_n)=f(r_1)$ by what we have already shown. We conclude that $f$ is a constant function.

Now suppose $c>1/4$. Then the sequence $x_n$ defined by $x_0=0$ and $x_{n+1}=x_n^2+c$ is strictly increasing and has no limit point. Thus if we define $f$ on $[x_0,x_1]$ as any continuous function with equal values on the endpoints, and extend the definition from $[x_n,x_{n+1}]$ to $[x_{n+1},x_{n+2}]$ by the relation $f(x)=f(x^2+c)$, and extend the definition further to $x<0$ by the relation $f(x)=f(-x)$, the resulting function has the desired property. Moreover, any function with that property clearly has this form.
 

FAQ: What are the properties of continuous functions satisfying $f(x)=f(x^2+c)$?

What are the properties of continuous functions satisfying $f(x)=f(x^2+c)$?

The properties of continuous functions satisfying $f(x)=f(x^2+c)$ are as follows:

What is the domain of such functions?

The domain of these functions is the set of all real numbers.

Can these functions have infinite solutions?

Yes, these functions can have infinite solutions. For example, the function $f(x)=x$ and any constant function $f(x)=c$ both satisfy the given equation.

What are some examples of continuous functions satisfying $f(x)=f(x^2+c)$?

Some examples of continuous functions satisfying $f(x)=f(x^2+c)$ are $f(x)=\sin(x)$, $f(x)=e^x$, and $f(x)=\frac{1}{x+1}$. These functions satisfy the given equation for any value of $c$.

Are these functions one-to-one?

No, these functions are not one-to-one. Since $x^2$ and $x^2+c$ have different domains, it is possible for different values of $x$ to map to the same value of $y$.

What is the significance of these functions in mathematics?

These functions are important in mathematics because they are examples of non-trivial solutions to functional equations. They also have various applications in fields such as number theory and dynamical systems.

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