# find all rational functions $f(x)$ such that $(f(x))^2-f(x^2)=constant$,

Find all $f(x)$ such that $$(f(x))^2-f(x^2)=constant$$

• I Think this proble is very nice, and Now I can't solution.Thank you everyone help – math110 Jul 5 '13 at 12:26
• What is the underlying field of constants? If it is of characteristic two, then, for example, all the functions $f(x)\in\mathbb{F}_2[x,1/x]$ are solutions ;-> – Jyrki Lahtonen Jul 5 '13 at 13:55
• Thank you ,what is $F_{2}[x,1/x]?$ – math110 Jul 5 '13 at 14:01
• $$\mathbb{F}_2[x,1/x]= \{\sum_{i=-n}^ma_ix^i\mid n,m\in\mathbb{Z}, a_i\in\mathbb{F}_2\,\text{for all i}\}.$$ – Jyrki Lahtonen Jul 5 '13 at 14:02
• In characteristic two we have, for example $$(x^3+x)^2=x^6+x^2,$$ so $f(x)=x^3+x$ is a solution in that case. I do expect that you mean that the field of constants is some subfield of the complex numbers. That's why I added the evil grin emoticon. But I do have the point that the term "rational function" does not specify the ground field. – Jyrki Lahtonen Jul 5 '13 at 14:05

Assume that $a$ is a pole of $f$ or order $n$ then $f^2$ has $a$ as a pole of order $2n$ and therefore so does $f(x^2)$. Therefore $a^2$ is a pole of $f$ of order $2n$. Therefore $f$ has no poles different from zero.

Let $g(x):=f(1/x)$. Notice that $g(x)$ also satisfy the equation. Subtract the two equations to get $$(f(x)-g(x))(f(x)+g(x))=f(x^2)-g(x^2).$$

Therefore, for the function $h(x):=f(x)-g(x)$ we have $h(x)(f(x)+g(x))=h(x^2)$. If $a$ is a zero of $h(x)$ of order $n$, then $a^2$ is a zero of $h(x)$ too. Therefore, either $a=0$ or $h(x)=constant$.

Now we have $f(x)=P(x)/x^k$ for some polynomial $P$. Then $P(x)/x^k-P(1/x)/x^{-k}=C$.

If $n$ is the degree of $P$, comparing degrees we get that $n=2k$. Comparing coefficients we get that $P$ is a symmetric polynomial (the coefficients can be put in reverse order), i.e. $P(x)=a_{2k}x^{2k}+a_{2k-1}x^{2k-1}+\ldots+a_{2k-1}x+a_{2k}$.

So, so far we have that $f$ is a linear combination of $x^k+1/x^k$. Plugging a linear combination into $f(x)^2-f(x^2)=constant$,

... to be continued, but from there you should get the solutions ...

• Nice! You should also add $f(x) =$ constant under $k = 0$. – hot_queen Jul 5 '13 at 12:59
• The constant should be $0$ or $1$. Oh, in the recapitulation forgot to mention $1$. Adding it. – OR. Jul 5 '13 at 13:04
• Okay. You are solving $f(x^2) - (f(x))^2 = 0$. It doesn't really matter. – hot_queen Jul 5 '13 at 13:08
• In view of Michael's comment, you probably want to fix the part that says "f can only have 0 as its pole and/or zero implies $f(x) = ax^k$" to account for solutions of form $x^k + 1/x^k$. – hot_queen Jul 5 '13 at 13:14
• I saw that poles of $g$ are not the zeros of $f$ long ago. Typing. – OR. Jul 5 '13 at 13:18

The earlier answers have forgotten $f(x)=x+1/x$

• yes, I think this problem have other – math110 Jul 5 '13 at 13:10
• @S.B. that wasn't the question. Look again. – Empy2 Jul 5 '13 at 13:31