Problem:
If $f$ is a polynomial with unknown roots $x_1,-x_1,x_2,-x_2\ldots,x_b,-x_b \quad (b \in \mathbb{N})$ and can be expressed as (known expansion):
$f(x)=\sum_{a=0}^{2b}f_ax^a\quad(b \in \mathbb{N})$
How to find $g$, such the roots of $g$ is just $x_1,x_2,\ldots,x_b$?
Details:
Looks like the main problem is solve
$g(x){(-1)}^bg(-x)=f(x)$. But I don't know if this is suficient to force $g$ be of the form $\prod_{i=1}^{b}(x-x_i)$. Looks like using this way, we get a indeterminated system, but if we can express all solutions there's no problem.
Using mathematica an easy problem couldn't be solved, see:
f[x_]:=(x-1)(x+1)
, RSolve[-g[x]g[-x]==f[x],g[x],x]
. The software calculate and calculate...I didn't get answer.
So, using the definition of $f$ above ($f(x)=\sum_{a=0}^{2b}f_ax^a$) how to find the polynomial $g$ with roots $x_1,x_2,\ldots,x_b$?
How can we solve this recursion ($g(x){(-1)}^bg(-x)=f(x)$) and find all $g$ to satisfies it?
If someone knows a computable way to do it, I'd like to know too. If is using Mathematica is more interesting to me.