<s>[INST] Find all polynomials such that $p(x^2-2x)=p(x-2)^2$ 
Find all polynomials $p\in \mathbb{C}[x]$ such that $$p(x^2-2x)=p(x-2)^2$$

We can not say anything specific about the degree since both sides are of the degree $2n$.
Also by copering the coeficients we see that the leading coefficent must be $1$.
Setting $$x^2-2x = x-2\implies x\in\{1,2\}$$
If $x= 2$ we get $p(0)=p(0)^2$ so $p(0)\in\{0,1\}$.
If $x= 1$ we get $p(-1)=p(-1)^2$ so $p(-1)\in\{0,1\}$.
Now it sems there is a lot of options. Also if $p$ is linear we have $$a(x^2-2x)+b= (ax-2a+b)^2 =a^2x^2+2a(b-2a)x+(b-2a)^2$$
so $a=a^2$ and $b=(b-2a)^2$ and $-2a=2a(b-2a)$ so $a=1$ and $b=1$ thus $p(x)=x+1$. But how to finish in general?
 [/INST] EDIT: (I made a change of variable $x-1\mapsto x$ as suggested by @Hw Chu.) We need to solve $\displaystyle p((x-1)^2-1)=p(x-1-1)^2$ or $p(x^2-1)=p(x-1)^2$ equivalently. Let  $$\displaystyle q(x) =p(x-1)= cx^k\prod_{1\le i\le N\\a_i\ne 0} (x-a_i)^{n_i},$$ where $c\ne 0$ and $a_j\ne a_k$ for $j\ne k$. Plugging this into the equation $q(x^2)=q(x)^2$, we obtain
$$
cx^{2k}\prod_{i=1}^N(x^2-a_i)^{n_i} =c^2x^{2k}\prod_{i=1}^N (x-a_i)^{2n_i},\\
\prod_{i=1}^N(x+\sqrt{a_i})^{n_i}(x-\sqrt{a_i})^{n_i}=c\prod_{i=1}^N (x-a_i)^{2n_i}.
$$ Now, we find that the LHS has $2N$ linear factors because $$
x+\sqrt{a_j}\ne x-\sqrt{a_j},\\
x\pm\sqrt{a_j} \ne x\pm\sqrt{a_k}
$$ for all $j\ne k$. Since the RHS has only $N$ linear factors, it follows $N=0$ and $c=1$. Thus, non-trivial solutions are $p(x)=q(x+1)=(x+1)^k$ for some $k\ge 0$.
</s>