Question 5
Let $k,n$ be positive integers with $k\lt n$. Find all real-coefficient polynomials $P$ of degree $kn$ with leading coefficient $1$ such that
$$Q(x)=P(x^n+1)-(P(x))^n$$
has degree at most $kn(n-1)$.
$$Q(x)=P(x^n+1)-(P(x))^n$$
has degree at most $kn(n-1)$.