Let $P$ be a polynom of degree $n \geq 5$ with integer coefficients given by $P(x)=a_{n}x^n+a_{n-1}x^{n-1}+\cdots+a_0 \quad$ with $a_i \in \mathbb{Z}$, $a_n \neq 0$.

Suppose that $P$ has $n$ different integer roots (elements of $\mathbb{Z}$) : $0,\alpha_2,\ldots,\alpha_n$. Find all integers $k \in \mathbb{Z}$ such that $P(P(k))=0$.
1. Given that \( P(x) \) is a polynomial of degree \( n \geq 5 \) with integer coefficients and \( n \) distinct integer roots \( 0, \alpha_2, \ldots, \alpha_n \), we can write:
   \[
   P(x) = a_n x (x - \alpha_2) (x - \alpha_3) \cdots (x - \alpha_n)
   \]
   where \( a_n \neq 0 \) and \( a_i \in \mathbb{Z} \).

2. We need to find all integers \( k \in \mathbb{Z} \) such that \( P(P(k)) = 0 \). This implies that \( P(k) \) must be one of the roots of \( P(x) \), i.e., \( P(k) \in \{0, \alpha_2, \ldots, \alpha_n\} \).

3. First, consider the case where \( P(k) = 0 \). This means:
   \[
   P(k) = a_n k (k - \alpha_2) (k - \alpha_3) \cdots (k - \alpha_n) = 0
   \]
   Since \( a_n \neq 0 \), \( k \) must be one of the roots of \( P(x) \), i.e., \( k \in \{0, \alpha_2, \ldots, \alpha_n\} \).

4. Next, consider the case where \( P(k) = \alpha_i \) for some \( i \in \{2, 3, \ldots, n\} \). This means:
   \[
   a_n k (k - \alpha_2) (k - \alpha_3) \cdots (k - \alpha_n) = \alpha_i
   \]
   Since \( a_n, \alpha_i, k \neq 0 \), we have \( k \mid \alpha_i \). This implies that \( k \) is a divisor of \( \alpha_i \).

5. For \( k \) to be a divisor of \( \alpha_i \), we must have \( k \mid k - \alpha_i \). This implies \( k - \alpha_i \mid \alpha_i \) and \( k - \alpha_i \mid k \). Therefore, \( |k - \alpha_i| = |k| \), which means \( 2k = \alpha_i \).

6. Plugging \( k = \frac{\alpha_i}{2} \) back into the polynomial equation, we get:
   \[
   a_n \left( \frac{\alpha_i}{2} \right) \left( \frac{\alpha_i}{2} - \alpha_2 \right) \cdots \left( \frac{\alpha_i}{2} - \alpha_n \right) = \alpha_i
   \]
   Since \( \alpha_i \) is an integer, \( \frac{\alpha_i}{2} \) must also be an integer, which implies that \( \alpha_i \) must be even.

7. However, since \( \alpha_i \) are distinct integers and there are at least four distinct values among \( \alpha_2, \ldots, \alpha_n \), it is impossible for all \( k - \alpha_i \) to be distinct and belong to the set \( \{-2, -1, 1, 2\} \) without contradiction.

8. Therefore, the only possible values for \( k \) are the roots of \( P(x) \), i.e., \( k \in \{0, \alpha_2, \ldots, \alpha_n\} \).

The final answer is \( \boxed{ k \in \{0, \alpha_2, \ldots, \alpha_n\} } \).