Find all non-negative integer numbers $n$ for which there exists integers $a$ and $b$ such that $n^2=a+b$ and $n^3=a^2+b^2.$
To find all non-negative integer numbers \( n \) for which there exist integers \( a \) and \( b \) such that \( n^2 = a + b \) and \( n^3 = a^2 + b^2 \), we will proceed as follows:

1. **Check for \( n = 0 \):**
   - For \( n = 0 \):
     \[
     n^2 = 0^2 = 0 \quad \text{and} \quad n^3 = 0^3 = 0
     \]
     - We need to find \( a \) and \( b \) such that:
       \[
       0 = a + b \quad \text{and} \quad 0 = a^2 + b^2
       \]
     - The only solution to these equations is \( a = 0 \) and \( b = 0 \).
     - Therefore, \( n = 0 \) is a solution.

2. **Check for \( n = 1 \):**
   - For \( n = 1 \):
     \[
     n^2 = 1^2 = 1 \quad \text{and} \quad n^3 = 1^3 = 1
     \]
     - We need to find \( a \) and \( b \) such that:
       \[
       1 = a + b \quad \text{and} \quad 1 = a^2 + b^2
       \]
     - Let's try \( a = 0 \) and \( b = 1 \):
       \[
       1 = 0 + 1 \quad \text{and} \quad 1 = 0^2 + 1^2
       \]
     - This satisfies both equations.
     - Therefore, \( n = 1 \) is a solution.

3. **Check for \( n = 2 \):**
   - For \( n = 2 \):
     \[
     n^2 = 2^2 = 4 \quad \text{and} \quad n^3 = 2^3 = 8
     \]
     - We need to find \( a \) and \( b \) such that:
       \[
       4 = a + b \quad \text{and} \quad 8 = a^2 + b^2
       \]
     - Let's try \( a = 2 \) and \( b = 2 \):
       \[
       4 = 2 + 2 \quad \text{and} \quad 8 = 2^2 + 2^2
       \]
     - This satisfies both equations.
     - Therefore, \( n = 2 \) is a solution.

4. **Check for \( n \geq 3 \):**
   - For \( n \geq 3 \), we need to solve:
     \[
     n^2 = a + b \quad \text{and} \quad n^3 = a^2 + b^2
     \]
     - Let's assume \( a \leq b \). Then \( a \) and \( b \) are non-negative integers.
     - Since \( a + b = n^2 \), \( a \) and \( b \) must be less than or equal to \( n^2 \).
     - We also have \( a^2 + b^2 = n^3 \). For \( n \geq 3 \), the values of \( a \) and \( b \) that satisfy both equations become increasingly difficult to find without violating the non-negative integer constraint.
     - For example, if \( n = 3 \):
       \[
       3^2 = 9 \quad \text{and} \quad 3^3 = 27
       \]
       - We need \( a \) and \( b \) such that:
         \[
         9 = a + b \quad \text{and} \quad 27 = a^2 + b^2
         \]
       - Testing possible values for \( a \) and \( b \) (e.g., \( a = 0, 1, 2, \ldots, 9 \)) does not yield any integer solutions that satisfy both equations.

Therefore, the only non-negative integer solutions for \( n \) are \( n = 0, 1, 2 \).

The final answer is \( \boxed{0, 1, 2} \).