The wording is just ever so slightly different, however the problem is identical.

Problem 3. Determine all functions $f: \mathbb{N} \to \mathbb{N}$ such that $n^2 + f(n)f(m)$ is a multiple of $f(n) + m$ for all natural numbers $m, n$.
1. **Define the assertion**: Let \( P(m, n) \) be the assertion that \( f(n) + m \mid n^2 + f(n)f(m) \).

2. **Initial condition**: Consider \( P(1, 1) \):
   \[
   f(1) + 1 \mid 1^2 + f(1)f(1) \implies f(1) + 1 \mid 1 + f(1)^2
   \]
   Since \( f(1) + 1 \mid f(1)^2 - 1 \), we have:
   \[
   f(1) + 1 \mid (f(1) - 1)(f(1) + 1)
   \]
   This implies \( f(1) + 1 \mid f(1) - 1 \). The only natural number solution is \( f(1) = 1 \).

3. **Prime condition**: For a prime \( p \), consider \( P(p, p) \):
   \[
   f(p) + p \mid p^2 + f(p)f(p) \implies f(p) + p \mid p^2 + f(p)^2
   \]
   This implies \( f(p) + p \mid f(p)^2 - p^2 \). Therefore:
   \[
   f(p) + p \mid (f(p) - p)(f(p) + p)
   \]
   Since \( f(p) + p \) divides itself, it must divide \( f(p) - p \). Thus, \( f(p) - p \) must be a multiple of \( f(p) + p \). The only natural number solution is \( f(p) = p \).

4. **General condition**: Consider \( P(m, 1) \):
   \[
   f(m) + 1 \mid m^2 + f(m)f(1) \implies f(m) + 1 \mid m^2 + f(m)
   \]
   Since \( f(1) = 1 \), we have:
   \[
   f(m) + 1 \mid m^2 + f(m)
   \]
   This implies \( f(m) + 1 \mid m^2 - 1 \).

5. **Another general condition**: Consider \( P(1, m) \):
   \[
   f(1) + m \mid 1^2 + f(1)f(m) \implies 1 + m \mid 1 + f(m)
   \]
   Since \( f(1) = 1 \), we have:
   \[
   1 + m \mid 1 + f(m)
   \]
   This implies \( 1 + m \mid f(m) - m \).

6. **Combining conditions**: From the two conditions:
   \[
   f(m) + 1 \mid m^2 - 1 \quad \text{and} \quad 1 + m \mid f(m) - m
   \]
   We see that \( f(m) = m \) satisfies both conditions. 

7. **Verification**: Verify that \( f(n) = n \) satisfies the original condition:
   \[
   f(n) + m \mid n^2 + f(n)f(m) \implies n + m \mid n^2 + nm
   \]
   Since \( n + m \mid n(n + m) \), the condition is satisfied.

Thus, the only function that satisfies the given condition is \( f(n) = n \).

The final answer is \( \boxed{ f(n) = n } \).