Find all functions $f:\mathbb N\to\mathbb N$ such that
$$f(x)+f(y)\mid x^2-y^2$$holds for all $x,y\in\mathbb N$.
To find all functions \( f: \mathbb{N} \to \mathbb{N} \) such that 
\[ f(x) + f(y) \mid x^2 - y^2 \]
holds for all \( x, y \in \mathbb{N} \), we will proceed with the following steps:

1. **Assertion and Initial Observations:**
   Let \( P(x, y) \) denote the assertion \( f(x) + f(y) \mid x^2 - y^2 \).

2. **Unboundedness of \( f \):**
   Suppose \( f \) is bounded, i.e., there exists some \( M \) such that \( f(n) < M \) for all \( n \in \mathbb{N} \). Consider a sufficiently large prime \( p = 2k + 1 \). Using \( P(k+1, k) \), we have:
   \[ f(k+1) + f(k) \mid (k+1)^2 - k^2 = 2k + 1 = p. \]
   Since \( 1 < f(k+1) + f(k) < 2M < p \), this implies \( f(k+1) + f(k) \) must be a divisor of \( p \), which is a contradiction because \( p \) is a prime number and \( f(k+1) + f(k) \) cannot be both greater than 1 and less than \( p \). Hence, \( f \) must be unbounded.

3. **Injectivity of \( f \):**
   Suppose \( f(a) = f(b) \) for some \( a > b \). Using \( P(y, a) \) and \( P(y, b) \), we get:
   \[ f(a) + f(y) \mid a^2 - y^2 \quad \text{and} \quad f(b) + f(y) \mid b^2 - y^2. \]
   Subtracting these, we have:
   \[ f(a) + f(y) \mid a^2 - b^2. \]
   Since \( f(a) = f(b) \), this implies:
   \[ f(a) + f(y) \mid (a - b)(a + b). \]
   Given that \( f \) is unbounded, this would lead to a contradiction unless \( a = b \). Therefore, \( f \) must be injective.

4. **Form of \( f \):**
   We will now show that \( f(n) = n \) for all \( n \in \mathbb{N} \). First, consider \( P(1, 2) \), \( P(1, 3) \), and \( P(2, 3) \):
   \[ f(1) + f(2) \mid 1^2 - 2^2 = -3, \]
   \[ f(1) + f(3) \mid 1^2 - 3^2 = -8, \]
   \[ f(2) + f(3) \mid 2^2 - 3^2 = -5. \]
   Since \( f \) is injective and unbounded, we can deduce that \( f(1) = 1 \).

5. **Inductive Step:**
   Suppose \( f(i) = i \) for all \( i < N \). Consider \( P(N, N-1) \):
   \[ f(N) + f(N-1) \mid N^2 - (N-1)^2 = 2N - 1. \]
   Since \( f(N-1) = N-1 \), we have:
   \[ f(N) + (N-1) \mid 2N - 1. \]
   This implies:
   \[ f(N) + N - 1 \mid 2N - 1. \]
   Since \( f \) is injective and \( f(N) \leq N \), the only solution is \( f(N) = N \).

By induction, we conclude that \( f(n) = n \) for all \( n \in \mathbb{N} \).

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