Find all a positive integers $a$ and $b$, such that

$$\frac{a^b+b^a}{a^a-b^b}$$

is an integer
1. We are given the expression \(\frac{a^b + b^a}{a^a - b^b}\) and need to find all positive integers \(a\) and \(b\) such that this expression is an integer.
2. Clearly, \(a \neq b\) because if \(a = b\), the denominator \(a^a - b^b\) would be zero, making the expression undefined.
3. Without loss of generality (WLOG), assume \(a > b\). This implies \(a \geq 2\) since both \(a\) and \(b\) are positive integers.
4. We need to ensure that \(\frac{a^b + b^a}{a^a - b^b}\) is an integer. This requires that \(a^b + b^a\) is divisible by \(a^a - b^b\).
5. Since \(a > b\), we have \(a^a > b^b\). Therefore, \(a^a - b^b\) is a positive integer.
6. Consider the inequality \(a^b + b^a \geq a^a - b^b\). Since \(b \leq a - 1\), we can write:
   \[
   a^{a-1} + (a-1)^a \geq a^a - (a-1)^{a-1}
   \]
7. Rearrange the inequality:
   \[
   a^{a-1} + (a-1)^a \geq a^a - (a-1)^{a-1}
   \]
   This simplifies to:
   \[
   a^{a-1} + (a-1)^a + (a-1)^{a-1} \geq a^a
   \]
8. Dividing both sides by \(a^{a-2}\), we get:
   \[
   a + \left(\frac{a-1}{a}\right)^a + \left(\frac{a-1}{a}\right)^{a-1} \geq a^2
   \]
9. For \(a \geq 3\), the term \(\left(\frac{a-1}{a}\right)^a\) and \(\left(\frac{a-1}{a}\right)^{a-1}\) are both less than 1, making the left-hand side less than \(a^2\), which contradicts the inequality. Therefore, \(a\) must be less than 3.
10. Check \(a = 2\):
    - If \(a = 2\), then \(b = 1\):
      \[
      \frac{2^1 + 1^2}{2^2 - 1^1} = \frac{2 + 1}{4 - 1} = \frac{3}{3} = 1
      \]
      This is an integer.
11. By symmetry, if \(a = 1\) and \(b = 2\):
    - If \(a = 1\), then \(b = 2\):
      \[
      \frac{1^2 + 2^1}{1^1 - 2^2} = \frac{1 + 2}{1 - 4} = \frac{3}{-3} = -1
      \]
      This is also an integer.
12. Therefore, the solutions are \((a, b) = (2, 1)\) and \((a, b) = (1, 2)\).

The final answer is \( \boxed{ (a, b) = (2, 1) } \) and \((a, b) = (1, 2)\).