Do there exist positive integers $a$ and $b$ such that $a^n+n^b$ and $b^n+n^a$ are relatively prime for all natural $n$?
To determine whether there exist positive integers \(a\) and \(b\) such that \(a^n + n^b\) and \(b^n + n^a\) are relatively prime for all natural \(n\), we will analyze the given conditions and use properties of the greatest common divisor (gcd).

1. **Assume \(a\) and \(b\) are positive integers:**
   Suppose \(a, b \in \mathbb{N}\).

2. **Consider the gcd of \(a\) and \(b\):**
   Let \(d = \gcd(a, b)\). If \(d > 1\), then \(d\) divides both \(a\) and \(b\). For \(n = d\), we have:
   \[
   d \mid a^d + d^b \quad \text{and} \quad d \mid b^d + d^a
   \]
   This implies \(d\) divides both \(a^d + d^b\) and \(b^d + d^a\), which contradicts the requirement that \(a^n + n^b\) and \(b^n + n^a\) are relatively prime for all \(n\). Therefore, \(d = 1\), meaning \(a\) and \(b\) must be coprime.

3. **Case when \(a\) and \(b\) are both odd:**
   If \(a\) and \(b\) are both odd, consider \(n = q - 1\) where \(q\) is a prime number such that \(\gcd(a, q) = 1\) and \(\gcd(b, q) = 1\). Then:
   \[
   q \mid a^{q-1} + (q-1)^b \quad \text{and} \quad q \mid b^{q-1} + (q-1)^a
   \]
   By Fermat's Little Theorem, \(a^{q-1} \equiv 1 \pmod{q}\) and \(b^{q-1} \equiv 1 \pmod{q}\). Thus:
   \[
   q \mid 1 + (q-1)^b \quad \text{and} \quad q \mid 1 + (q-1)^a
   \]
   Since \((q-1)^b \equiv (-1)^b \pmod{q}\) and \((q-1)^a \equiv (-1)^a \pmod{q}\), we have:
   \[
   q \mid 1 + (-1)^b \quad \text{and} \quad q \mid 1 + (-1)^a
   \]
   If \(a\) and \(b\) are both odd, then \((-1)^a = -1\) and \((-1)^b = -1\), so:
   \[
   q \mid 1 - 1 \quad \text{which is false}
   \]
   This leads to a contradiction, so \(a\) and \(b\) cannot both be odd.

4. **Case when \(a\) is even and \(b\) is odd:**
   Let \(a\) be even and \(b\) be odd. Consider \(n = 2\):
   \[
   a^2 + 2^b \quad \text{and} \quad b^2 + 2^a
   \]
   Since \(a\) is even, \(a^2\) is even, and since \(b\) is odd, \(2^b\) is even. Thus, \(a^2 + 2^b\) is even. Similarly, since \(b\) is odd, \(b^2\) is odd, and since \(a\) is even, \(2^a\) is even. Thus, \(b^2 + 2^a\) is odd. An even number and an odd number are always relatively prime.

5. **General case for \(a\) even and \(b\) odd:**
   For any natural number \(n\), if \(a\) is even and \(b\) is odd, \(a^n\) is even and \(n^b\) is even if \(n\) is even, and odd if \(n\) is odd. Similarly, \(b^n\) is odd and \(n^a\) is even if \(n\) is even, and odd if \(n\) is odd. Therefore, \(a^n + n^b\) and \(b^n + n^a\) will always be one even and one odd, making them relatively prime.

Conclusion:
There exist positive integers \(a\) and \(b\) such that \(a^n + n^b\) and \(b^n + n^a\) are relatively prime for all natural \(n\) if and only if \(a\) is even and \(b\) is odd.

The final answer is \( \boxed{ a } \) is even and \(b\) is odd.