A positive integer $m$ is perfect if the sum of all its positive divisors, $1$ and $m$ inclusive, is equal to $2m$. Determine the positive integers $n$ such that $n^n + 1$ is a perfect number.
To determine the positive integers \( n \) such that \( n^n + 1 \) is a perfect number, we need to verify the conditions under which \( n^n + 1 \) equals the sum of all its positive divisors, which is \( 2(n^n + 1) \).

1. **Verification for small values of \( n \):**
   - For \( n = 1 \):
     \[
     1^1 + 1 = 2
     \]
     The divisors of 2 are 1 and 2, and their sum is \( 1 + 2 = 3 \neq 2 \times 2 \). Thus, \( n = 1 \) is not a solution.
   - For \( n = 2 \):
     \[
     2^2 + 1 = 5
     \]
     The divisors of 5 are 1 and 5, and their sum is \( 1 + 5 = 6 \neq 2 \times 5 \). Thus, \( n = 2 \) is not a solution.
   - For \( n = 3 \):
     \[
     3^3 + 1 = 28
     \]
     The divisors of 28 are 1, 2, 4, 7, 14, and 28, and their sum is \( 1 + 2 + 4 + 7 + 14 + 28 = 56 = 2 \times 28 \). Thus, \( n = 3 \) is a solution.

2. **Assume \( n \geq 4 \) and \( n^n + 1 \) is a perfect number:**

   **Case 1: \( n \) is odd**
   - Suppose \( n \) is odd and \( n^n + 1 = 2^{p-1}(2^p - 1) \) for some prime \( p \).
   - We have \( v_2(n + 1) = p - 1 \), implying \( n \geq 2^{p-1} - 1 \).
   - Therefore, \( 2^{p-1}(2^p - 1) > (2^{p-1} - 1)^3 \).
   - This inequality is impossible for \( n \geq 4 \).

   **Case 2: \( n \) is even**
   - We need the following two lemmas:
     - **Lemma 1:** Any odd perfect number \( N \) is of the form \( N = q^a s^2 \) where \( q \) is a prime and \( q \equiv a \equiv 1 \pmod{4} \).
     - **Lemma 2:** \( N \not\equiv 2 \pmod{3} \) for any perfect number \( N \).

   - From Lemma 2, \( n \equiv 0 \pmod{3} \). Let \( n = 3m \) for some positive integer \( m \).
   - Then,
     \[
     n^n + 1 = (n^m + 1)(n^{2m} - n^m + 1)
     \]
   - Since \( \gcd(n^m + 1, n^{2m} - n^m + 1) = 1 \) and by Lemma 1, either \( n^m + 1 \) or \( n^{2m} - n^m + 1 \) must be a perfect square.
   - This is impossible due to the following inequalities:
     \[
     (n^{m/2})^2 < n^m + 1 < (n^{m/2} + 1)^2
     \]
     \[
     (n^m - 1)^2 < n^{2m} - n^m + 1 < (n^m)^2
     \]

Conclusion:
The only solution is \( n = 3 \).

The final answer is \( \boxed{ n = 3 } \).