Let $ d(n)$ be the number of positive divisors of the natural number $ n$.   Find all $ n$ such that  $ \frac {n} {d(n)}\equal{}p$   where $ p$ is a prime number

Daniel
Given the problem, we need to find all natural numbers \( n \) such that \( \frac{n}{d(n)} = p \), where \( p \) is a prime number and \( d(n) \) is the number of positive divisors of \( n \).

1. **Express \( n \) in terms of its prime factorization:**
   Suppose \( n = p^r p_1^{r_1} p_2^{r_2} \cdots p_k^{r_k} \), where \( p, p_1, p_2, \ldots, p_k \) are distinct primes. The number of divisors \( d(n) \) is given by:
   \[
   d(n) = (r+1)(r_1+1)(r_2+1) \cdots (r_k+1)
   \]
   Given \( \frac{n}{d(n)} = p \), we can write:
   \[
   n = p \cdot d(n)
   \]
   Substituting the expressions for \( n \) and \( d(n) \), we get:
   \[
   p^r p_1^{r_1} p_2^{r_2} \cdots p_k^{r_k} = p (r+1)(r_1+1)(r_2+1) \cdots (r_k+1)
   \]
   Dividing both sides by \( p \), we obtain:
   \[
   p^{r-1} p_1^{r_1} p_2^{r_2} \cdots p_k^{r_k} = (r+1)(r_1+1)(r_2+1) \cdots (r_k+1)
   \]

2. **Case \( k = 0 \):**
   If \( k = 0 \), then \( n = p^r \) and the equation simplifies to:
   \[
   p^{r-1} = r+1
   \]
   We need to find \( p \) and \( r \) such that this holds:
   - For \( p = 2 \):
     \[
     2^{r-1} = r+1
     \]
     Testing values of \( r \):
     - \( r = 1 \): \( 2^0 = 1+1 \) (False)
     - \( r = 2 \): \( 2^1 = 2+1 \) (False)
     - \( r = 3 \): \( 2^2 = 3+1 \) (True)
     Thus, \( n = 2^3 = 8 \).
   - For \( p = 3 \):
     \[
     3^{r-1} = r+1
     \]
     Testing values of \( r \):
     - \( r = 1 \): \( 3^0 = 1+1 \) (False)
     - \( r = 2 \): \( 3^1 = 2+1 \) (True)
     Thus, \( n = 3^2 = 9 \).

3. **Case \( k \geq 1 \):**
   If \( k \geq 1 \), we need to consider the contributions of other primes \( p_1, p_2, \ldots, p_k \).

   - If \( p_1 \geq 3 \), then \( p_i^{r_i} > r_i+1 \) for all \( i \). This implies:
     \[
     p^{r-1} < r+1
     \]
     This can only occur if \( r = 1 \) or \( (p, r) = (2, 2) \). However, \( r = 1 \) leads to \( p_1 = 2 \), a contradiction. Hence, \( p = 2 \) and \( r = 2 \), implying \( p_1 = 3 \). Now:
     \[
     2 \cdot 3^{r_1} \leq 3 (r_1 + 1)
     \]
     This holds if and only if \( k = 1 \) and \( r_1 = 1 \). Therefore, \( n = 2^2 \cdot 3 = 12 \).

   - If \( p_1 = 2 \) and \( r = 1 \), then:
     \[
     2^{r_1 - 1} \leq r_1 + 1
     \]
     This implies \( r_1 = 1, 2, \) or \( 3 \). Testing these:
     - \( r_1 = 1 \): Contradiction.
     - \( r_1 = 2 \): \( p_2 = 3 \), and:
       \[
       2 \cdot 3^{r_2 - 1} \leq r_2 + 1
       \]
       This holds if \( r_2 = 1 \) and \( k = 2 \). Thus, \( n = 12p \).
     - \( r_1 = 3 \): \( k = 1 \). Thus, \( n = 8p \).

   - If \( p_1 = 2 \) and \( r > 1 \), then:
     \[
     p^{r-1} \geq r+1 \quad \text{and} \quad p_i^{r_i} > r_i+1 \quad \text{for all} \quad i > 1
     \]
     This implies:
     \[
     2^{r_1} \leq r_1 + 1
     \]
     The only solution is \( r_1 = 1 \), making \( k = 1 \) and \( p^{r-1} = r+1 \) (which occurs only when \( p = 3 \) and \( r = 2 \)). Therefore, \( n = 18 \).

In summary:
1. If \( p = 2 \), then \( n = 8 \) or \( n = 12 \).
2. If \( p = 3 \), then \( n = 9 \), \( n = 18 \), or \( n = 24 \).
3. If \( p > 3 \), then \( n = 8p \) or \( n = 12p \).

The final answer is \( \boxed{ n = 8, 9, 12, 18, 24, 8p, 12p } \) where \( p \) is a prime number greater than 3.