The real-valued function $f$ is defined for all positive integers. For any integers $a>1, b>1$ with $d=\gcd (a, b)$, we have
\[f(ab)=f(d)\left(f\left(\frac{a}{d}\right)+f\left(\frac{b}{d}\right)\right) \]
Determine all possible values of $f(2001)$.
1. Let \( P(x, y) \) be the assertion \( f(xy) = f(\gcd(x, y)) \left( f \left( \frac{x}{\gcd(x, y)} \right) + f \left( \frac{y}{\gcd(x, y)} \right) \right) \).
2. Let \( f(1) = a \). We will consider two cases: \( a = 0 \) and \( a \neq 0 \).

### Case 1: \( a = 0 \)
3. If \( a = 0 \), then \( P(3, 667) \) implies:
   \[
   f(2001) = f(\gcd(3, 667)) \left( f \left( \frac{3}{\gcd(3, 667)} \right) + f \left( \frac{667}{\gcd(3, 667)} \right) \right) = f(1) \left( f(3) + f(667) \right) = 0 \left( f(3) + f(667) \right) = 0
   \]
   Therefore, \( f(2001) = 0 \).

### Case 2: \( a \neq 0 \)
4. Let \( x > 1 \). Consider \( P(x, x) \):
   \[
   f(x^2) = f(x) \left( f(1) + f(1) \right) = f(x) \cdot 2a = 2a f(x)
   \]
5. Next, consider \( P(x^2, x) \):
   \[
   f(x^3) = f(x) \left( f(x) + f(1) \right) = f(x) \left( f(x) + a \right) = f(x)^2 + a f(x)
   \]
6. Now, consider \( P(x^3, x) \):
   \[
   f(x^4) = f(x) \left( f(x^2) + f(1) \right) = f(x) \left( 2a f(x) + a \right) = 2a f(x)^2 + a f(x)
   \]
7. Also, consider \( P(x^2, x^2) \):
   \[
   f(x^4) = f(x^2) \left( f(1) + f(1) \right) = f(x^2) \cdot 2a = 4a^2 f(x)
   \]
8. Equating the two expressions for \( f(x^4) \):
   \[
   2a f(x)^2 + a f(x) = 4a^2 f(x)
   \]
   Simplifying, we get:
   \[
   f(x) \left( 2a f(x) + a - 4a^2 \right) = 0
   \]
   Therefore, either \( f(x) = 0 \) or \( 2a f(x) + a - 4a^2 = 0 \).

9. If \( f(x) \neq 0 \), then:
   \[
   2a f(x) + a - 4a^2 = 0 \implies f(x) = 2a - \frac{1}{2}
   \]
10. Thus, \( f(x) \) can either be \( 0 \) or \( 2a - \frac{1}{2} \).

11. If \( f(x) = 2a - \frac{1}{2} \), then:
    \[
    f(x^2) = 2a f(x) = 2a \left( 2a - \frac{1}{2} \right) = 4a^2 - a
    \]
    But we also have \( f(x^2) = 2a f(x) = 2a \left( 2a - \frac{1}{2} \right) = 4a^2 - a \).

12. Therefore, \( 2a f(x) = 2a - \frac{1}{2} \) must hold for all \( x \). This implies:
    \[
    2a - \frac{1}{2} = 2a - \frac{1}{2}
    \]
    which is always true.

13. Hence, \( f(x) = 2a - \frac{1}{2} \) for all \( x \neq 0 \).

14. Solving for \( a \):
    \[
    (2a - 1) \left( 2a - \frac{1}{2} \right) = 0
    \]
    This gives \( a = \frac{1}{2} \).

15. Therefore, \( f(x) = \frac{1}{2} \) for all \( x \).

16. Since the solution \( f(x) = \frac{1}{2} \) for all \( x \) is indeed a solution, we get the result:
    \[
    \boxed{f(2001) \in \{0, \frac{1}{2}\}}
    \]