The polynomial $1976(x+x^2+ \cdots +x^n)$ is decomposed into a sum of polynomials of the form $a_1x + a_2x^2 + \cdots + a_nx^n$, where $a_1, a_2, \ldots , a_n$ are distinct positive integers not greater than $n$. Find all values of $n$ for which such a decomposition is possible.
1. We start with the polynomial \( 1976(x + x^2 + \cdots + x^n) \) and decompose it into a sum of polynomials of the form \( a_1x + a_2x^2 + \cdots + a_nx^n \), where \( a_1, a_2, \ldots, a_n \) are distinct positive integers not greater than \( n \).

2. Let \( 1976(x + x^2 + \cdots + x^n) \) be decomposed into the sum of \( t \) polynomials \( P_i \), i.e., 
   \[
   1976(x + x^2 + \cdots + x^n) = \sum_{i=1}^t P_i(x),
   \]
   where \( P_k(x) = \sum_{i=1}^n a_{k,i} x^i \).

3. Since \( a_{k,i} \) are \( n \) distinct positive integers, the set \( \{a_{k,1}, a_{k,2}, \ldots, a_{k,n}\} \) is a permutation of the set \( \{1, 2, \ldots, n\} \). Hence,
   \[
   a_{k,1} + a_{k,2} + \cdots + a_{k,n} = \frac{n(n+1)}{2}.
   \]

4. Since this is true for any \( k \), we have:
   \[
   \sum_{j=1}^t \sum_{i=1}^n a_{j,i} = \sum_{i=1}^t P_i(1) = \frac{tn(n+1)}{2}.
   \]
   Therefore,
   \[
   1976n = \frac{tn(n+1)}{2}.
   \]

5. Simplifying the above equation, we get:
   \[
   2 \cdot 1976 = t(n+1).
   \]
   Hence,
   \[
   n+1 \mid 2 \cdot 1976 = 2^4 \cdot 13 \cdot 19.
   \]

6. We now consider the possible values of \( n+1 \) that divide \( 2^4 \cdot 13 \cdot 19 \). These values are \( 1, 2, 4, 8, 16, 13, 19, 26, 38, 52, 76, 104, 152, 247, 304, 494, 988, 1976 \).

7. We need to ensure that \( t = \frac{2 \cdot 1976}{n+1} \) is an integer and that \( t \) is even. This implies \( v_2(n+1) \ge 3 \).

8. We now check the values of \( n+1 \) that satisfy \( v_2(n+1) \ge 3 \):
   - \( n+1 = 8 \) (i.e., \( n = 7 \))
   - \( n+1 = 16 \) (i.e., \( n = 15 \))
   - \( n+1 = 104 \) (i.e., \( n = 103 \))
   - \( n+1 = 304 \) (i.e., \( n = 303 \))
   - \( n+1 = 1976 \) (i.e., \( n = 1975 \))

9. For each of these values, we verify that \( t = \frac{2 \cdot 1976}{n+1} \) is even:
   - For \( n = 7 \), \( t = \frac{2 \cdot 1976}{8} = 494 \) (even)
   - For \( n = 15 \), \( t = \frac{2 \cdot 1976}{16} = 247 \) (odd, discard)
   - For \( n = 103 \), \( t = \frac{2 \cdot 1976}{104} = 38 \) (even)
   - For \( n = 303 \), \( t = \frac{2 \cdot 1976}{304} = 13 \) (odd, discard)
   - For \( n = 1975 \), \( t = \frac{2 \cdot 1976}{1976} = 2 \) (even)

10. Therefore, the possible values of \( n \) are \( 7, 103, \) and \( 1975 \).

The final answer is \( \boxed{7, 103, 1975} \)