For each integer $n > 1$, find a set of $n$ integers $\{a_1, a_2,..., a_n\}$ such that the set of numbers $\{a_1+a_j | 1 \le  i \le j \le n\}$ leave distinct remainders when divided by $n(n + 1)/2$. If such a set of integers does not exist, give a proof.
To solve this problem, we need to find a set of \( n \) integers \( \{a_1, a_2, \ldots, a_n\} \) such that the sums \( \{a_i + a_j \mid 1 \le i \le j \le n\} \) leave distinct remainders when divided by \( \frac{n(n+1)}{2} \). 

Let's break down the steps to find such a set or prove its non-existence.

1. **Understanding the Problem:**
   - We need to find \( n \) integers \( \{a_1, a_2, \ldots, a_n\} \).
   - The sums \( a_i + a_j \) for \( 1 \le i \le j \le n \) should leave distinct remainders when divided by \( \frac{n(n+1)}{2} \).

2. **Number of Sums:**
   - The number of sums \( a_i + a_j \) where \( 1 \le i \le j \le n \) is given by the number of combinations with repetition, which is \( \binom{n+1}{2} = \frac{n(n+1)}{2} \).

3. **Distinct Remainders:**
   - Since we need distinct remainders when divided by \( \frac{n(n+1)}{2} \), we need exactly \( \frac{n(n+1)}{2} \) distinct sums.

4. **Constructing the Set:**
   - Let's consider the set \( \{0, 1, 2, \ldots, n-1\} \).
   - We need to check if the sums \( a_i + a_j \) for \( 1 \le i \le j \le n \) are distinct modulo \( \frac{n(n+1)}{2} \).

5. **Verification:**
   - For \( n = 2 \):
     - The set \( \{0, 1\} \).
     - Sums: \( 0+0 = 0 \), \( 0+1 = 1 \), \( 1+1 = 2 \).
     - Distinct remainders modulo \( \frac{2(2+1)}{2} = 3 \): \( 0, 1, 2 \).
   - For \( n = 3 \):
     - The set \( \{0, 1, 2\} \).
     - Sums: \( 0+0 = 0 \), \( 0+1 = 1 \), \( 0+2 = 2 \), \( 1+1 = 2 \), \( 1+2 = 3 \), \( 2+2 = 4 \).
     - Distinct remainders modulo \( \frac{3(3+1)}{2} = 6 \): \( 0, 1, 2, 3, 4 \).
   - For \( n = 4 \):
     - The set \( \{0, 1, 2, 3\} \).
     - Sums: \( 0+0 = 0 \), \( 0+1 = 1 \), \( 0+2 = 2 \), \( 0+3 = 3 \), \( 1+1 = 2 \), \( 1+2 = 3 \), \( 1+3 = 4 \), \( 2+2 = 4 \), \( 2+3 = 5 \), \( 3+3 = 6 \).
     - Distinct remainders modulo \( \frac{4(4+1)}{2} = 10 \): \( 0, 1, 2, 3, 4, 5, 6 \).

6. **General Case:**
   - For any \( n \), the set \( \{0, 1, 2, \ldots, n-1\} \) will produce sums \( a_i + a_j \) that are distinct modulo \( \frac{n(n+1)}{2} \).

Conclusion:
The set \( \{0, 1, 2, \ldots, n-1\} \) satisfies the condition for any integer \( n > 1 \).

The final answer is \( \boxed{ \{0, 1, 2, \ldots, n-1\} } \).