Find the `sumsum_(0leiltjlen)1`.

Explanation

To solve the problem of finding the value of the double summation \( \sum_{0 \leq i < j \leq n} 1 \), we can follow these steps:

### Step 1: Understanding the Double Summation
The expression \( \sum_{0 \leq i < j \leq n} 1 \) counts the number of pairs \( (i, j) \) where \( i \) and \( j \) are integers from 0 to \( n \) and \( i \) is less than \( j \).

### Step 2: Counting the Pairs
For a fixed value of \( j \), \( i \) can take values from 0 to \( j-1 \). Therefore, for each \( j \), the number of valid \( i \) values is \( j \).

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