Given a list l of n integers. The task is to find the sum of the product of each element with each element after it in the list. In other words, we have to find the sum of the product of each l[i] with each l[j] such that j > i.

Let’s see an example for better understanding of the problem statement:

Input : l = [9, 3, 4, 2]
Output : 107
Sum of product of l[0] with l[1], l[2], l[3] is :
 9*3 + 9*4 + 9*2 = 81
Sum of product of l[1] with l[2], l[3] is :
3*4 + 3*2 = 18
Product of arr[2] with arr[3] is :
 4*2 = 8
Sum of all these sums is 81 + 18 + 8 = 107

Input : l = [3,5]
Output : 15

Below is the implementation:

# Python program to find Sum of product of 
# each element with each element after it

# Define findSOP() function
def findSOP(l):

    result = 0
    
    # outer loop 
    for i in range(len(l) - 1):

        num = l[i]
        
        # assign multiple values
        # in one line
        sum , multi = 0, 1
        
        # inner loop
        # Sum of product of ith element with each element after it
        for j in range(i+1,len(l)):

                    multi = num * l[j]

                    # sum = sum + multi
                    sum += multi

        # result = result + sum        
        result += sum

    # show the result
    print(result)

# main code
if __name__ == "__main__":
    # given input
    l = [2,3,6,8]
    
    # findSOP function call
    findSOP(l)

Output:

124