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I get confused with double summation!
Double sum is nothing more than sum of a sum. If the elements of the sum have two indices and you want to add the index one by one, then you use double sums. In calculating double summations, here are the steps
 First, the outersum index is hold and increment the inner index
 After all the innersum index has been used, then increment the outersum index.
 Repeat the two steps above for the entire outersum index.
It is easier to see the steps above by example below. Before that, you may remember one nice property of double summation:
We can interchange order of double summation
Example
We have matrix
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See Also:
Kardi Teknomo's Tutorial
This tutorial is copyrighted .
Preferable reference for this tutorial is
Teknomo, Kardi (2015) Summation Tutorial. http://people.revoledu.com/kardi/tutorial/BasicMath/Sum/