How do double sigmas operate on the variable? Can’t find it on google.

You start the index on the outside summation at the lower limit, then you  cycle through all the indices on the inside summation (the second one); then you increment the outside index and do another cycle.   If the outer summation is 1 to n  and the inner one is 1 to the current value of the outer index, for example, then you  would do an increasing number of iterations of the inside summation.

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