# Google Sheets Sigma notation to calculate series

I have to calculate a series but since the `Sigma` icon represents ALL possible formulas in the UI, it's very difficult to find a documentation on the actual "mathematical" summation symbol.

Let's say I want to calculate something like the growth of an hypothetical animal that weighs 200g over 3 years.

My mathematical notation would be the following But I don't know how to represent that in Google Sheets. What I did find was `SERIESSUM` however the documentation left me with more question than answers, I'm not sure if the "power sum" they mention is what I am looking for.

The Microsoft documentation includes a mathematical notation for this issue, it helped me understand how the formula works.

If anyone else runs into this problem, check: https://support.microsoft.com/en-us/office/seriessum-function-a3ab25b5-1093-4f5b-b084-96c49087f637

In my case, this translates to

`=SERIESSUM(0.5,0,1,{200,200,200,200})`

There is probably a cleaner way of generating the array for the last parameter since we don't need variable coefficients for this.

Try one of these formulas — they will all give the same result:

``````=arrayformula( sum( 200 * 0.5 ^ { 0, 1, 2, 3 } ) )

=arrayformula( sum( 200 * 0.5 ^ sequence(1, 4, 0, 1) ) )

=seriessum( 0.5, 0, 1, sequence(1, 4, 200, 0) )
``````

The first one is probably the easiest as it does not require you to memorize the roles of various function parameters.