In FileMaker Advanced, you could use a recursive function to loop through the 13 values of n.
without Advanced, you can use a looping script.
If the number of iterations is fixed at 0 to 13 and not subject to user specified variations, you could expand the iteration into multiple addition terms.
The looping script:
Set Variable [$Sigma ; value: $SIgma + 100 * ( 8 * $n / 3 + 8 ) ]
Exit Loop If [ $n = 12 ]
Set Variable [ $n ; value: $n + 1 ]
Thanks Phil. I was hoping I just wasn't seeing the function in the reference section. : )