I have a series with an integral inside the series. I have worked the problem 2 different ways using sum vs Sum. The integration variables are independent of the series variables so swapping the order of operation should not matter, but in the case, (S2), I do get a difference & I do not understand why. The explanation fo INERT vs ACTIVE I do not think explains this. The reason why I say this is because S1 the ACTIVE sum concurs with the INERT expressions S3 & S4. S2 is swapping the order of operation for the ACTIVE sum does not yield the same result as the other 3 cases. Why is this? I am at a loss so including examples would be helpful to me.