I'm taking an algebra course in which we put an apostrophe after a sum to indicate that it is finite (not sure how standard this is), and I'm having trouble texing this. When writing a sum in math mode, I try
\sum\limits_i' a_i x_i
but it ends up putting the apostrophe on top of the sum instead of after it. Is there any way to put the index under the sum but the apostrophe to the side of it? Thanks.
EDIT: I want the apostrophe to appear after the capital Sigma, i.e. in the same place as it does if I were to write
$ \sum' a_i x_i $