So the textbook Sigma notation is one which has i=1 on the subscript and the n on the top of the sigma notation, whereas in some texts there is an i on the subscript and no "=1" following it nor an "n" on top of it.
In any case, do both mean the same thing? I.e. even without a =1 or an n, does a variable underneath the sigma sign mean that the units that are being summed vary from 1...to a finite (or infinite) n?