The upper limit of summation is usually omitted when the index is obvious from the context.
Example: If we know [tex]a_n[/tex] is an infinite sequence (i.e. a function [tex]a_n:\mathbb N\to\mathbb R[/tex], for instance), then we can denote the summation of all the terms in the sequence by
[tex]\displaystyle\sum_{n=1}^\infty a_n[/tex]
or by
[tex]\displaystyle\sum_n a_n[/tex]
