Summation Sign

The sum of a set of terms typically distinguished by a subscript. For example
\[ \sum_{i=0}^n x_i = x_0 + x_1 + \dots + x_{n-1} + x_n \]
The sum of an infinite number of terms starting at the \(o^{th}\) will be written as
\[ \sum_{i=o}^\infty x_i \]
and, if the number of terms can be implied from the context of the expression, as
\[ \sum_{i} x_i \]