\varSigma
The capital Greek letter Sigma (\varSigma) is often used in mathematics and science to represent the sum of a series of numbers or elements. It is also used in statistics to denote the sum of a set of values.
Sum of the first n natural numbers.
\varSigma_{i=1}^{n} i = \frac{n(n+1)}{2}
General representation of the sum of a sequence.
\varSigma_{k=1}^{m} a_k = a_1 + a_2 + \cdots + a_m
Sum of squared deviations from the mean, used in variance calculations.
\varSigma_{i=1}^{n} (x_i - \bar{x})^2