Capital Sigma Symbol
The capital Greek letter Sigma (Σ), also known as the summation symbol, is used in algebra to represent the summation operator.
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Unicode  Σ 

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The capital letter Sigma Σ represents the summation operation in mathematics, often called sum for short. A sum has three parts: an initial value, an end value, and the expression being summed. The summation starts at the initial value and iterates, adding one to the value for each iteration, stopping when the value reaches the end value.
To calculate the expected value of a discrete distribution multiply all of the events of the distribution with the probability of the element occuring.
The population mean formula gives the average value of the whole population.
A series is the operation of adding an infinite number expressions together. A series is denoted using the summation operator and the index variable k.
The sample standard deviation formula is denoted by the greek lower case sigma symbol in the case of the population and the latin letter s for the sample.