What is the sum sign?

The sum sign, represented by the Greek letter sigma (Σ), is used in mathematics to denote summation. It indicates the addition of a series of numbers or terms, typically defined by an index that specifies the range of values to be summed. For example, Σ from i=1 to n represents the sum of terms a_i for i starting at 1 and ending at n. This notation is commonly used in calculus, statistics, and various mathematical contexts.