Sigma

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Sigma ( ; uppercase Σ, lowercase σ, lowercase in word-final position ς; ) is the eighteenth letter of the Greek alphabet. When used at the end of a letter-case word (one that does not use all caps), the final form (ς) is used. In ' (Odysseus), for example, the two lowercase sigmas (σ) in the center of the name are distinct from the word-final sigma (ς) at the end.

In the system of Greek numerals, sigma has a value of 200. In general mathematics, uppercase Σ''' is used as an operator for summation. The Latin letter S derives from sigma while the Cyrillic letter Es derives from a lunate form of this letter. Provided by Wikipedia