Sigma (upper case Σ, lower case σ, lowercase in word-final position ς) is the 18th letter of the Greek alphabet. In the system of Greek numerals it has a value of 200. When used at the end of a word, and the word is not upper case, the final form (ς) is used.

. However, its name derives from Samekh.

In Eastern forms of Greek writing (as opposed to the Western Greek alphabet used in the European Greek colonies) and in the Middle Ages, the lunate sigma (upper case Ϲ, lower case ϲ) — which resembles, but which is not at all related to, the Latin letter C — was often used. Lunate sigma was frequently used for writing Medieval Greek, and it can still sometimes be seen in inscriptions in Greek Orthodox churches, but also in certain printed editions of classical authors. The form of the Cyrillic letter С ^* derives from lunate sigma.

A dotted lunate sigma (sigma periestigmenon, encoded at U+03FE ) is used by Aristarchus of Samothrace as an editorial sign indicating that the line so marked is at an incorrect position. Similarly, an antisigma (encoded at U+03FD ) may mark a line that is out of place. A dotted antisigma (antisigma periestigmenon, encoded at U+03FF ) may indicate a line after which rearrangements should be made, or to variant readings of uncertain priority.

The uppercase form of sigma was borrowed into the Latin alphabet to serve as the uppercase of esh (lowercase: ).

Science

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Upper case Σ is used as a symbol for:

• the summation operator
• a class of baryons in particle physics

Lower case σ is used for ...

• sigma bonds in chemistry
• the parametric standard deviation of a population (as opposed to a sample) in statistics
• the divisor function in number theory
• the Stefan-Boltzmann constant
• the "sigma factor" of RNA polymerase
• a measure of electrical conductivity
• Normal stress in continuum mechanics
• a measure of horniness by Lawrence Waterhouse in Neal Stephenson's Cryptonomicon

Politics

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During the 1930s, an upper case Σ was in use as the symbol of the Ação Integralista Brasileira, a fascist party in Brazil.

Popular culture

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In a practice similar to faux Cyrillic, capital sigmas are sometimes used in place of Roman E to give a Greek flavour to titles or text (for example, the film My Big Fat GRΣΣK Wedding and the Greek characters in Asterix comics ).

Fictional names

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• Sigma is the name of the main villain in Capcom's Mega Man X.

• Sigma Octanus IV is a fictional planet in the The Fall of Reach.

• Sigma is the name of third server in the series of .hack//IMOQ (PS2 video games)

See also

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• Antisigma

Greek letters

Σ | Sigma (lizherenn) | Сигма | Sigma | Sigma (bogstav) | Sigma | Σίγμα | Σ | Sigma | Sigma | Sigma | Σ | Sigma (lettera) | סיגמא | Sigma (letter) | Σ | Sigma | Sigma | Sigma | Σ | Sigma | Sigma | Sigma | Σ