Sigma (upper case Σ, lower case σ, lower case in word-final position ς; Greek σίγμα) is the eighteenth letter of the Greek alphabet, and carries the ‘S’ sound. 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 all upper case, the final form (ς) is used, e.g. Ὀδυσσεύς (Odysseus) – note the two sigmas in the center of the name, and the word-final sigma at the end. It is not to be confused with the cedilla ç.
The name of sigma, according to one theory, may continue that of Phoenician Samekh. According to a different theory, its original name may have been "San" (the name today associated with another, obsolete letter), while "Sigma" was a Greek innovation that simply meant "hissing", based on a nominalization of a verb σίζω (sízō, from earlier *sig-jō, meaning ‘I hiss’).
Uppercase of esh
In handwritten Greek during the Hellenistic period (4th and 3rd centuries BC), the epigraphic form of Σ was simplified into a C-like shape. It is also found on coins from the fourth century BC onwards. This became the universal standard form of Sigma during late antiquity and the Middle Ages. It is today known as lunate sigma (upper case Ϲ, lower case ϲ), because of its crescent-like shape.
It is still widely used in decorative typefaces in Greece, especially in religious and church contexts, as well as in some modern print editions of classical Greek texts. The forms of the Cyrillic letter С (representing /s/) and Coptic letter Ⲥ sima are derived from lunate sigma.
A dotted lunate sigma (sigma periestigmenon, encoded at U+03FE Ͼ) was used by Aristarchus of Samothrace as an editorial sign indicating that the line so marked is at an incorrect position. Similarly, an antisigma or reversed sigma (Ͻ) may mark a line that is out of place. A dottedantisigma or dotted reversed sigma (antisigma periestigmenon: Ͽ) may indicate a line after which rearrangements should be made, or to variant readings of uncertain priority.
Science and mathematics
Upper case Σ is used as a symbol for:
- the summation operator
- a class of baryons in particle physics
- macroscopic cross sections in nuclear and particle physics
- self-energy in condensed matter physics
- the balance of the invoice classes and the overall amount of the debts and demands in economics
- the set of symbols that form an alphabet in linguistics and computer science
Lower case σ is used for:
- sigma bonds in chemistry
- the sigma receptor in biology
- the standard deviation of a population or probability distribution in statistics
- a quality model for business, Six Sigma, based on the standard deviation, often referred to as "6σ"
- sigma-algebras, sigma-fields and sigma-finiteness in measure theory; more generally, the symbol σ serves as a shorthand for "countably", e.g. a σ-compact topological space is one that can be written as a countable union of compact subsets.
- the generated sigma-algebra of a set A is denoted σ(A)
- the sum-of-divisors function in number theory
- the Stefan–Boltzmann constant
- the "sigma factor" of RNA polymerase
- a measure of electrical conductivity
- the Surface charge density in electrostatics
- Normal stress in continuum mechanics
- volatility of a stock generally needed for options pricing
- a syllable in phonology
- the spectrum of a matrix A, denoted as σ(A), in applied mathematics
- surface tension
- the unary operation of selection on a database relation in relational algebra
- the Pauli matrices in quantum mechanics
- a target’s radar cross-section (RCS) in radar jamming or Electronic Warfare (EW)
- Mercury-Atlas 8 was called Sigma 7.
- Upper-case Σ is sometimes substituted jocularly for upper-case E to impart a "Greek" air; posters for the film My Big Fat Greek Wedding and promotional materials for the television series Greek, for instance, have "GRΣΣK" instead.
- Sigma 2,3,5,6,7,8,9 computers produced by Xerox Data Systems in the 1970s following purchase of SDS, Scientific Data Systems, by it. Successful range of scientific computers similar to IBM 370s.
- Sigma is the name of a system for ontology development and reasoning
- Project Sigma was the name of a proposal for a "spontaneous university" by the Scottish writer Alexander Trocchi.
- The French mathematician and poet Jacques Roubaud used the symbol as a title for his first book of poems, Σ, published in 1967.
- The main antagonist in the Mega Man X series is named Sigma.
- In the video game Lux-Pain, Sigma is often mentioned as a power used to destroy "Silent".
- Doctor Who‘s Ood Sigma was a primary character in Planet of the Ood he also played a minor role in The Waters of Mars and The End of Time. He has the letter Σ on his tunic. Sigma was also part of the name which the Doctor used whilst living on Gallifrey, the name being Theta Sigma.
- The last part of the colon in the human body is called " sigmoid" as it is "S" shaped, as it descends down into the pelvis.
- Six Sigma is a popular business management strategy.