The Greek letter σ (sigma) was derived from the ancient Phoenician letter 𐤔 (šīn) which meant “tooth”.
ς is the lower-case letter sigma (σίγμα τελικό) when used as the final letter in a word.
The lower-case σ has been widely adopted in maths and sciences, for instance:
- in chemistry: the σ bonds
- in continuum mechanics: the Cauchy stress tensor σ
- in biochemistry: σ factor of RNA polymerase
- in mathematics: σ(z; Λ), the Weierstrass sigma function
- in organic chemistry: σ-value (the substituent constant) of the Hammett equation
- in particle physics: cross section
- in pharmacology: the σ–receptors
- in physics: the surface charge density, σq
- in physics: a symbol for conductivity (the reciprocal of resistivity ρ)
- in statistics: the standard deviation
- in thermodynamics: the Stefan–Boltzmann constant
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