Lower-case σ is used for:
the standard deviation of a population or probability distribution in statistics
sigma bonds in chemistry
to represent an unknown angle in mathematics
cross section (physics)
velocity dispersion in astronomy
sigma constant in organic chemistry
the sigma receptor in biology
a quality model for business, Six Sigma (6σ), based on the standard deviation
sigma-algebras, sigma-fields, and sigma-finiteness in measure theory; in general terms, 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 {\displaystyle A} A is denoted σ ( A ) {\displaystyle \sigma (A)} \sigma (A)
the sum-of-divisors function in number theory
the Stefan–Boltzmann constant
the "sigma factor" of RNA polymerase
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