TOPICS
Search

Sigma


Sigma is the eighteenth letter of the ancient Greek alphabet. As an upper case letter (Sigma), it is used as a symbol for sums and series.

As a lower case letter (sigma) it is a prefix used in several contexts to indicate that a term is referred in some way to countable unions. For example, a sigma-algebra is a collection of sets closed under countable union and a sigma-compact topological space is a topological space which is the union of countably many compact subsets. The symbol sigma_k(n) is also used to denote the divisor function.

Another common usage of the symbol sigma is to denote the standard deviation (i.e., square root of the variance) of a statistical distribution. However, some care is needed in interpreting the meaning of sigma, since this symbol is also commonly used as a parameter related to but not equivalent to the square root of the variance, for example in the log normal distribution, Maxwell distribution, and Rayleigh distribution.


See also

Borel Sigma-Algebra, Divisor Function, Sigma-Algebra, Sigma-Discrete Family, Sigma-Locally Finite Family, Sigma-Compact Topological Space, Sigmoid Function, Variance

This entry contributed by Margherita Barile

Explore with Wolfram|Alpha

Cite this as:

Barile, Margherita. "Sigma." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/Sigma.html

Subject classifications