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Upper-case Σ is used as a symbol for:


the summation operator
Sigma is the addition version of factorial.
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
the covariance matrix of a set of random variables in probability theory and statistics, sometimes in the form | Σ {\displaystyle \;|\!\!\!\Sigma } {\displaystyle \;|\!\!\!\Sigma } to distinguish it from the summation operator.
The busy beaver function