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Sigma
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Greek alphabet  

Αα  Alpha  Νν  Nu 
Ββ  Beta  Ξξ  Xi 
Γγ  Gamma  Οο  Omicron 
Δδ  Delta  Ππ  Pi 
Εε  Epsilon  Ρρ  Rho 
Ζζ  Zeta  Σσς  Sigma 
Ηη  Eta  Ττ  Tau 
Θθ  Theta  Υυ  Upsilon 
Ιι  Iota  Φφ  Phi 
Κκ  Kappa  Χχ  Chi 
Λλ  Lambda  Ψψ  Psi 
Μμ  Mu  Ωω  Omega 
Other letters  
Ϝϝ  Digamma  Ⱶⱶ  Heta 
Ϻϻ  San  Ϙϙ  Koppa 
Ϡϡ  Sampi  Ϛϛ  Stigma 
Ϸϸ  Sho  

Sigma (uppercase/lowercase Σ σ ς) is the 18th letter of the Greek alphabet. It is used to represent the "s" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 200. Letters that came from it include the Roman S and Cyrillic С.
Mathematics
The uppercase Σ is the shorthand symbol for summation (the symbol is also known as the summation operator).^{[1]} It is often used to condense Taylor series and to write out trigonometric functions such as sine, cosine, and tangent as series.
The lowercase σ is used to represent population standard deviation in statistics, and permutation function in abstract algebra.^{[1]}^{[2]}
Related pages
References
 ↑ ^{1.0} ^{1.1} "Greek/Hebrew/Latinbased Symbols in Mathematics" (in enUS). 20200320. https://mathvault.ca/hub/highermath/mathsymbols/greekhebrewlatinsymbols/.
 ↑ "Sigma Symbol in Greek Alphabet" (in en). https://www.greeksymbols.net/sigmasymbol.