Sigma (Σ σ) in LaTeX
In LaTeX, \sigma gives the lowercase sigma (σ) and \Sigma gives the uppercase sigma (Σ), both in maths mode; \varsigma produces the final-sigma variant (ς).
LaTeX command
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Lowercase sigma, \sigma, is the standard symbol for standard deviation, with \sigma^2 the variance. In physics it denotes mechanical stress, electrical conductivity and a scattering cross-section, and in mathematics a sigma-algebra and the Pauli matrices \sigma_x, \sigma_y, \sigma_z. It is maths-mode only and italic by default.
Uppercase sigma, \Sigma, is the letter form; in statistics it names a covariance matrix. The large summation operator in \sum_{i=1}^{n} x_i is the separate command \sum, not \Sigma: \sum grows with its limits, whereas \Sigma stays letter-sized. Reaching for \Sigma when you mean a sum is a common mistake.
The variant \varsigma gives the final sigma (ς), the form Greek uses at the end of a word. It is rarely needed in mathematics but is available when you want a distinct second sigma. For a bold sigma use \boldsymbol{\sigma}, or for the covariance matrix \boldsymbol{\Sigma}.
Other forms
\sigmalowercase sigma\Sigmauppercase sigma (Σ)\varsigmacurly / variant formExamples
$\sigma$lowercase sigma inline
$\sigma^2 = \frac{1}{n}\sum_{i=1}^{n}(x_i - \mu)^2$the variance
$\Sigma$uppercase sigma (the letter, not \sum)
$X \sim \mathcal{N}(\mu, \Sigma)$a multivariate normal with covariance matrix Sigma
$\varsigma$the final-sigma variant
Tips and common mistakes
- The summation operator is
\sum, not\Sigma;\sumenlarges and takes limits,\Sigmais a plain letter. \varsigmais the final sigma (ς); it is rarely used in maths but available as a distinct variant.- All sigma commands are maths-mode only; wrap them in .
- For a bold covariance matrix use
\boldsymbol{\Sigma};\mathbfalso bolds an uppercase Greek letter like this one, but\boldsymbolworks for every Greek letter. - Write the variance as
\sigma^2, not (\sigma)^2; the exponent sits on the letter.
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Convert your maths to LaTeX →FAQ
What is the difference between \Sigma and \sum?
\Sigma is the uppercase letter sigma (Σ), letter-sized. \sum is the summation operator that grows to fit its limits, as in \sum_{i=1}^{n} x_i. Use \sum for sums.
How do I write standard deviation and variance?
Use \sigma for the standard deviation and \sigma^2 for the variance, for example .
What is \varsigma?
\varsigma is the final sigma (ς), used at the end of Greek words. It is seldom needed in mathematics but is available as a variant of \sigma.