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The LaTeX command for Sigma (Σ) is \Sigma. The lowercase form σ is \sigma.
\SigmaWrite \Sigma inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
$\Sigma(x) = \Sigma_0$Prefer to see it live? Paste it into the LaTeX preview and it renders as you type: no compiler, no signup.
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Use \Sigma in math mode; it needs no extra package. For example, $\Sigma(x) = \Sigma_0$.
None: \Sigma is built into LaTeX, so no extra package is required.