Xi in LaTeX (\xi)

Where it is used

Lowercase Greek xi, a squiggly letter used for random variables, dummy variables, and damping.

Probability texts often use \xi for a random variable or noise term, and analysis uses it for an intermediate point, as in the mean value theorem f(b) - f(a) = f'(\xi)(b - a). Fourier analysis writes frequency variables as \xi in \hat{f}(\xi). Some engineering texts use \xi for the damping ratio instead of \zeta. Because \xi and \zeta are hard to tell apart when handwritten, check which letter a source means. Its uppercase form \Xi is a separate command.

Minimal compilable example

Paste this into an empty .tex file and compile it with pdfLaTeX, XeLaTeX or LuaLaTeX.

\documentclass{article}
\begin{document}
Xi: $\xi$

\[ f(b) - f(a) = f'(\xi)(b - a), \quad \xi \in (a, b) \]
\end{document}

Related symbols

• Xi (Upper)\Xi
• Nu\nu
• Pi\pi
• Mu\mu
• Pi (Upper)\Pi
• Lambda (Upper)\Lambda

Frequently asked questions

How do I write Xi in LaTeX?

Type \xi in math mode, for example $f(b) - f(a) = f'(\xi)(b - a), \quad \xi \in (a, b)$. It works in plain LaTeX without loading any package.

Which package do I need for \xi?

None. \xi is part of standard LaTeX, so it compiles with nothing in the preamble but the document class.

Can I use \xi in text mode, or only in math mode?

Only in math mode. Outside $...$ LaTeX stops with "Missing $ inserted", so write $\xi$ in running text. If you only need the glyph in text, paste the Unicode character ξ instead (with XeLaTeX or LuaLaTeX and a font that has it).