[Tex/LaTex] How to insert Greek letters directly in math environment

math-modeunicodexetex

Why are non-Latin characters not displayed in formulas, even when using XeLaTeX? 

\documentclass{article}
\usepackage{xltxtra}

\begin{document}

$\omega$ %displyed  
$ω$ %not displyed

\end{document}

A complementary for following answers:

You can use the following sites to copy and paste a lot of math symbols directly to your tex file, using unicode-math package:

Beside more readability in your markup you have not to search for a specific package that provide a special symbol.

Best Answer

You can't use non-latin alphabets in math formulas for font reasons, even XeTeX use traditional Type1/MetaFont math fonts by default.

You can, however, use unicode-math package with OpenType math fonts:

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{Latin Modern Math}
\begin{document}

$α + β + γ + δ = ε$

\[
∫f(x)dx = ∑_k Λ_k
\]

\end{document}
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