# Why can't I get a bold Greek letter? [duplicate]

My MWE is as follows:

\documentclass[paper=a4, fontsize=12pt]{article} % A4 paper and 11pt
% font size
\usepackage[english]{babel} % English language/hyphenation
\usepackage{amsmath,amsfonts,amsthm} % Math packages

\begin{document}
\begin{align*}
(\mathbf{n}\cdot \mathbf{\sigma}) \sigma&=\mathbf{0}
\end{align*}
\end{document}


But the results show \mathbf{\sigma} and \sigma the same. Why?

## marked as duplicate by CarLaTeX, Zarko, Schweinebacke, Stefan Pinnow, user36296Mar 1 '17 at 12:20

You can use the bm-package

\documentclass[paper=a4, fontsize=12pt]{article} % A4 paper and 11pt
% font size
\usepackage[english]{babel} % English language/hyphenation
\usepackage{amsmath,amsfonts,amsthm,bm} % Math packages

\begin{document}
\begin{align*}
(\mathbf{n}\cdot \bm{\sigma}) \sigma&=\mathbf{0}
\end{align*}
\end{document}


• Thank you, it works. But do you know why the $\mathbf{\sigma}$ not work? Is it because I lack some fonts that supports the $\mathbf{\sigma}$ – yuxuan Sep 30 '15 at 19:36
• @yuxuan: sigma is defined in a way that it "switch back" to the normal font. You should use \mathbf only for normal chars, not for symbols. – Ulrike Fischer Sep 30 '15 at 20:35

Uf, Ulrike Fisher is faster than me ... so I will escape the same prepared solution and add one other possibilities, for symbols instead of \mathbf{..} you can use \boldsymbol{...}:

\documentclass[paper=a4, fontsize=12pt]{article} % A4 paper and 11pt
\usepackage{amsmath,amsfonts,amsthm} % Math packages

\begin{document}
\begin{align*}
(\mathbf{n}\cdot \boldsymbol{\sigma}) \sigma &=\mathbf{0}
\end{align*}
\end{document}


You will get almost the same result:

• The documentation of amsbsy (that's automatically loaded by amsmath) says that using bm is better. – egreg Sep 30 '15 at 23:38