# How to write letters in bold in the math mode?

In mathematics, a letter, say, g, can stand for a scalar, while a letter in bold, e.g., g stands for a vector.

Here is my question:

How can one turn the letter into bold in the math mode?

For example, how can I turn g here into bold?

$$-\nabla p+\mu\nabla^2u+g\delta(x-x_0)=0$$

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It is preferrable to use $ and $ as discussed here. Also see this for more on bold math symbols. –  Peter Grill Jul 9 '11 at 2:31
@Peter: +1. Thanks for the link. –  Jack Jul 9 '11 at 3:41

Using the bm package. The \mathbf command can be used only for non-italic symbols, instead the amsmath package defines the \boldsymbol command that can be used whit every literal symbol or math operator. The bm package redefines the \boldsymbol command providing the \bm command and is indipendent from amsmath. So you can use:

\documentclass{article}
\usepackage{amsmath}
\usepackage{bm}
\begin{document}
$\boldsymbol{g} \qquad \bm{g}$
\end{document}

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Use \mathbf{g} for bold in math mode.

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