Why can't I get my blackboard bold C to work?

I just downloaded MiKTeX today and \mathbb{C} won't work nor will \C. Can somebody explain what I need to do/or download? Thanks!

• Welcome to TeX.SX! You need the bbm package (for example), but as far as I know, \C will not work anyway. \mathbbm{C} however does! – user31729 Dec 10 '15 at 23:37
• With \usepackage{amssymb}, you have \mathbb. – egreg Dec 10 '15 at 23:41
• I have noticed \C already defined Error while trying to redefine \C in a document, and that was because hyperref with unicode option. – Sunilkumar KS Dec 11 '15 at 7:07

You need to load a package such as amsfonts or amssymb which provide the additional maths alphabet required. For example:

\documentclass{article}
\usepackage{amsfonts}
\begin{document}
$\mathbb{C}$
\end{document}


Here's bbm - package version -- if this is not nice enough choose amssymb (which is apparently preferred by some fellows here;-) )

\documentclass{article}

\usepackage{bbm}
\begin{document}

$\mathbbm{C}$

Will not work: \verb+\C+

\end{document}


• Why not \mathbb with amssymb? amsfonts suffices, though. – egreg Dec 10 '15 at 23:41
• While I can't say I like \mathbb, I surely dislike \bbm. Consider also that the font is only bitmap. – egreg Dec 10 '15 at 23:47

I always define the command \C to be \mathbb{C}. You can too:

\documentclass{article}

\usepackage{amssymb}
\newcommand{\C}{\mathbb{C}}
\newcommand{\R}{\mathbb{R}}
\begin{document}
Let $z\in\C$. Then $z\cdot\overline{z}\in\R$.
\end{document}


If you are using one of the Unicode TeX engines, say either xelatex or lualatex, then this approach might come in handy.

The unicode-math package will load the Latin Modern Math font by default, which also has various blackboard bold glyphs. The can be accessed as usual via \mathbb. Moreover unicode-math defines shorthands for all glyphs (which avoids loading huge token lists). These can be looked up in unimath-symbols. For blackboard bold glyphs these shorthands are \BbbA for A and so on.

\documentclass{article}
\usepackage{unicode-math}
\begin{document}
$\mathbb{C}$ or $\BbbC$
\end{document}


• +1. You may want to state explicitly which LaTeX engines can handle, or cannot handle, this approach. – Mico Dec 15 '15 at 10:04