Blackboard Bold in bbm package

Question

The link over here http://cs.brown.edu/about/system/managed/latex/doc/bbm.pdf says

By typing \mathbold or \mathversion{bold} you switch to the bold variant of some math symbols.

But when i type \mathbold, I always get an error. Can someone please help with the commands to generate blackboard "bold" fonts using the bbm package?

Answer 1

You’ve found an error in the bbm manual. They meant \boldmath, a standard command to select \mathversion{bold}. So, you could set your title formatting to \bfseries\boldmath or \fontseries{bx}\mathversion{bold}\selectfont. This would give you bold math symbols to match your bold text.

A more-convenient way to select the bold version of a single math symbol is the \boldsymbol command from amsmath:

\documentclass{article}
\usepackage{amsmath, bm, bbm}

\newcommand\mathbb[1]{\mathbbm{#1}}
\newcommand\mathbbb[1]{\boldsymbol{\mathbb{#1}}}

\begin{document}
\begin{gather*}
  \mathbb{N}\mathbbb{N} \\
  \mathbb{R}\mathbbb{R} \\
  \mathbb{C}\mathbbb{C} \\
  \mathbb{Q}\mathbbb{Q}
\end{gather*}
\end{document}

This aliases the bbm commands to more-standard ones used by other packages, such as mathalpha.

You can switch to sans-serif by changing the definition of \mathbb to

\newcommand\mathbb[1]{\mathbbmss{#1}}

Which gets you

Note that the bbm fonts are in an obsolete format, and will be pixelated in a PDF file. If you want bold double-struck letters, you can load an OpenType math font with a bold version in unicode-math (such as Libertinus Math or XITS Math), and \boldsymbol{\mathbb{N}} will work. In PDFTeX, you can load

\usepackage[bb=pxtx]{mathalpha}

This supports \mathbbb{N} if you select a \mathbb font that comes in bold, and will get you a Type 1 outline font instead of a bitmap.