Redefine mathbbm to print a colored output

Question

I need \mathbbm to print a colored output.

I tried:

\documentclass[11pt]{article}
\usepackage{amsmath}
\usepackage{xcolor}
\usepackage{bbm}
\let\oldmathbbm\mathbbm
\def\mathbbm#1#{\mathbbmaux{#1}}
\newcommand*\mathbbmaux[2]{{\color{red}\oldmathbbm#1{#2}}}
\pagestyle{empty}
\begin{document}
$\mathbbm{i}$
${\mathbbm i}$
\end{document}

but the string ${\mathbbm i}$ returns an error. Where Am I wrong?

I need this trick to spot if authors used \mathbbm, directly in the pdf file. (I pass this strings to the pdflatex engine without modifying the source .tex file.)

The error comes because with \def\mathbbm#1#{...} you are defining \mathbbm so that it must find an opening brace. Why are you doing that at all? — campa, Mar 12 '21 at 21:20
@campa I updated my question giving an explanation of my purpose. — Gabriele, Mar 12 '21 at 21:29
I understand that, but I still don't understand the reason for the brace trick #1#{. — campa, Mar 12 '21 at 21:30
Honestly, I don't remember... it's an old piece of code I've been using for a while, that never gave a problem until this case. Since I use it in a weird way I though \renewcommand didn't work. — Gabriele, Mar 12 '21 at 21:35

score 3 · Answer 1 · answered Mar 12 '21 at 21:06

I would rather use renewcommand:

%\documentclass[11pt]{article}
\documentclass[border=3.141592]{standalone}
\usepackage{xcolor}
\usepackage{bbm}
\let\oldmathbbm\mathbbm
\renewcommand\mathbbm[1]{\color{red}\oldmathbbm{#1}}
\begin{document}
$\mathbbm{i}$
${\mathbbm i}$ 
$\oldmathbbm{i}$
\end{document}

Davislor · Answer 2 · 2021-03-12T21:17:50.963

I do not recommend bbm, as its obsolete font format does not display correctly in a PDF. (And nobody prints DVIs on a laser printer any more.)

This lets you choose a more meaningful name, instead of redefining \mathbbm to mean something other than what someone reading your source would assume. This has the happy side-effects of simplifying your code and solving your problem.

\documentclass[11pt]{article}
\usepackage{amsmath}
\usepackage[svgnames]{xcolor}
\usepackage[bb=dsserif]{mathalpha}
\newcommand\mathredbb[1]{\textcolor{Red}{\mathbb{#1}}}
\pagestyle{empty}
\begin{document}
$i \in \mathredbb{i}$,
$i \in {\mathredbb i}$
\end{document}

Other nice things you can do with mathalpha are use a heavier blackboard bold as \mathbbb, select from an extensive list of math alphabets, and scale the font with the bbscaled= package opton.

In LuaLaTeX or XeLaTeX, you could use the same code with unicode-math rather than mathalpha.

indeed I need this trick to spot if authors used mathbbm directly in the pdf file. (I pass this strings to the pdflatex engine without modifying the source .tex file.) — Gabriele, Mar 12 '21 at 21:26
@GabrieleNicolardi Ah! Classic XY problem! That makes more sense. Other methods: remove the mathbbm package, redefine \mathbbm to give an error message, do a search. — Davislor, Mar 12 '21 at 22:31

Redefine mathbbm to print a colored output

2 Answers2