Render to pdf fails if \bm command is included

[retodomax]

Description

I try to compile the following quarto file

---
title: "Test"
format:
  pdf:
    include-in-header:
      - text: |
          \usepackage{bm}
---

## Math with bm

$$
\bm{y}
$$

So far this was working, but since yesterday it fails with the following error:

updating tlmgr

updating existing packages
ERROR: 
compilation failed- error
Extended mathchar used as mathchar (14799974).
<to be read again> 
                   \relax 
l.175 \bm{y}
            
A mathchar number must be between 0 and "7FFF.
I changed this one to zero.

Using tinytex to compile LaTeX documents which contain \bm{} works. Only within a Quarto document, it creates problems.
Thanks for your help!

I use

• Quarto version 1.7.29
• Windows 11
• I tried with MiKTeX and tinytex as LaTeX installation for Quarto

[mcanouil]

Add keep-tex: trueto your YAML frontmatter, then investigate the intermediate LaTeX document being generated.
The issue is a LaTeX issue as it occurs after Quarto did its job.
It's likely a package conflict because you added the bm package.

You can also very easily look for the LaTeX error online which has plenty of thread about that particular error when using bm.

First search hit:

• https://tex.stackexchange.com/questions/431013/error-extended-mathchar-used-as-mathchar-when-using-bm

[cscheid]

EDIT: I'm seeing the same error, though, I don't understand why.

Worse yet: If I add keep-tex: true and then run pdflatex on the file, it compiles correctly.

[cscheid]

I can repro this as a discrepancy between pdflatex and xelatex's behavior in the keep-tex: true output.

[cscheid]

Here's a minimal example:

\documentclass{article}
\usepackage{unicode-math}
\usepackage{bm}
\begin{document}
$\bm{y}$
\end{document}

It seems that bm is incompatible with unicode-math. I don't know if this ever worked, but unfortunately it's not a Quarto issue.

[mcanouil]

FYI, from my first reply:

• https://tex.stackexchange.com/questions/431013/error-extended-mathchar-used-as-mathchar-when-using-bm

[cscheid]

To work around this issue, try pdf-engine: pdflatex.

[cscheid]

To summarize the situation for other people who find this thread, the issue is that Quarto (through the latex templates we inherit from Pandoc) enables unicode-math by default in xelatex, and bm doesn't work with unicode-math.

So, here are some possible workarounds:

• add pdf-engine: pdflatex to your document, although this will disable unicode support (because pdflatex doesn't support unicode)
• change the fonts.latex pandoc partial (consult our documentation)
• add mathspec: true to your format: pdf: configuration (though TBH I don't really understand what this does)

[mcanouil]

Thanks Carlos!

For reference:

• Boldface in equations #4403

[mcanouil]

mathspec allows to change a component in the Pandoc LaTeX template, namely the use unicode-math, which conflicts with bm when using engine such as xelatex.
It all summarise to the handling of fonts in math mode.

• quarto-cli/src/resources/formats/pdf/pandoc/fonts.latex

  Lines 7 to 15 in 79a1b18

  	$if(mathspec)$
  	\ifXeTeX
  	\usepackage{mathspec} % this also loads fontspec
  	\else
  	\usepackage{unicode-math} % this also loads fontspec
  	\fi
  	$else$
  	\usepackage{unicode-math} % this also loads fontspec
  	$endif$

• quarto-cli/src/resources/formats/pdf/pandoc/font-settings.latex

  Lines 43 to 53 in 79a1b18

  	$if(mathfont)$
  	$if(mathspec)$
  	\ifXeTeX
  	\setmathfont(Digits,Latin,Greek)[$for(mathfontoptions)$$mathfontoptions$$sep$,$endfor$]{$mathfont$}
  	\else
  	\setmathfont[$for(mathfontoptions)$$mathfontoptions$$sep$,$endfor$]{$mathfont$}
  	\fi
  	$else$
  	\setmathfont[$for(mathfontoptions)$$mathfontoptions$$sep$,$endfor$]{$mathfont$}
  	$endif$
  	$endif$

[mcanouil]

Since unicode-math and the ams* already provide options, you could also use those native packages (to the template):

• See unimath-symbols.pdf from https://ctan.org/pkg/unicode-math

---
format:
  pdf: default
---

$$
\mbfY = \mathbf{Y} = Y
$$

$$
\mbfOmega = \Omega
$$