Multi-line math fails in Quarto

Quarto 1.2.269, Pandoc 2.19.2, XeTeX 3.141592653-2.6-0.999994

index.qmd

---
title: "Why do I get a warning and failure to convert?"
---

This works:

$$
a = \pi r^2
$$

These do not:

$$
a_n = \begin{cases}
          3n + 1  & \text{ if } n \text{ is odd} \\
          n / 2 & \text{ otherwise}
      \end{cases}
$$

$$
1 + 2 = 3 \\
3 + 2 = 5
$$

_quarto.yml

project:
  type: book

book:
  title: "MRE"
  chapters:
    - index.qmd

format:
  epub:
    toc: true  # whatever

Then, attempt rendering...

$ quarto render

pandoc 
  to: epub
  output-file: MRE.epub
  toc: true
  number-sections: true
  default-image-extension: png
  
metadata
  crossref:
    chapters: true
  title: MRE
  
[WARNING] Could not convert TeX math 
      a_n = \begin{cases}
            3n + 1  & \text{ if } n \text{ is odd } \\
            n / 2 & \text{ otherwise }
        \end{cases}
  , rendering as TeX
[WARNING] Could not convert TeX math 
  1 + 2 = 3 \\
  3 + 2 = 5
  , rendering as TeX:
  1 + 2 = 3 \\
  ^
  unexpected control sequence \\
  expecting "%", "\\label", "\\tag", "\\nonumber" or whitespace
Output created: _book/MRE.epub

...and the resulting epub looks like this:

What am I doing wrong? How do I use cases environment or multi-line math in Quarto?

Note: This works with project type website, but not with project type book.

Solution

You could try to wrap it in LateX environments, e.g. equation or align (works both in PDF and HTML outputs)

\begin{equation*}
    a_n = \begin{cases}
              3n + 1  & \text{ if } n \text{ is odd} \\
              n / 2 & \text{ otherwise}
          \end{cases}
\end{equation*}

\begin{align*}
1 + 2 = 3 \\
3 + 2 = 5
\end{align*}

Here for instance as HTML Book output: