R help pages with multiline LaTeX equations

I am writing R package documentation with roxygen2. I want to insert the following multiline equation into a help page, but my LaTeX code is not being rendered.

#' hello2
#'
#' @description
#' \deqn{
#' F(t)= \begin{cases}\alpha(t) f_{L}(t)+[1-\alpha(t)] f_{C}(t) & t_{L}<t<t_{C} \\ \beta(t) f_{C}(t)+[1-\beta(t)] f_{R}(t) & t_{C}<t<t_{R}\end{cases}
#' }
#' 
#' @export
hello2 <- function() {}

New answer

Support for KaTeX in HTML rendering of Rd files was added in R 4.2.0. Support for amsmath in PDF rendering of Rd files was added in R 4.2.2. Hence packages with Depends: R (>= 4.3) can safely specify multiline equations using \deqn. No need for mathjaxr.

---

Original answer

Just to demonstrate that it's possible once you've integrated mathjaxr correctly:

tmp <- tempfile()
dir.create(tmp)
cwd <- setwd(tmp)

pkgname <- "foo"
usethis::create_package(pkgname, rstudio = FALSE, open = FALSE,
                        fields = list(Imports = "mathjaxr", RdMacros = "mathjaxr"))
setwd(pkgname)
text <- "#' A title
#' 
#' \\loadmathjax{}
#' A description.
#' 
#' @param a,b Arguments.
#' 
#' @details
#' An irrelevant equation:
#' \\mjtdeqn{F(t) = \\left\\lbrace\\begin{array}{ll} \\alpha(t) f_{L}(t) + \\lbrack 1 - \\alpha(t) \\rbrack f_{C}(t)\\,, & t_{L} < t < t_{C}\\,, \\cr \\beta(t) f_{C}(t) + \\lbrack 1 - \\beta(t) \\rbrack f_{R}(t)\\,, & t_{C} < t < t_{R}\\,. \\end{array}\\right.}{%
#'           F(t) = \\begin{cases} \\alpha(t) f_{L}(t) + \\lbrack 1 - \\alpha(t) \\rbrack f_{C}(t)\\,, & t_{L} < t < t_{C}\\,, \\cr \\beta(t) f_{C}(t) + \\lbrack 1 - \\beta(t) \\rbrack f_{R}(t)\\,, & t_{C} < t < t_{R}\\,. \\end{cases}}{%
#'           ... a plain text translation ...}
#' 
#' @noMd
#' @export
#' @importFrom mathjaxr preview_rd
add <- function(a, b) a + b
"
cat(text, file = file.path("R", "add.R"))
roxygen2::roxygenize(".")

PDF output

mathjaxr::preview_rd("add.Rd", type = "pdf")

HTML output

mathjaxr::preview_rd("add.Rd", type = "html")

Plain text output

mathjaxr::preview_rd("add.Rd", type = "txt")

A few remarks

• We are only escaping back slashes because we are typing the contents of add.R into a string. The text file created by cat does not contain the escapes.
• \loadmathjax{} injects a MathJax script into the HTML file. It is usually placed at the top of the description, so that you can typeset math everywhere after that.
• We are using macro \mjtdeqn to supply LaTeX for the PDF manual, LaTeX for the HTML help page, and plain text for the plain text help page, in that order. There are other macros that you can use in simpler cases. They are all documented in the README, which you should read carefully.
• In the PDF field, you do not have access to AMS extensions, so you have to implement cases yourself with array.
• In both the PDF and HTML fields, you need to use \cr in place of the usual \\ in multiline environments. I'm not sure why.
• I haven't figured out how to encode line breaks in the plain text field (yet). It's going to be hard to translate multiline equations to plain text without line breaks...
• I disabled Markdown support for this header with @noMd because, in my experience, the Markdown parser doesn't always play nicely with the mathjaxr macros. In my own packages, I disable Markdown support globally via DESCRIPTION.
• You need @importFrom mathjaxr preview_rd somewhere in your package to circumvent R CMD check warnings about having mathjaxr in your Imports without using any functions exported by mathjaxr.
• To preview help pages that use MathJax without installing your package, you need to use mathjaxr::preview_rd. The devtools preview will show you unrendered LaTeX code.
• You'll have to decide whether MathJax support is worth having mathjaxr in your Imports. Anyone who tries to install your package is going to have to install mathjaxr, too.

Cleaning up

setwd(cwd)
unlink(tmp, recursive = TRUE)