# mathjaxr-package: Using MathJax in Rd Files In mathjaxr: Using 'Mathjax' in Rd Files

## Description

The mathjaxr package allows for easy inclusion of MathJax equations in Rd files. Package authors wanting to make use of the package and its functionality need to:

1. install the mathjaxr package,

2. add mathjaxr to Suggests or Imports in the ‘DESCRIPTION’ file of their package,

3. add mathjaxr to RdMacros in the ‘DESCRIPTION’ file of their package (or add RdMacros: mathjaxr if the ‘DESCRIPTION’ file does not yet contain a RdMacros entry)

One can then enable the use of MathJax by calling the \loadmathjax macro (that is provided by the mathjaxr package) within the \description{} section of an Rd file (or within the @description section if you use roxygen2).

An inline equation can then be added with the \mjeqn{latex}{ascii} macro, with the LaTeX commands for the equation given between the first set of curly brackets (which will be rendered in the HTML and PDF help pages) and the plain-text version of the equation given between the second set of curly brackets (which will be shown in the plain text help). With the \mjdeqn{latex}{ascii} macro, one can add ‘displayed equations’ (as in LaTeX's displaymath environment).

Single argument versions of these macros, \mjseqn{latexascii} and \mjsdeqn{latexascii}, are also available. For the relatively rare case that one must specify different LaTeX commands for the PDF and HTML pages, there are also triple argument versions of these macros, namely \mjteqn{pdflatex}{htmllatex}{ascii} and \mjtdeqn{pdflatex}{htmllatex}{ascii}.

## Details

The Javascript code for MathJax is contained in this package. If a user viewing a help page has mathjaxr installed, it will be retrieved from there, otherwise it will be retrieved from the CDN site https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml-full.js. To force use of the CDN site, the user can set the environment variable MATHJAXR_USECDN to any non-blank value (e.g., Sys.setenv(MATHJAXR_USECDN=TRUE)). The URL for a diferent CDN can be specified via the environment variable MATHJAXR_CDN.

Package authors who want to ensure that users can see the rendered equations in the HTML help pages also when offline should add mathjaxr to Imports. To avoid the note from ‘R CMD check’ that ‘All declared Imports should be used’, one can add import(mathjaxr) to ‘NAMESPACE’.

## Issues

Care must be taken when using the less-than and greater-than symbols in equations as these might get interpreted by the browser as HTML tags. See here for further details. Adding space around these symbols should solve this problem (e.g., instead of writing \mjseqn{i<j}, one should write \mjseqn{i < j}). Do not use the \lt and \gt macros provided by MathJax as these will cause problems when rendering the PDF help pages.

Curly braces/brackets in equations also cause problems. Using \lbrace and \rbrace (possibly in combination with \left and \right to make them sufficiently large) is a solution (e.g., \mjeqn{\left\lbrace ... \right\rbrace}{\{...\}} should render nicely in the PDF/HTML help pages and the plain-text version).

Using the percent symbol (i.e., %) inside of equations is also problematic. The percent symbol needs to be ‘escaped’ by using a backslash, but backslashes need to be escaped as well. For this to work, we need to use the correct number of backslashes, which works slightly differently for producing the PDF, HTML, and plain-text help pages. The equation \mjteqn{100\\\%}{100\\\\\\\%}{100\%} should be rendered correctly in all three help pages.

Finally, while MathJax supports a large number of LaTeX commands, only the math-mode commands are implemented. See here for a list of the supported commands.

## Example

The probability density function of a normal distribution is given by \mjdeqnf(x) = \frac1\sqrt2\pi \sigma e^-\frac12\left(\fracx-\mu\sigma\right)^2,f(x) = 1/(sqrt(2 pi) sigma) e^-((x - mu)^2/(2 sigma^2)), where \mjseqn\mu denotes the mean of the distribution and \mjseqn\sigma its standard deviation.

## Author(s)

Wolfgang Viechtbauer wvb@wvbauer.com https://www.wvbauer.com/

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