Rendering mathematical symbols in papaja()

I'm trying to render what I think should be a very simple table using kable() within papaja(). One cell in the table includes mathematical symbols, and I get an error message when trying to render the document to pdf.

  # some data
  subj <- c(1, 2, 3, 4, 5)
  var1 <- c(1, 1, 2, 1, 2)
  var2 <- c("hfg", "hrfg", "thflk", "plht", "sdrpv")
  sum_sq_diffs <- c("Σ√(total))", NA, NA, NA, NA) 

  df <- data.frame(subj, var1, var2, sum_sq_diffs)

When I try to render this in papaja(), the Σ and √ will not render. I have tried to re-code these using equation notation, e.g. $sum$ $sqrt$(total) but I always get error messages:

 ! Missing { inserted.
 <to be read again> 
              $
l.129 (Σ √
                              (total)) & NA & NA & NA...

I've also tried $\\sum$ $\\sqrt$(total) in my code, and have changed the front matter to the following:

 latex_engine: xelatex
 header-includes:
 - \usepackage{newunicodechar}
 - \newunicodechar{Σ}{\ensuremath{\sum}}
 - \newunicodechar{√}{\ensuremath{\sqrt}}

Note that the equation won't render properly when I view it as a dataframe either (though the original symbols do work in this case), it just shows the string as is specified in the code, with $$s all in place.

Solution

I would recommend using LaTeX math directly; the trick here is to use escape = FALSE in kable() and to ensure that the column names are valid LaTeX expressions (e.g., no _). Consider the following example:

 # some data
subj <- c(1, 2, 3, 4, 5)
var1 <- c(1, 1, 2, 1, 2)
var2 <- c("hfg", "hrfg", "thflk", "plht", "sdrpv")
sum_sq_diffs <- c("$\\sum\\sqrt\\text{total}$", NA, NA, NA, NA)

data.frame(subj, var1, var2, sum_sq_diffs) |>
  knitr::kable(
    col.names = c("Subject", "Var 1", "Var 2", "Sum of sqaured differences")
    , escape = FALSE
  )