Add the definitions for various useful LaTeX equation symbols for statistics to an RMarkdown document.
define_latex_stats()The function returns NULL invisibly. The command definitions
are output as a side effect.
Run this function from within a code chunk in a RMarkdown document
with options results = "asis" and echo = FALSE (see "Examples").
It only works for pdf output.
It defines the following macros: \E, \P, \Var, \Cov, \Cor,
\SD, \SE, \Xb, \Yb.