Defining a math symbol of variable sizes in XeTeX with \Umathchardef

Question

I need to use a character in a TrueType font as a math symbol. Having seen morbusg's answer, I tried something like this:

\font\mymathtfont="Times New Roman"
\font\mymathsfont="Times New Roman/S=7" at 7pt
\font\mymathssfont="Times New Roman/S=5" at 5pt
\newfam\mymathfam
\textfont\mymathfam=\mymathtfont
\scriptfont\mymathfam=\mymathsfont
\scriptscriptfont\mymathfam=\mymathssfont
\Umathchardef\mysym 3 \mymathfam "1D76

The command \mysym does print the symbol, but when the font size in the article changes (for example by adding \fontsize{14.4pt}{1.3\baselineskip}\selectfont after \begin{document}), the size of the symbol printed by \mysym remains the same. I speculate that it is because the font sizes are fixed when the font is selected, so what is the correct way to select the font in this case? Thanks!

Answer 1

Setting up math fonts is a delicate matter and you don't need it for just a few symbols.

\documentclass{article}
\usepackage{amsmath}
\usepackage{fontspec}

\newfontfamily{\timesnewroman}{Times New Roman}

\DeclareRobustCommand\mysym{%
  \mathrel{\text{\normalfont\timesnewroman\symbol{"1D76}}}%
}

\begin{document}

$a\mysym b+\sum_{a\mysym b}X$

\Huge

$a\mysym b$

\end{document}