LuaTeX: Use single symbol from other font in math mode

Question

I try to use specific symbols from other (TrueType) fonts (several different fonts) in math mode (as math letters, math operators, ...). So the symbol should be defined via DeclareMathSymbol, I guess. $\mysymbol$ in the TeX file produces the error message "Font shape `U/MyFont/m/n' undefined". I could not repair it via DeclareSymbolFont and DeclareFontShape. (\MyFont{10509} and $\text{\MyFont{10509}}$ does function here, but that's not what I need.) Thanks for help.

%\listfiles
\documentclass{article}
\usepackage{geometry}
\geometry{
 a4paper,
 total={170mm,257mm},
 left=20mm,
 top=20mm,
}
\usepackage{fontspec}
\usepackage{mathtools}

\title{Formula Font Test}
\author{Stefan}
\date{2017-03-15}

\newfontfamily\MyFontFamily[Path=C:/tmp/fonts/]{cambria.ttc}

% \DeclareFontFamily {} {} {}
%\DeclareFontFamily{U}{MyFamilyFont}{m}

\newcommand\MyFont[1]{\MyFontFamily\symbol{#1}}

% \DeclareSymbolFont {} {} {} {} {}
\DeclareSymbolFont{MyFontSymb}{U}{MyFont}{m}{n}

% \DeclareFontShape{U}{MyFontSymb}{m}{n}{  MyFont }{}
%\DeclareFontShape{MyFontSymb}{m}{n}{\UnicodeFontFile{cambria}{\UnicodeFontTeXLigatures}}{}

\DeclareMathSymbol{\othersymbol}{\mathalpha}{letters}{"0B}
\DeclareMathSymbol{\mysymbol}{\mathalpha}{MyFontSymb}{10509}

\usepackage{luacode}
\begin{document}

\maketitle

\MyFont{10509}

$
\othersymbol
\text{ \MyFont{10509} }
\mysymbol
$

\end{document}

Answer 1

You can do it like this

\documentclass{article}

\usepackage{fontspec}
\usepackage{mathtools}



\newfontfamily\MyFontFamily{cambria.ttc}[NFSSFamily=cambria]
\newcommand\MyFont[1]{{\MyFontFamily\symbol{#1}}}

\DeclareSymbolFont{MyFontSymb}{TU}{cambria}{m}{n}
\Umathchardef\mysymbol="3 \symMyFontSymb 10509


\begin{document}

\MyFont{10509}
$
x_{\mysymbol} {\mysymbol}
$

\end{document}

But I doubt a bit if it is really needed to set up a new symbol font. A simpler way is in this answer https://tex.stackexchange.com/a/344462/2388