[Tex/LaTex] Specific font for specific characters with mathspec package

fontsfontsizemathspecxetex

I am trying to manipulate the font, font size and thickness of the mathematical characters individually. I came into this solution at stackexchange;
XeTeX/mathspec punctuation issue

...
\makeatletter
\ernewcommand\eu@MathPunctuation@symfont{Latin:m:n}
\DeclareMathSymbol{,}{\mathpunct}{\eu@MathPunctuation@symfont}{`,}
...

To make the +-= bold and plain other I tried this way (which did not work);

\documentclass[9pt]{article}
\usepackage{extsizes}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathspec}


\setmathfont(Digits,Latin)[Numbers={Lining,Proportional},Lowercase={Regular},Uppercase={Regular}]{Arial}
\setmathfont(Greek)[Scale=1.15, Lowercase={Regular},Uppercase={Regular}]{Symbol}
\setmainfont[Ligatures=TeX]{Arial}

\makeatletter
\ernewcommand\eu@MathPunctuation@symfont{Latin:bx:n}
\DeclareMathSymbol{+}{\mathpunct}{\eu@MathPunctuation@symfont}{`+}
\DeclareMathSymbol{-}{\mathord}{\eu@MathPunctuation@symfont}{`-}
\DeclareMathSymbol{=}{\mathrel}{\eu@MathPunctuation@symfont}{`=}
\makeatother


\begin{document}
\thispagestyle{empty}

$+-=$+-=

\end{document}

Question 1) How can we make only characters "+-=" bold and other characters plain.
Question 2) Can we set the font size of the "a" in the expression "x^a". I want my normal text to be 9pt and sub, sup, frac and frac{frac} characters to be fixed at 8pt not shrinking as the level increases.
Any help is appreciated. Thank you all reading or responding.

Best Answer

You're forgetting to declare the suitable math symbol font; you have also to use \XeTeXDeclareMathSymbol if you want a real minus sign.

\documentclass[9pt]{article}
\usepackage{extsizes}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathspec}


\setmathfont(Digits,Latin)[
  Numbers={Lining,Proportional},
  Lowercase=Regular,
  Uppercase=Regular
]{Arial}
\setmathfont(Greek)[
  Scale=1.15,
  Lowercase=Regular,
  Uppercase=Regular
]{Symbol}
\setmainfont[Ligatures=TeX]{Arial}

% subscripts are 8pt at any level
\DeclareMathSizes{9}{9}{8}{8}

\makeatletter
\DeclareSymbolFont{Latin:bx:n}{EU1}{\eu@Latinmathsfont}{bx}{n}
\ernewcommand\eu@MathPunctuation@symfont{Latin:bx:n}
\DeclareMathSymbol{+}{\mathbin}{\eu@MathPunctuation@symfont}{`+}
\DeclareMathSymbol{=}{\mathrel}{\eu@MathPunctuation@symfont}{`=}
% fight against amsmath using the mathcode of `-`
\AtBeginDocument{%
  \XeTeXDeclareMathSymbol{-}{\mathbin}{\eu@MathPunctuation@symfont}{"2212}%
}
\makeatother


\begin{document}
\thispagestyle{empty}

$+-=$+-=

$a^{a^a}$

\end{document}

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