in order to go over the "too many alphabets" usual error, I am creating fake math commands somehow.
\documentclass[fleqn,a4paper,12pt,twoside]{book}
\usepackage[top=3.2cm,bottom=3.2cm,left=3.3cm,right=3.3cm,a4paper]{geometry}
\usepackage[MnSymbol]{mathspec}
\usepackage{xltxtra}
\usepackage{esvect}
\renewcommand{\vec}{\vv}
\defaultfontfeatures{Numbers=OldStyle,Scale=MatchLowercase}
\setmainfont[Mapping=tex-text]{Minion Pro}
\setmathfont(Digits,Latin,Greek)[Uppercase=Regular,Lowercase=Italic]{Minion Pro}
% correct commands yielding the math alphabet error messages when uncommented
\setmathrm{Minion Pro}
\DeclareMathOperator{\dive}{div}
\begin{document}
\begin{equation}
\mathcal{A}=\cos \pi\quad\vec{e}=3\vec{\dive}(u)\mathfrak{B}
\end{equation}
\end{document}
This code as such will not compile. As a work-around, when the two lines defining the Math Alphabet of the document are replaced (you have to comment them) by :
% fake cosinus and divergence commands to get similar results without the error message
\renewcommand{\cos}{\text{cos}\,}
\newcommand{\dive}{\text{div}\,}
the code will compile with very similar results: what is wrong then, with this trick?
Thank you