% !TEX TS-program = xelatex 
% !TEX encoding = UTF-8
\documentclass{minimal}

\usepackage{unicode-math}

\begin{document}
\noindent					Computer Modern	\(f = f_k\) \\
\setmathfont{Asana Math}		Asana Math		\(f = f_k\) \\
\setmathfont{Cambria Math}	Cambria Math		\(f = f_k\) \\
\setmathfont{XITS Math}		XITS Math			\(f = f_k\) \\
\setmathfont[math-style=upright]{Neo Euler} Neo Euler	\(f = f_k\)
\end{document}
