%#!lualatex
\documentclass{article}
\usepackage[a4paper]{geometry}

\usepackage{luatexja-fontspec}

\usepackage{luatexja}
\makeatletter\ltj@alljachar %←全部の（>=U+0080な）文字を和文文字扱いに！

\usepackage{unicode-math}
\setmathfont{XITSMath}
\setmainjfont{IPAGothic}

\begin{document}\makeatletter

\[
 \frac1{1^2}+\frac1{2^2}+\cdots=
\sum_{n=1}^\infty \frac1{n^s}=\zeta(2)=\frac{\pi^2}{6}
=\frac16\cdot \pi\times\pi,\qquad a_1,\dots,a_n.
\]

$
 \frac1{1^2}+\frac1{2^2}+\cdots=
\sum_{n=1}^\infty \frac1{n^s\zeta}=\zeta(2)=\frac{\pi^2}{6}
=\frac16\cdot \pi\times\pi,\qquad a_1,\dots,a_n. \hbar
$

{\tt\meaning\sum}


○×○漢\ltjjachar`\×漢\ltjalchar`\×漢\times 漢
え\ltjjachar`\“え\ltjalchar`\“え\textquotedblleft え“え
←数式外では和文文字扱いのままになっている．
\end{document}
