\documentclass[oneside,phd]{article}
\usepackage{amsthm,amssymb,amsmath,amsfonts}
\usepackage{tensor}
\usepackage{xepersian}

\begin{document}
فرمول بدون خطا:
\begin{align*}
\tensor[_a]D{_x^{\mu}}\tensor[_a]D{_x^{-\mu}}u(x)& \\
& =D^n \tensor[_a]D{_x^{\mu-n}}\tensor[_a]D{_x^{-\mu}} u(x)\\
& =D^n \tensor[_a]D{_x^{-n}}u(x)=u(x{^a_b}).
\end{align*}
خطا در فرمول زیر:
\begin{align*}
\indices{_a}D{_x^{\mu}}\indices{_a}D{_x^{-\mu}}u(x)& \\
& =D^n \indices{_a}D{_x^{\mu-n}}\indices{_a}D{_x^{-\mu}} u(x)\\
& =D^n \tensor[_a]D{_x^{-n}}u(x)=u(x{^a_b}).
\end{align*}
\end{document}