Here is my MWE:
\documentclass[a4paper,openany,12pt]{book}
\usepackage[a]{esvect}
\usepackage[utf8x]{inputenc}
\usepackage{amsmath,amssymb,amsthm,amsfonts}
\usepackage{mathrsfs,mathtools}
\usepackage{fontenc}
\usepackage{geometry}
\usepackage{fontspec}
\RequirePackage{hyperref}
\usepackage{etoolbox}
\usepackage[computeautoilg=off]{xepersian}
\settextfont[Scale=1.1]{IRXLotus}
\setdigitfont[Scale=1.1]{IRXLotus}
\setlatintextfont[Scale=1.1]{Times New Roman}
\begin{document}
\chapter{بردارهای ویژه و مقادیر ویژه}
بردار $\mathbf{x}$ و $\mathbf{y}$ بر حسب پایهای متعامد از فضای $\mathfrak{L}$ با مجموع حاصلضرب دوبهدو متناظر مختصات آنها بهدست میآید. حال $\mathbf{x}$ را از $\mathfrak{L}$ بگیرید. اگر $\mathop{\xi_i=(\mathbf{x},\mathbf{e}_i)}_{(1 \leqslant i \leqslant n)}$ فرض شود، آنگاه
\begin{equation*}
\mathbf{x}=\xi_1 \mathbf{e}_1 +\cdots +\xi_n \mathbf{e}_n
\end{equation*}
و علت این است که
\begin{equation*}
(\mathbf{x},\mathbf{e}_i)=\displaystyle\xi_1 (\mathbf{e}_1,\mathbf{e}_i) +\cdots +\xi_n (\mathbf{e}_n,\mathbf{e}_i)=\xi_i (\mathbf{e}_i,\mathbf{e})=\xi_i (\mathbf{e}_i ,\mathbf{e}_i)=\xi_i (\mathbf{e}_i,\mathbf{e}_i )= \xi_i.
\end{equation*}
\end{document}
I would like to import certain open math fonts for mathematical objects as suggested in this neat answer. It seems that they work under unicode-math package but, in this minimal work it crashed. May I ask you what to do? Thanks for your effort and the TIME!

\usepackage[utf8x]{inputenc}with xelatex (it does nothing other than generate a warning that it should not be used) – David Carlisle Aug 12 '21 at 15:40\displaystyledoes nothing at the point you have used it (equations are in displaystyle by default) – David Carlisle Aug 12 '21 at 15:54