\begin{scriptsize}
\begin{footnotesize}
•
\end{footnotesize}
\end{scriptsize}\documentclass[12pt]{book}
\usepackage{graphicx,amsmath,geometry,amssymb,amsthm,amssymb,hyperref,geometry}
%\usepackage{graphicx,fancyhdr,color,xspace,amsmath,amssymb,,hyperref,geometry,xepersian}
\usepackage{xepersian}
%\usepackage[twoside,top=26mm,bottom=21mm,left=20mm,right=20mm]{geometry}
\newcommand{\Q}{QIEP}
\newcommand{\nn}{$n\times n$}
%\newcommand{\mck}{$M$،  $C$، و$K$ }
\newcommand{\mck}{$(M,C,K)$}
\newcommand{\pl}{\subset \hspace{-4mm}+}
\newcommand{\Ql}{Q(\lambda)}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\geometry{verbose,a4paper, tmargin=3cm, bmargin=2.5cm, lmargin=2.5cm, rmargin=4cm,footskip=1.6cm}
\renewcommand{\baselinestretch}{1.9} 
\renewcommand{\baselinestretch}{1.9}%فاصله بین خطوط  ۶.۱ اندازه استاندارد 

\settextfont[Scale=1.1]{B Nazanin}
\setlatintextfont[Scale=1]{Times New Roman}
\setdigitfont[Scale=0.9]{Yas}
\newfontinstance\nastaliq[Script=Arabic,Scale=1.5]{IranNastaliq}
\newcommand{\translation}[1]{\LTRfootnote{\rmfamily #1}}
\newcommand{\Eng}[1]{{\rmfamily #1}}
%\newcommand{\EW}[1]{\lr{\scriptsize #1}}
%\graphicspath{{Figures/}}
\numberwithin{equation}{section}

%\setlatintextfont[ExternalLocation,BoldFont={lmroman10-bold},BoldItalicFont={lmroman10-bolditalic},ItalicFont={lmroman10-italic}]{lmroman10-regular}

\let\iranic\it
\let\khabide\sl
\def\siahir{\siah\iranic} 
\def\siahkh{\siah\khabide}
\let\tookhali\pookfamily
\let\sayedar\sayehfamily
\let\farsi\Persian
\let\english\Latin 
\let\farmbox\mbox
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%5
\newtheorem{de}{\siah{\large\rl{{تعریف }}}}[chapter]
\newtheorem{thh}{\siah{\large\rl{{ قضیه }}}}[chapter]
\newenvironment{proof}{{\bf اثبات. }  \rm }{\hfill{$\Box$}\\}
\newtheorem{rem}{\siah{\large\rl{{ نتیجه }}}}[chapter]
\newtheorem{remm}{\siah{\large\rl{{ نتیجه‌گیری }}}}[chapter]
\newtheorem{lee}{\siah{\large\rl{{لم }}}}[chapter]
\newtheorem{note}{\siah{\large\rl{{توجه}}}}[chapter]
\newtheorem{exa}{\siah{\large\rl{{مثال }}}}[chapter]
\newtheorem{alg}{\siah{\large\rl{{الگوریتم }}}}[chapter]
%\pagenumbering{}
%\pagenumbering{roman}
\vspace{6cm}

%%%%%%%%%%%%%%%%%%%%%%%%5      
\begin{document}
\pagestyle{empty}
برای هر $ k=0,1,.. $.\\ از آن‌جایی که $ 0\leq\lambda \leq 1 $ هنگامی که $ k \rightarrow \infty $ میل می‌کند، \begingroup\makeatletter\def\f@size{11}\check@mathfonts $\max_{0 \leq i \leq n\  0\leq j\leq m} \vert \xi^{(k+1)}_{i,j}-\xi^{*}_{i,j}\vert$  \endgroup صفر می‌شود.
\end{document}