\documentclass[12pt , a4paper]{book} 
\usepackage{fancyhdr} \pagestyle{fancy}
\lhead{LEFT} \chead{CENTER} \rhead{RIGHT}

\renewcommand\headrulewidth{2pt}
\usepackage{amsthm}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{mathtools}
\usepackage{blkarray} 

\usepackage{graphicx}
\usepackage{fancybox}

\usepackage{tocbibind}
\usepackage{xepersian}
\fancyhead[R]{\leftmark}
\pagestyle{fancyplain}
\usepackage[top=6cm, bottom=5cm, left=4cm, right=4cm]{geometry}



\theoremstyle{definition}
\newtheorem{definition}{تعریف}[section]
\theoremstyle{theorem}
\newtheorem{theorem}[definition]{قضیه}
\newtheorem{lemma}[definition]{لم}
\newtheorem{proposition}[definition]{گزاره}
\newtheorem{corollary}[definition]{نتیجه}
\newtheorem{remark}[definition]{ملاحظه}
\newtheorem{example}[definition]{مثال}



\setdigitfont{PGaramond}


\renewcommand{\baselinestretch}{1.50}
\renewcommand{\sectionmark}[1]