\documentclass{book}
\usepackage{xepersian}
\settextfont{B Nazanin}
\usepackage[a4paper]{geometry}
\usepackage{graphicx}
\usepackage{algorithm}
\usepackage{algorithmic}
\usepackage{tabularx}
\usepackage{amsthm,amssymb,amsmath}
\theoremstyle{plain}
\newtheorem{theorem}{theorem}
\newtheorem{lemma}{lemma}
\newtheorem{proposition}{proposition}
\theoremstyle{definition}
\newtheorem{definition}{definition}
\newtheorem{example}{example}
\newtheorem{prob}{prob}
\theoremstyle{remark}
\newtheorem{corollary}{corollary}
\newtheorem{remark}{remark}

\begin{document}
 
 \lr{L,N and B}

\end{document}