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I wish to add a symbol from MS Symbol to my document. Specifically, I need to add the

\plus

character and only that to my document below.

\documentclass[a4paper,12pt,twoside]{book}

\usepackage[left=2.50cm,right=2.50cm,top=2.50cm,bottom=2.75cm]{geometry}
\usepackage{amsmath,amssymb,amscd,amsbsy,array,color}
\usepackage{fancyhdr,framed,latexsym,multicol,pstricks,slashed,xcolor}
\usepackage{picture}
\usepackage{indentfirst}
\usepackage{enumitem}

\usepackage[amsmath,framed,thmmarks]{ntheorem}
\usepackage{changes}


\newtheorem{Theorem}{Theorem}

\theoremclass{Theorem}
\theoremstyle{break}
\theoreminframepreskip{0pt}
\theoreminframepostskip{0pt}
\theoremframepreskip{1cm}
\theoremframepostskip{1cm}
\theoremstyle{break}
\def\theoremframecommand{%
\psshadowbox[fillstyle=solid,fillcolor=blue,linecolor=black]}
\newshadedtheorem{them}[section]{Theorem}

\theoremstyle{nonumberplain}
\theoremsymbol{\rule{1ex}{1ex}}
\def\theoremframecommand{%
\psshadowbox[fillstyle=solid,fillcolor=red,linecolor=black]}
\newshadedtheorem{proof}{Proof}


\title{Real and Complex Analysis}
\author{Michael Dykes and Croix Snapp}

\begin{document}
\maketitle

\begin{them}[Arithmetic of Natural Numbers]
There are operations \textbf{addition} $\plus$,   \textbf{multiplication} $\cdot$ and a partial order $\leq$ on $\bn$ which are uniquely determined by the following conditions:
\begin{enumerate}
    \item Addition is associative, commutative, and has the identity element $0$.
    \item Multiplication is associative, commutative, and has $1 \eqdef \sigma(0)$ as its identiy element.
    \item The distributive law holds:
    \[
    (\ell + m) \cdot n = \ell \cdot n + m \cdot n, \q \q \ell,m,n \in bn.
    \]
    \end{enumerate}
    \end{them}

    \end{document}

When I compile I get an error of course. I don't know how to import just that one symbol into my document without changing all my other symbols. Please help.

marked as duplicate by Steven B. Segletes, user36296, Reinstate Monica - M. Schröder, Mike Renfro, barbara beeton Sep 15 '16 at 15:09

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