title: latex error Missing $ inserted
body: <p>I'm trying to use LaTeX to write my math notes. But I have some problems. </p>
  
  <p>The error is written:</p>
  
  <blockquote>
    <p>LaTeX error Missing $ inserted</p>
  </blockquote>
  
  <p>And it doesn't compile correctly:</p>
  
  <p><img src="https://i.imgur.com/Rma87Zu.jpg" alt=""></p>
  
  <pre><code>\documentclass[french,12pt,a4paper]{article}
  \usepackage[french]{babel}
  \usepackage[T1]{fontenc}
  \usepackage[utf8]{inputenc}
  \usepackage{amsmath}
  \usepackage{fullpage}
  
  \author{
    Alexandre Monterroso\\
   Université de Fribourg\\
  }
  \newtheorem{de}{Définition}[subsection] 
  \newtheorem{theo}{Théorème}[section]
  \newtheorem{prop}[theo]{Proposition}
  
  
  \begin{document}
  \section{Combinatoris}
  \subsection{Ordered choices}
  \begin{itemize}
      \item With repetition
  \end{itemize}
  \begin{theo}
  The number of lists (a_{1}, ..., a_{k}) of k not necessarly distincts objects elements (ie. with repetition) from a set n elements is a^{k} (n, k \epsilon N).
  \end{theo}
  \end{document}
  </code></pre>