\documentclass{amsart}
\usepackage[english]{babel}
\usepackage[utf8]{inputenc}

%%%%%%%%%% Start TeXmacs macros
\newcommand{\of}{:}
\newcommand{\tmdfn}[1]{\textbf{#1}}
\newcommand{\tmstrong}[1]{\textbf{#1}}
\newtheorem{definition}{Definition}
\newtheorem{theorem}{Theorem}
%%%%%%%%%% End TeXmacs macros

\begin{document}

\title{Groups}

{\maketitle}

Recall the definition of groups.

\begin{definition}
  A {\tmdfn{group}} is a set $G$ along with a binary operation $(-) \cdot (-)
  \of G \times G \rightarrow G$ satisfying the following axioms.
  \begin{enumerate}
    \item {\tmstrong{Associativity}}. For every triple $(a, b, c) \in G^3$, we
    have $(ab) c = a (bc)$.
    
    \item {\tmstrong{Identity}}. There exists an element $1 \in G$ such that,
    for every $g \in G$, we have $1 g = g 1 = g$.
    
    \item {\tmstrong{Inverse}}. For every $x, y \in G$, there exists an
    element $z \in G$ such that $y = xz$.
  \end{enumerate}
\end{definition}

We now study {\tmdfn{finite groups}}.

\begin{theorem}
  ABC
\end{theorem}

\begin{theorem}
  DEF
\end{theorem}

\begin{theorem}
  GHI
\end{theorem}

\end{document}
