\documentclass[12pt,a4paper]{article}

%  These commands bring in all the nice fonts and symbols
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsmath}

\usepackage{amsthm}

\usepackage[german]{babel}

\newcommand{\Z}{\mathbb Z}
\newcommand{\N}{\mathbb N}
\newcommand{\F}{\mathbb F}
\newcommand{\Q}{\mathbb Q}
\newcommand{\R}{\mathbb R}
\newcommand{\C}{\mathbb C}

%  Usually journal style files will have their own theorem environments
\newtheorem{theorem}{Satz}[section]
%\newenvironment{proof}{\noindent \textbf{Proof.}}
%\newtheorem{definition}{Definition}%[section]
%\newtheorem{theorem}{Theorem}[section]
\newtheorem{definition}[theorem]{Definition}
%\newtheorem{lemma}[theorem]{Lemma}
%\newtheorem{corollary}[theorem]{Corollary}
\newtheorem{fact}[theorem]{Fact}
\newtheorem{example}[theorem]{Example}



\begin{document}

\title{Ein paar Versuche}

%\author{}

\date{\today}

\maketitle

\section{Einleitung}
\begin{definition}
abc
\end{definition}

\begin{theorem}
test
\end{theorem}

\begin{proof}

test
\end{proof}




\begin{thebibliography}{}

\bibitem{boneh} D. Boneh,
The decision Diffie-Hellman problem,
in J. Buhler (ed.), ANTS III, Springer LNCS 1423
(1998) 48--63.

\end{thebibliography}

\end{document}

