\documentclass[11pt,a4paper,twoside]{article}

\usepackage{german}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{theorem}

\theoremstyle{change}

\newtheorem{satz}{Satz.}[subsection]
\newtheorem{lem}[satz]{Lemma.}
\newtheorem{defin}[satz]{Definition.}
\newtheorem{kor}[satz]{Korollar.}

\newenvironment{beweis}
               {\textbf{Beweis.}}
	       {\hfill \(\Box\) \medskip
	       
	       }

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

\begin{document}


\end{document}
