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+%
+% ebene.tex -- Punktgruppen in der Ebene
+%
+% (c) 2021 Prof Dr Andreas Müller, OST Ostschweizer Fachhochschule
+%
+\bgroup
+\begin{frame}[t]
+\setlength{\abovedisplayskip}{5pt}
+\setlength{\belowdisplayskip}{5pt}
+\frametitle{Punktgruppen in der Ebene}
+\vspace{-20pt}
+\begin{columns}[t,onlytextwidth]
+\begin{column}{0.48\textwidth}
+\begin{block}{Zyklische Gruppen}
+\begin{center}
+\begin{tikzpicture}[>=latex,thick]
+\def\a{40}
+\def\r{2}
+\def\R{2.5}
+\fill[color=blue!20] (0,0) -- (0:{1.1*\R}) arc (0:\a:{1.1*\R}) -- cycle;
+\node[color=blue] at ({0.5*\a}:{0.8*\r}) {$\displaystyle\frac{2\pi}n$};
+\fill (0,0) circle[radius=0.08];
+\draw[color=red] (0:\r) -- (0:\R)
+	-- ({1*\a}:\r) -- ({1*\a}:\R)
+	-- ({2*\a}:\r) -- ({2*\a}:\R)
+	-- ({3*\a}:\r) -- ({3*\a}:\R)
+	-- ({4*\a}:\r) -- ({4*\a}:\R)
+	-- ({5*\a}:\r) -- ({5*\a}:\R)
+	-- ({6*\a}:\r) -- ({6*\a}:\R)
+	-- ({7*\a}:\r) -- ({7*\a}:\R)
+	-- ({8*\a}:\r) %-- ({8*\a}:\R)
+;
+\end{tikzpicture}
+\end{center}
+\[
+C_n
+=
+\{\text{Drehungen um Winkel $2\pi/n$}\}
+\]
+\end{block}
+\end{column}
+\begin{column}{0.48\textwidth}
+\begin{block}{Diedergruppen}
+\begin{center}
+\begin{tikzpicture}[>=latex,thick]
+\def\a{40}
+\def\r{2}
+\def\R{2.5}
+\fill[color=blue!20] (0,0) -- (0:{1.1*\R}) arc (0:\a:{1.1*\R}) -- cycle;
+\node[color=blue] at ({0.5*\a}:{0.8*\r}) {$\displaystyle\frac{2\pi}n$};
+\fill (0,0) circle[radius=0.08];
+\draw[color=red] (0:\r) -- ({0.5*\a}:\R)
+	-- ({1*\a}:\r) -- ({1.5*\a}:\R)
+	-- ({2*\a}:\r) -- ({2.5*\a}:\R)
+	-- ({3*\a}:\r) -- ({3.5*\a}:\R)
+	-- ({4*\a}:\r) -- ({4.5*\a}:\R)
+	-- ({5*\a}:\r) -- ({5.5*\a}:\R)
+	-- ({6*\a}:\r) -- ({6.5*\a}:\R)
+	-- ({7*\a}:\r) -- ({7.5*\a}:\R)
+	-- ({8*\a}:\r) %-- ({8.5*\a}:\R)
+;
+\end{tikzpicture}
+\end{center}
+\begin{align*}
+D_n
+&=
+\langle\text{Spiegelung},
+\text{Drehungen}\rangle
+\\
+&=
+C_2
+\ltimes
+C_n
+\end{align*}
+\end{block}
+\end{column}
+\end{columns}
+\end{frame}
+\egroup