\documentclass[a4paper,11pt]{article}

\usepackage{ngerman}
\usepackage{amsmath,amsfonts,amssymb,amsthm}

\usepackage{tikz}
\usetikzlibrary{arrows,chains,matrix,positioning,scopes}
\usetikzlibrary{arrows.meta}
\usetikzlibrary{math}

\def\O{\mathcal{O}}
\def\sort{\mathrm{sort}}
\def\scan{\mathrm{scan}}
\def\dist{\mathrm{dist}}
\def\Z{\mathbb{Z}}
\def\Q{\mathbb{Q}}
\def\R{\mathbb{R}}
\def\E{\mathbb{E}}


\begin{document}


\textbf{Aufgabe 4}
\begin{center}
	\begin{tikzpicture}
	[scale=0.8,auto=left,every node/.style={circle,fill=none,draw=black!80,line width=1pt,minimum size=.6cm}]
	\node (a) at (0,0) {$7$};
	\node (b) at (2,0) {$8$};
	\node (c) at (4,0) {$9$};
	\node (d) at (0,2) {$4$};
	\node (e) at (2,2) {$5$};
	\node (f) at (4,2) {$6$};
	\node (g) at (0,4) {$1$};
	\node (h) at (2,4) {$2$};
	\node (i) at (4,4) {$3$};
	
	\path[-] 
	(a) edge (b) edge (d)
	(b) edge (c) edge (e) edge (f)
	(c) edge (f)
	(d) edge (e) edge (g) edge (h)
	(e) edge (f) edge (h)
	(f) edge (i)
	(g) edge (h)
	(h) edge (i);
	\end{tikzpicture}
\end{center}


\end{document}