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\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{pgf, tikz}
\usetikzlibrary{arrows , automata , positioning}
\begin{document}

\begin{center}
\begin{tikzpicture}[>=stealth',shorten >=1pt,auto,node distance=2.5cm]
%Knoten
\node (a) [state, thick] {a};
\node (b) [state, thick, right of= a, below of=a] {b};
\node (c) [state, thick, left of= a, below of=a] {c};
\node (d) [state, thick, below of=b, right of=b] {d};

\node (e) [state, thick, below of=c, left of=c] {e};

\node (g) [state, thick, below of=d, left of=d] {g};

\node (f) [state, thick, below of=e, right of=e] {f};

%Verbindungen
\path[thick,->]
(a) edge [bend angle=20, bend left,below] (c)
(a) edge [bend angle=20, bend left,below] (e)
(a) edge [bend angle=20, bend left,below] (f)
(a) edge [bend angle=20, bend left,below] (g)
(b) edge [bend angle=20, bend left,below] (a)
(b) edge [bend angle=20, bend left,below] (d)
(b) edge [bend angle=20, bend left,below] (f)
(b) edge [bend angle=20, bend left,below] (g)
(c) edge [bend angle=20, bend left,below] (c)
(c) edge [bend angle=20, bend left,below] (g)
(d) edge [bend angle=20, bend left,below] (d)
(d) edge [bend angle=20, bend left,below] (e)
(d) edge [bend angle=20, bend left,below] (f)
(e) edge [bend angle=20, bend left,below] (a)
(e) edge [bend angle=20, bend left,below] (c)
(e) edge [bend angle=20, bend left,below] (d)
(e) edge [bend angle=20, bend left,below] (e)
(e) edge [bend angle=20, bend left,below] (f)
(e) edge [bend angle=20, bend left,below] (g)
(f) edge [bend angle=20, bend left,below] (c)
(f) edge [bend angle=20, bend left,below] (d)
(f) edge [bend angle=20, bend left,below] (e)
(f) edge [bend angle=20, bend left,below] (f)
(g) edge [bend angle=20, bend left,below] (a)
(g) edge [bend angle=20, bend left,below] (c)
(g) edge [bend angle=20, bend left,below] (d)
(g) edge [bend angle=20, bend left,below] (e)
(g) edge [bend angle=20, bend left,below] (f)
(g) edge [bend angle=20, bend left,below] (g)
(a) edge [bend angle=20, bend left,below, style thick] (`)
(`) edge [bend angle=20, bend left,below, style thick] (`)
(`) edge [bend angle=20, bend left,below, style thick] (`)
(`) edge [bend angle=20, bend left,below, style thick] (`)
(`) edge [bend angle=20, bend left,below, style thick] (`)
(`) edge [bend angle=20, bend left,below, style thick] (`)
;
\end{tikzpicture}
\end{center}
\end{document}
