Visual representation of a function and its inverse, with the y=x line as the axis of symmetry. Classic illustration of inverse functions. From Sarmate's technical TikZ gallery (sources: MartinThoma/LaTeX-examples MIT, TeXample.net LPPL) — see also our 900+ figures on /tikz-gallery.php.
\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes,snakes,shapes.geometric,positioning,decorations.text}
\begin{document}
\tikzstyle{arrow}=[bend left,->,very thick, line cap=round]
\begin{tikzpicture}[node distance=2cm]
\node[ellipse,draw,minimum width=80pt,minimum height=20pt,fill=gray!5] (a) {1\\2\\3};
\node[ellipse,draw,minimum width=80pt,minimum height=20pt,fill=gray!5,below of=a] (u) {a\\b\\c};
% Arrows
\draw[arrow,postaction={decorate,decoration={text along path,raise=0.08cm,text align=center,text={Aktion {$a_k$}}}}] (a.east) to node {} (u.east);
\draw[arrow,postaction={decorate,decoration={text along path,raise=0.08cm,text align=center,text={Zustand {$x_k$}}}}] (u.west) to node {} (a.west);
\end{tikzpicture}
\end{document}
