I was in need of a diagram of a futile cycle. I tried illustator but I didn’t have the right LaTeX fonts so the figure did blend well with the rest of the document. I decided to make one using TiKz. And here it is.

```
\documentclass{article}
```

\usepackage{tikz}
\usetikzlibrary{arrows}
\begin{document}
\begin{tikzpicture}[>=latex',node distance = 2.5cm]
\node (S2) {$S_2$};
\node [left of = S2] (S1) {$S_1$};
\node [left of = S1, node distance = 1.5cm] (Xo) {};
\draw[->,thick] (Xo) to node[above] {$v_1$} (S1);
\node [right of = S2, node distance = 1.5cm] (X1) {};
\draw[->,thick] (S2) to node[above] {$v_4$} (X1);
\draw [->,thick] (S2) to [bend left=40] node[below] {$v_3$} (S1);
\draw [->,thick] (S1) to [bend left=40] node[above] {$v_3$} (S2);
\end{tikzpicture}
\end{document}