Ivan's wiki

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\documentclass[border=5pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows.meta,positioning}
\begin{document}
\begin{tikzpicture}[node distance=18mm,>={Stealth[round]}]
  \node[draw,circle] (a) {$a$};
  \node[draw,circle,right=of a] (b) {$b$};
  \node[draw,circle,right=of b] (c) {$c$};
  \draw[->,thick] (a) -- node[above]{$f$} (b);
  \draw[->,thick] (b) -- node[above]{$g$} (c);
  \draw[->,thick,bend right=40] (a) to node[below]{$g\circ f$} (c);
\end{tikzpicture}
\end{document}