View source for Seifert-Van Kampen theorem

== Statement ==
Let <math>X</math> be a topological space, and <math>U, V\subset X</math> be open sets such that <math>X = U\cup V</math>, and <math>U</math>, <math>V</math> and <math>U\cap V</math> are path-connected. Take a basepoint <math>x_0\in U\cap V</math> with inclusion maps:

<math display="block">i\colon U\cap V\hookrightarrow U,\quad j\colon U\cap V\hookrightarrow V,\quad k\colon U\hookrightarrow X,\quad l\colon V\hookrightarrow X,</math>

then the following diagram is a pushout:
<div style="display: flex; justify-content: center; gap: 40px;">
<kroki lang="tikz">
\documentclass[tikz]{standalone}
\usepackage{quiver}
\begin{document}
\begin{tikzpicture}[baseline=(current bounding box.center)]  
\node[scale=1.5] {
\begin{tikzcd}
	{\pi_1(U\cap V,x_0)} & {\pi_1(U,x_0)} \\
	{\pi_1(V,x_0)} & {\pi_1(X,x_0)}
	\arrow["{i_*}", from=1-1, to=1-2]
	\arrow["{j_*}"', from=1-1, to=2-1]
	\arrow["{k_*}", from=1-2, to=2-2]
	\arrow["{l_*}"', from=2-1, to=2-2]
\end{tikzcd}
  };
\end{tikzpicture}

\end{document}
</kroki>
</div>