finished multivar calc

chapters/sections/contents_measures.tex

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+% !TeX root = ../../script.tex
+\documentclass[../../script.tex]{subfiles}
+
+\begin{document}
+\section{Contents and Measures}
+
+\begin{defi}
+    A set $M$ is said to be countable if there exists a surjective mapping from $\natn$ to $M$, i.e.
+    \[
+        \exists \seq{x} \subset M: ~~\forall y \in M ~\exists n \in \natn: ~~x_n = y
+    \]
+    A set $M$ is said to be countably infinite if it is countable and unbounded.
+\end{defi}
+\end{document}