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\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}
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