Let $X$ be a D16: Countable set.

Then
\begin{equation}
|X|
= \sum_{x \in X} I_X(x)
\end{equation}

Result on D41: Indicator function

Formulation 0

Let $X$ be a D16: Countable set.

Then
\begin{equation}
|X|
= \sum_{x \in X} I_X(x)
\end{equation}

Also known as

Countable cardinality equals sum of indicator functions

Proofs

<No proofs for this assertion yet>