Let $M = (X, \mathcal{F})$ be a D1700: Discrete measurable space such that

(i) | $\mu : \mathcal{F} \to [0, \infty]$ is a D4105: Standard counting measure on $M$ |

(ii) | $I_X : X \to \{ 0, 1 \}$ is an D41: Indicator function on $M$ with respect to $X$ |

Then
\begin{equation}
\forall \, E \in \mathcal{F} :
\mu(E) = \sum_{x \in E} I_X(x)
\end{equation}