Let $f : X \to \mathbb{R}^D$ be a D4363: Euclidean real function such that

(i)	$E_0, E_1, E_2, \ldots \subseteq X$ are each a D78: Subset of $X$
(ii)	$E_0, E_1, E_2, \ldots$ is a D5143: Set partition of $X$

Then \begin{equation} f = \sum_{n = 0}^{\infty} f I_{E_n} \end{equation}