Mapping Defines Additive Function of Subalgebra of Power Set

Theorem
Let $\mathbb A$ be one of the standard number systems $\N,\Z,\Q,\R,\C$.

Let $S$ be a finite set.

Let $f : S \to \mathbb A$ be a mapping.

Let $B$ be an algebra of sets over $S$.

Define $\Sigma : B \to \mathbb A$ using summation over finite sets as:
 * $\Sigma(T) = \displaystyle \sum_{t \mathop \in T} f(t)$

for $T\subseteq S$.

Then $\Sigma$ is an additive function on $B$.

Proof
Note that by Subset of Finite Set is Finite, $B$ consists of finite sets.

The result now follows from Sum over Disjoint Union of Finite Sets.

Also see

 * Inclusion-Exclusion Principle