Inclusion-Exclusion Principle/Corollary

Corollary to Inclusion-Exclusion Principle
Let $\mathcal S$ be an algebra of sets.

Let $A_1, A_2, \ldots, A_n$ be finite sets which are pairwise disjoint.

Let $f: \mathcal S \to \R$ be an additive function.

Then:


 * $\displaystyle \map f {\bigcup_{i \mathop = 1}^n A_i} = \sum_{i \mathop = 1}^n \map f {A_i}$

Proof
As $A_1, A_2, \ldots, A_n$ are pairwise disjoint, their intersections are all empty.

Then Inclusion-Exclusion Principle holds.

However, from Cardinality of Empty Set, all the terms apart from the first vanish.

Comment
This result is usually quoted in the context of combinatorics, where $f$ is the cardinality function.

It is also seen in the context of probability theory, in which $f$ is taken to be a probability measure.