Cardinality is an Additive Function
From ProofWiki
Theorem
Let $S$ be a finite set.
Let $\mathcal P \left({S}\right)$ be the power set of $S$.
The function $C: \mathcal P \left({S}\right) \to \R$, where $C$ is defined as the cardinality of a set, is an additive function.
Proof
We have that $\mathcal P \left({S}\right)$ is an algebra of sets.
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