Measure

Map $\mu :\Sigma \to {\mathbb{R}}_0^{+}$ where:

• $\Sigma$ is a sigma-algebra over $X$.
• Non-negativity: $\mu (E)\ge 0\forall E\in \Sigma$
• $\mu (\varnothing )=0$
• Countable additivity: $\forall \{E_k{\}}_k$ pairwise disjoint countable sets, $\mu ({\cup }_k{E}_k)={\sum }_k\mu (E_k)$.