The collection of Lebesgue measurable subsets of $\mathbb{R}^n$ is a $\sigma$-algebra containing all open subsets of $\mathbb{R}^n$. The measure $\mathcal{L}^n$ is countably additive on this $\sigma$-algebra. If $(E_k)_{k=1}^\infty$ are pairwise disjoint Lebesgue measurable sets, then