Let $\mathcal{K}$ be a sequential covering class of $X$, and let $\lambda:\mathcal{K}\to[0,+\infty]$ be such that $\lambda(\emptyset)=0$. For each $A\subseteq X$, let $$\mu^{*}(A):=\inf\left\{ \sum_{k=1}^{\infty}\lambda(E_{k}):(E_{k})_{k\ge 1}\subseteq K\text{ s.t. }A\subseteq \bigcup_{k=1}^{\infty}E_{k} \right\}$$then $\mu^{*}$ is an outer measure on $X$. # Theorem (437) For any interval $I=|a,b|\subset \mathbb{R}$ let $\mathscr{l}(I)=b-a$ denote its length. For any subset $E\subset \mathbb{R}$ we define the Lebesgue Outer Measure as: $$\lambda^{*}(E)=\inf\left\{ \sum_{k=0}^{\infty}\lambda(I_{k}):(I_{k})_{k\in\mathbb{N}}\in R_{I}\ \& \ E \subset \bigcup_{k=0}^\infty I_{k} \right\}$$$\forall A\in\mathcal{P}(X)$.