Definition 14.5.2 (Outer Regular). Let $X$ be a topological space, $\mu: \cb_{X} \to [0, \infty]$ be a Borel measure, and $E \in \cb_{X}$, then $\mu$ is outer regular on $E$ if
\[\mu(E) = \sup\bracs{\mu(U)| U \in \cn^o(A)}\]
Definition 14.5.2 (Outer Regular). Let $X$ be a topological space, $\mu: \cb_{X} \to [0, \infty]$ be a Borel measure, and $E \in \cb_{X}$, then $\mu$ is outer regular on $E$ if