Definition 14.3.1 (Semifinite). Let $(X, \cm, \mu)$ be a measure space, then the following are equivalent:
For any $E \in \cm$ with $\mu(E) = \infty$, there exists $F \subset E$ with $0 < \mu(F) < \infty$.
For any $E \in \cm$,
\[\mu(E) = \sup\bracs{\mu(F)| F \in \cm, F \subset E, \mu(F) < \infty}\]
If the above holds, then $\mu$ is a semifinite measure.
Proof. $(1) \Rightarrow (2)$: If $\mu(E) < \infty$, then the result holds directly. Assume that $\mu(E) = \infty$, and suppose for contradiction that
Let $\seq{F_n}\subset \cm$ such that $F \subset E$, $\mu(F) < \infty$, and $\mu(F_{n}) \upto M$. Let $F = \bigcup_{n \in \nat}F_{n}$, then $\mu(F) = M$, and $\mu(E \setminus F) = \infty$. By (1), there exists $G \in \cm$ with $G \subset E \setminus F$ and $0 < \mu(G) < \infty$. Thus $F \cup G \subset E$ with $M < \mu(F) + \mu(G) = \mu(F \cup G) < \infty$. This contradicts the fact that $M$ is the supremum.$\square$