Definition 5.17.1 ($\sigma$-Compact).label Let $X$ be a topological space, then $X$ is $\sigma$-compact if there exits $\seq{K_n}\subset 2^{X}$ compact such that $X = \bigcup_{n \in \natp}K_{n}$.
Definition 5.17.1 ($\sigma$-Compact).label Let $X$ be a topological space, then $X$ is $\sigma$-compact if there exits $\seq{K_n}\subset 2^{X}$ compact such that $X = \bigcup_{n \in \natp}K_{n}$.