Definition 4.2.2 (Filter Base). Let $X$ be a set, $\fF \subset 2^{X}$ be a filter, and $\fB \subset \fF$, then $\fB$ is a filter base for $\fF$ if for every $F \in \fF$, there exists $E \in \fB$ such that $E \subset F$.
Definition 4.2.2 (Filter Base). Let $X$ be a set, $\fF \subset 2^{X}$ be a filter, and $\fB \subset \fF$, then $\fB$ is a filter base for $\fF$ if for every $F \in \fF$, there exists $E \in \fB$ such that $E \subset F$.