## Base for a topology

### Set

 context $\langle X,T\rangle$ … topological space definiendum $B\in$ it inclusion $B\subseteq T$ for all $U\in T$ exists $C\subseteq B$ postulate $U=\bigcup C$

### Discussion

A base $B$ for a topology is a collection its open sets, which suffice to cover any open set.