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## 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 other open set.