[0KM] Let \({\mathcal B}\) be a base for a topology \(π\) on \(X\); chosen an open set \(Aβ π\), for every \(xβ A\) we can choose a \(B_ xβ {\mathcal B}\) with \(xβ B_ x\), and such that \(A=β_{xβ A}B_ x \).
[0KN]βΊβ»
