Closed G-delta set as a zero set
= Closed G-delta set as a zero set
A closed subset $A$ of a normal space is a countable intersection of open sets exactly when it is the zero set of some continuous function $f:X\to[0,1]$.
= Closed G-delta set as a zero set
A closed subset $A$ of a normal space is a countable intersection of open sets exactly when it is the zero set of some continuous function $f:X\to[0,1]$.