 G-delta set (source code)

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= G-delta set
{c}
{title2=$G_\delta=\bigcap_{n\ge1}U_n\quad(U_n\text{ open})$}

A G-delta set is a countable intersection of open subsets of a topological space. In a complete <metric> space these are precisely the <topologically complete> subspaces, by the <G-delta criterion for topological completeness>.

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