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G-delta set
Formulation 0
Let $T = (X, \mathcal{T})$ be a D1106: Topological space.
A D11: Set $E \subseteq X$ is a G-delta set in $T$ if and only if \begin{equation} \exists \, U_0, U_1, U_2, \dots \in \mathcal{T} : E = \bigcap_{n \in \mathbb{N}} U_n \end{equation}
Dual definition
» F-sigma set
Results
» R4187: Complement of an F-sigma set is a G-delta set