Let $T = (X, \mathcal{T})$ be a D1106: Topological space such that

(i)	$\mathcal{T}^{\text{op}}$ is the D2439: Set of closed sets in $T$

A D11: Set $E \subseteq X$ is an F-sigma set in $T$ if and only if \begin{equation} \exists \, F_0, F_1, F_2, \dots \in \mathcal{T}^{\text{op}} : E = \bigcup_{n \in \mathbb{N}} F_n \end{equation}

Let $T = (X, \mathcal{T})$ be a D1106: Topological space.
A D11: Set $E \subseteq X$ is an F-sigma set in $T$ if and only if \begin{equation} \exists \, (X \setminus F_0), (X \setminus F_1), (X \setminus F_2), \dots \in \mathcal{T} : E = \bigcup_{n \in \mathbb{N}} F_n \end{equation}