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

**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 such that

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}

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