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}