Let $X$ be a D11: Set.
A D11: Set $E \subseteq X$ is cofinite within $X$ if and only if $X \setminus E$ is a D17: Finite set.

Let $X$ be a D11: Set.
A D11: Set $E \subseteq X$ is cofinite within $X$ if and only if \begin{equation} |X \setminus E| < \infty \end{equation}