Definition:Cantor-Bendixson Rank

Definition
Let $\struct {X, \tau}$ be a topological space.

Let $S \subseteq X$.

For each ordinal $\alpha$, let $S^{\paren \alpha}$ be the $\alpha$th Cantor-Bendixson derivative of $S$.

Then the Cantor-Bendixson rank of $S$ is the least ordinal $\alpha$ such that:
 * $S^{\paren {\alpha^+} } = S^{\paren \alpha}$

if such an ordinal exists.