Perfect set

In mathematics, in the field of topology, a subset of a topological space is perfect if it is closed and has no isolated points. Equivalently: the set ${\displaystyle S}$ is perfect if ${\displaystyle S=S'}$, where ${\displaystyle S'}$ denotes the set of all limit points of ${\displaystyle S}$, also known as the derived set of ${\displaystyle S}$.

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