derived setDerivedSet2003-04-23 17:10:552007-08-09 01:27:11Definition% this is the default PlanetMath preamble. as your knowledge
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% there are many more packages, add them here as you need them
% define commands here{\bf Definition.}
Let $X$ be a topological space and let $A$ be a subset of $X$.
The \emph{derived set} $A'$ of $A$ is the set of all limit points
of $A$ in $X$ \cite{kelley}.
\begin{thebibliography}{9}
\bibitem{kelley}
J.L. Kelley,
\emph{General Topology},
D. van Nostrand Company, Inc., 1955.
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