of a subspace $A$ of a topological space $X$
A point $a\in A$ such that the intersection of some neighbourhood of $a$ with $A$ consists of the point $a$ alone.
A subset $A$ with no isolated points is dense-in-itself; a closed dense-in-itself subset is a perfect set.
|||Steen, Lynn Arthur; Seebach, J. Arthur Jr. (1978). Counterexamples in Topology (second edition). Berlin, New York: Springer-Verlag. ISBN 978-0-486-68735-3. MR 507446. Zbl 0386.54001.|
Isolated point. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Isolated_point&oldid=33534