In topology, a branch of mathematics, a point x of a set S is called an isolated point of S if there exists a neighborhood of x not containing other points of S.
In particular, in a Euclidean space (or in a metric space), x is an isolated point of S, if one can find an open ball around x which contains no other points of S.
Equivalently, a point x in S is an isolated point of S if and only if it is not a limit point of S.
A set which is made up only of isolated points is called a discrete set. Any discrete subset of Euclidean space is countable, since the isolation of each of its points (together with the fact the rationals are dense in the reals) means that it may be mapped 1-1 to a set of points with rational co-ordinates, of which there are only countably many. However, a set can be countable but not discrete, e.g. the rational numbers with the absolute difference metric. See also discrete space.
A set with no isolated point is said to be dense-in-itself. A closed set with no isolated point is called a perfect set.
The number of isolated points is a topological invariant, i.e. if two topological spaces and are homeomorphic, the number of isolated points in each is equal.
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Famous quotes containing the words isolated and/or point:
“There is a city myth that country life was isolated and lonely; the truth is that farmers and their families then had a richer social life than they have now. They enjoyed a society organic, satisfying and whole, not mixed and thinned with the life of town, city and nation as it now is.”
—Rose Wilder Lane (1886–1965)
“It used to be said that, socially speaking, Philadelphia asked who a person is, New York how much is he worth, and Boston what does he know. Nationally it has now become generally recognized that Boston Society has long cared even more than Philadelphia about the first point and has refined the asking of who a person is to the point of demanding to know who he was. Philadelphia asks about a man’s parents; Boston wants to know about his grandparents.”
—Cleveland Amory (b. 1917)