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A sequence enumerating all positive rational numbers.Each positive real number is a cluster point.. Let be a subset of a topological space. A point in is a limit point or cluster point or accumulation point of the set if every neighbourhood of contains at least one point of different from itself.
This definition allows a limit to be defined at limit points of the domain S, if a suitable subset T which has the same limit point is chosen. Notably, the previous two-sided definition works on int S ∪ iso S c , {\displaystyle \operatorname {int} S\cup \operatorname {iso} S^{c},} which is a subset of the limit points of S .
The concept of a limit of a sequence is further generalized to the concept of a limit of a topological net, and is closely related to limit and direct limit in category theory. The limit inferior and limit superior provide generalizations of the concept of a limit which are particularly relevant when the limit at a point may not exist.
The definition of a point of closure of a set is closely related to the definition of a limit point of a set.The difference between the two definitions is subtle but important – namely, in the definition of a limit point of a set , every neighbourhood of must contain a point of other than itself, i.e., each neighbourhood of obviously has but it also must have a point of that is not equal to ...
Therefore, if the limit set exists it contains the points and only the points which are in all except finitely many of the sets of the sequence. Since convergence in the discrete metric is the strictest form of convergence (i.e., requires the most), this definition of a limit set is the strictest possible.
A limit of a sequence of points () in a topological space is a special case of a limit of a function: the domain is in the space {+}, with the induced topology of the affinely extended real number system, the range is , and the function argument tends to +, which in this space is a limit point of .
A deleted neighbourhood of a given point is not in fact a neighbourhood of the point. The concept of deleted neighbourhood occurs in the definition of the limit of a function and in the definition of limit points (among other things). [4]
A closed set with no isolated point is called a perfect set (it contains all its limit points and no isolated points). The number of isolated points is a topological invariant, i.e. if two topological spaces X, Y are homeomorphic, the number of isolated points in each is equal.