1. Definition

1.1. Accumulation Points of a Set

Let [math]\displaystyle{ S }[/math] be a subset of a topological space [math]\displaystyle{ X. }[/math] A point [math]\displaystyle{ x }[/math] in [math]\displaystyle{ X }[/math] is a limit point or cluster point or accumulation point of the set [math]\displaystyle{ S }[/math] if every neighbourhood of [math]\displaystyle{ x }[/math] contains at least one point of [math]\displaystyle{ S }[/math] different from [math]\displaystyle{ x }[/math] itself.

It does not make a difference if we restrict the condition to open neighbourhoods only. It is often convenient to use the "open neighbourhood" form of the definition to show that a point is a limit point and to use the "general neighbourhood" form of the definition to derive facts from a known limit point.

If [math]\displaystyle{ X }[/math] is a [math]\displaystyle{ T 1 }[/math] space (such as a metric space), then [math]\displaystyle{ x \in X }[/math] is a limit point of [math]\displaystyle{ S }[/math] if and only if every neighbourhood of [math]\displaystyle{ x }[/math] contains infinitely many points of [math]\displaystyle{ S. }[/math]^[1] In fact, [math]\displaystyle{ T_1 }[/math] spaces are characterized by this property.

If [math]\displaystyle{ X }[/math] is a Fréchet–Urysohn space (which all metric spaces and first-countable spaces are), then [math]\displaystyle{ x \in X }[/math] is a limit point of [math]\displaystyle{ S }[/math] if and only if there is a sequence of points in [math]\displaystyle{ S \setminus \{ x \} }[/math] whose limit is [math]\displaystyle{ x. }[/math] In fact, Fréchet–Urysohn spaces are characterized by this property.

The set of limit points of [math]\displaystyle{ S }[/math] is called the derived set of [math]\displaystyle{ S. }[/math]

Types of accumulation points

If every neighbourhood of [math]\displaystyle{ x }[/math] contains infinitely many points of [math]\displaystyle{ S, }[/math] then [math]\displaystyle{ x }[/math] is a specific type of limit point called an ω-accumulation point of [math]\displaystyle{ S. }[/math]

If every neighbourhood of [math]\displaystyle{ x }[/math] contains uncountably many points of [math]\displaystyle{ S, }[/math] then [math]\displaystyle{ x }[/math] is a specific type of limit point called a condensation point of [math]\displaystyle{ S. }[/math]

If every neighbourhood [math]\displaystyle{ U }[/math] of [math]\displaystyle{ x }[/math] satisfies [math]\displaystyle{ \left| U \cap S\right| = \left| S \right|, }[/math] then [math]\displaystyle{ x }[/math] is a specific type of limit point called a complete accumulation point of [math]\displaystyle{ S. }[/math]

1.2. Accumulation Points of Sequences and Nets

A sequence enumerating all positive rational numbers. Each positive real number is a cluster point.

In a topological space [math]\displaystyle{ X, }[/math] a point [math]\displaystyle{ x \in X }[/math] is said to be a cluster point or accumulation point of a sequence [math]\displaystyle{ x_{\bull} = \left(x_n\right)_{n=1}^{\infty} }[/math] if, for every neighbourhood [math]\displaystyle{ V }[/math] of [math]\displaystyle{ x, }[/math] there are infinitely many [math]\displaystyle{ n \in \mathbb{N} }[/math] such that [math]\displaystyle{ x_n \in V. }[/math] It is equivalent to say that for every neighbourhood [math]\displaystyle{ V }[/math] of [math]\displaystyle{ x }[/math] and every [math]\displaystyle{ n_0 \in \mathbb{N}, }[/math] there is some [math]\displaystyle{ n \geq n_0 }[/math] such that [math]\displaystyle{ x_n \in V. }[/math] If [math]\displaystyle{ X }[/math] is a metric space or a first-countable space (or, more generally, a Fréchet–Urysohn space), then [math]\displaystyle{ x }[/math] is a cluster point of [math]\displaystyle{ x_{\bull} }[/math] if and only if [math]\displaystyle{ x }[/math] is a limit of some subsequence of [math]\displaystyle{ x_{\bull}. }[/math] The set of all cluster points of a sequence is sometimes called the limit set.

Note that there is already the notion of limit of a sequence to mean a point [math]\displaystyle{ x }[/math] to which the sequence converges (that is, every neighborhood of [math]\displaystyle{ x }[/math] contains all but finitely many elements of the sequence). That is why we do not use the term limit point of a sequence as a synonym for accumulation point of the sequence.

The concept of a net generalizes the idea of a sequence. A net is a function [math]\displaystyle{ f : (P,\leq) \to X, }[/math] where [math]\displaystyle{ (P,\leq) }[/math] is a directed set and [math]\displaystyle{ X }[/math] is a topological space. A point [math]\displaystyle{ x \in X }[/math] is said to be a cluster point or accumulation point of a net [math]\displaystyle{ f }[/math] if, for every neighbourhood [math]\displaystyle{ V }[/math] of [math]\displaystyle{ x }[/math] and every [math]\displaystyle{ p_0 \in P, }[/math] there is some [math]\displaystyle{ p \geq p_0 }[/math] such that [math]\displaystyle{ f(p) \in V, }[/math] equivalently, if [math]\displaystyle{ f }[/math] has a subnet which converges to [math]\displaystyle{ x. }[/math] Cluster points in nets encompass the idea of both condensation points and ω-accumulation points. Clustering and limit points are also defined for filters.

2. Relation Between Accumulation Point of a Sequence and Accumulation Point of a Set

Every sequence [math]\displaystyle{ x_{\bull} = \left(x_n\right)_{n=1}^{\infty} }[/math] in [math]\displaystyle{ X }[/math] is by definition just a map [math]\displaystyle{ x_{\bull} : \mathbb{N} \to X }[/math] so that its image [math]\displaystyle{ \operatorname{Im} x_{\bull} := \left\{ x_n : n \in \mathbb{N} \right\} }[/math] can be defined in the usual way.

• If there exists an element [math]\displaystyle{ x \in X }[/math] that occurs infinitely many times in the sequence, [math]\displaystyle{ x }[/math] is an accumulation point of the sequence. But [math]\displaystyle{ x }[/math] need not be an accumulation point of the corresponding set [math]\displaystyle{ \operatorname{Im} x_{\bull}. }[/math] For example, if the sequence is the constant sequence with value [math]\displaystyle{ x, }[/math] we have [math]\displaystyle{ \operatorname{Im} x_{\bull} = \{ x \} }[/math] and [math]\displaystyle{ x }[/math] is an isolated point of [math]\displaystyle{ \operatorname{Im} x_{\bull} }[/math] and not an accumulation point of [math]\displaystyle{ \operatorname{Im} x_{\bull}. }[/math]

• If no element occurs infinitely many times in the sequence, for example if all the elements are distinct, any accumulation point of the sequence is an [math]\displaystyle{ \omega }[/math]-accumulation point of the associated set [math]\displaystyle{ \operatorname{Im} x_{\bull}. }[/math]

Conversely, given a countable infinite set [math]\displaystyle{ A \subseteq X }[/math] in [math]\displaystyle{ X, }[/math] we can enumerate all the elements of [math]\displaystyle{ A }[/math] in many ways, even with repeats, and thus associate with it many sequences [math]\displaystyle{ x_{\bull} }[/math] that will satisfy [math]\displaystyle{ A = \operatorname{Im} x_{\bull}. }[/math]

• Any [math]\displaystyle{ \omega }[/math]-accumulation point of [math]\displaystyle{ A }[/math] is an accumulation point of any of the corresponding sequences (because any neighborhood of the point will contain infinitely many elements of [math]\displaystyle{ A }[/math] and hence also infinitely many terms in any associated sequence).

• A point [math]\displaystyle{ x \in X }[/math] that is not an [math]\displaystyle{ \omega }[/math]-accumulation point of [math]\displaystyle{ A }[/math] cannot be an accumulation point of any of the associated sequences without infinite repeats (because [math]\displaystyle{ x }[/math] has a neighborhood that contains only finitely many (possibly even none) points of [math]\displaystyle{ A }[/math] and that neighborhood can only contain finitely many terms of such sequences).

3. Properties

Every limit of a non-constant sequence is an accumulation point of the sequence. And by definition, every limit point is an adherent point.

The closure [math]\displaystyle{ \operatorname{cl}(S) }[/math] of a set [math]\displaystyle{ S }[/math] is a disjoint union of its limit points [math]\displaystyle{ L(S) }[/math] and isolated points [math]\displaystyle{ I(S) }[/math]: [math]\displaystyle{ \operatorname{cl} (S) = L(S) \cup I(S), L(S) \cap I(S) = \varnothing. }[/math]

A point [math]\displaystyle{ x \in X }[/math] is a limit point of [math]\displaystyle{ S \subseteq X }[/math] if and only if it is in the closure of [math]\displaystyle{ S \setminus \{ x \}. }[/math]