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In topology and related areas of mathematics, a subset *A* of a topological space *X* is called **dense** (in *X*) if every point *x* in *X* either belongs to *A* or is a limit point of *A*; that is, the closure of *A* constitutes the whole set *X*.^{ [1] } Informally, for every point in *X*, the point is either in *A* or arbitrarily "close" to a member of *A*— for instance, the rational numbers are a dense subset of the real numbers because every real number either is a rational number or has a rational number arbitrarily close to it (see Diophantine approximation).

- Density in metric spaces
- Examples
- Properties
- Related notions
- See also
- References
- Notes
- General references

Formally, a subset *A* of a topological space *X* is dense in X if for any point *x* in *X*, any neighborhood of *x* contains at least one point from *A* (i.e., *A* has non-empty intersection with every non-empty open subset of *X*). Equivalently, *A* is dense in *X* if and only if the smallest closed subset of *X* containing *A* is *X* itself. This can also be expressed by saying that the closure of *A* is *X*, or that the interior of the complement of *A* is empty.

The **density** of a topological space *X* is the least cardinality of a dense subset of *X*.

An alternative definition of dense set in the case of metric spaces is the following. When the topology of *X* is given by a metric, the closure of *A* in *X* is the union of *A* and the set of all limits of sequences of elements in *A* (its *limit points*),

Then *A* is dense in *X* if

If is a sequence of dense open sets in a complete metric space, *X*, then is also dense in *X*. This fact is one of the equivalent forms of the Baire category theorem.

The real numbers with the usual topology have the rational numbers as a countable dense subset which shows that the cardinality of a dense subset of a topological space may be strictly smaller than the cardinality of the space itself. The irrational numbers are another dense subset which shows that a topological space may have several disjoint dense subsets (in particular, two dense subsets may be each other's complements), and they need not even be of the same cardinality. Perhaps even more surprisingly, both the rationals and the irrationals have empty interiors, showing that dense sets need not contain any non-empty open set. The intersection of two dense open subsets of a topological space is again dense and open.

By the Weierstrass approximation theorem, any given complex-valued continuous function defined on a closed interval [*a*, *b*] can be uniformly approximated as closely as desired by a polynomial function. In other words, the polynomial functions are dense in the space C[*a*, *b*] of continuous complex-valued functions on the interval [*a*, *b*], equipped with the supremum norm.

Every metric space is dense in its completion.

Every topological space is a dense subset of itself. For a set *X* equipped with the discrete topology, the whole space is the only dense subset. Every non-empty subset of a set *X* equipped with the trivial topology is dense, and every topology for which every non-empty subset is dense must be trivial.

Denseness is transitive: Given three subsets *A*, *B* and *C* of a topological space *X* with *A* ⊆ *B* ⊆ *C* ⊆ *X* such that *A* is dense in *B* and *B* is dense in *C* (in the respective subspace topology) then *A* is also dense in *C*.

The image of a dense subset under a surjective continuous function is again dense. The density of a topological space (the least of the cardinalities of its dense subsets) is a topological invariant.

A topological space with a connected dense subset is necessarily connected itself.

Continuous functions into Hausdorff spaces are determined by their values on dense subsets: if two continuous functions *f*, *g* : *X* → *Y* into a Hausdorff space *Y* agree on a dense subset of *X* then they agree on all of *X*.

For metric spaces there are universal spaces, into which all spaces of given density can be embedded: a metric space of density α is isometric to a subspace of C([0, 1]^{α}, **R**), the space of real continuous functions on the product of α copies of the unit interval. ^{ [2] }

A point *x* of a subset *A* of a topological space *X* is called a limit point of *A* (in *X*) if every neighbourhood of *x* also contains a point of *A* other than *x* itself, and an isolated point of *A* otherwise. A subset without isolated points is said to be dense-in-itself.

A subset *A* of a topological space *X* is called nowhere dense (in *X*) if there is no neighborhood in *X* on which *A* is dense. Equivalently, a subset of a topological space is nowhere dense if and only if the interior of its closure is empty. The interior of the complement of a nowhere dense set is always dense. The complement of a closed nowhere dense set is a dense open set. Given a topological space *X*, a subset *A* of *X* that can be expressed as the union of countably many nowhere dense subsets of *X* is called meagre. The rational numbers, while dense in the real numbers, are meagre as a subset of the reals.

A topological space with a countable dense subset is called separable. A topological space is a Baire space if and only if the intersection of countably many dense open sets is always dense. A topological space is called resolvable if it is the union of two disjoint dense subsets. More generally, a topological space is called κ-resolvable for a cardinal κ if it contains κ pairwise disjoint dense sets.

An embedding of a topological space *X* as a dense subset of a compact space is called a compactification of *X*.

A linear operator between topological vector spaces *X* and *Y* is said to be densely defined if its domain is a dense subset of *X* and if its range is contained within *Y*. See also continuous linear extension.

A topological space *X* is hyperconnected if and only if every nonempty open set is dense in *X*. A topological space is submaximal if and only if every dense subset is open.

If is a metric space, then a non-empty subset Y is said to be *ε-dense* if

One can then show that *D* is dense in if and only if it is ε-dense for every

In mathematics, more specifically in general topology, **compactness** is a property that generalizes the notion of a subset of Euclidean space being closed and bounded. Examples include a closed interval, a rectangle, or a finite set of points. This notion is defined for more general topological spaces than Euclidean space in various ways.

In mathematics, a **continuous function** is a function that does not have any abrupt changes in value, known as discontinuities. More precisely, sufficiently small changes in the input of a continuous function result in arbitrarily small changes in its output. If not continuous, a function is said to be *discontinuous*. Up until the 19th century, mathematicians largely relied on intuitive notions of continuity, during which attempts such as the epsilon–delta definition were made to formalize it.

In mathematical analysis, a metric space M is called **complete** if every Cauchy sequence of points in M has a limit that is also in M or, alternatively, if every Cauchy sequence in M converges in M.

In mathematics, a **metric space** is a set together with a metric on the set. The metric is a function that defines a concept of *distance* between any two members of the set, which are usually called points. The metric satisfies a few simple properties. Informally:

In mathematics, a topological space is called **separable** if it contains a countable, dense subset; that is, there exists a sequence of elements of the space such that every nonempty open subset of the space contains at least one element of the sequence.

This is a glossary of some terms used in the branch of mathematics known as topology. Although there is no absolute distinction between different areas of topology, the focus here is on general topology. The following definitions are also fundamental to algebraic topology, differential topology and geometric topology.

In mathematics, particularly in topology, an **open set** is an abstract concept generalizing the idea of an open interval in the real line. The simplest example is in metric spaces, where open sets can be defined as those sets which contain a ball around each of their points ; however, an open set, in general, can be very abstract: any collection of sets can be called open, as long as the union of an arbitrary number of open sets in the collection is open, the intersection of a finite number of open sets is open, and the space itself is open. These conditions are very loose, and they allow enormous flexibility in the choice of open sets. In the two extremes, every set can be open, or no set can be open but the space itself and the empty set.

In mathematics, the **real line**, or **real number line** is the line whose points are the real numbers. That is, the real line is the set **R** of all real numbers, viewed as a geometric space, namely the Euclidean space of dimension one. It can be thought of as a vector space, a metric space, a topological space, a measure space, or a linear continuum.

The **Baire category theorem** (BCT) is an important result in general topology and functional analysis. The theorem has two forms, each of which gives sufficient conditions for a topological space to be a Baire space.

In topology, a **discrete space** is a particularly simple example of a topological space or similar structure, one in which the points form a *discontinuous sequence*, meaning they are *isolated* from each other in a certain sense. The discrete topology is the finest topology that can be given on a set, i.e., it defines all subsets as open sets. In particular, each singleton is an open set in the discrete topology.

In mathematics, **general topology** is the branch of topology that deals with the basic set-theoretic definitions and constructions used in topology. It is the foundation of most other branches of topology, including differential topology, geometric topology, and algebraic topology. Another name for general topology is **point-set topology**.

In mathematics, a **Baire space** is a topological space such that every intersection of a countable collection of open dense sets in the space is also dense. Complete metric spaces and locally compact Hausdorff spaces are examples of Baire spaces according to the Baire category theorem. The spaces are named in honor of René-Louis Baire who introduced the concept.

In the mathematical field of topology, a **G _{δ} set** is a subset of a topological space that is a countable intersection of open sets. The notation originated in German with

In mathematical analysis, a family of functions is **equicontinuous** if all the functions are continuous and they have equal variation over a given neighbourhood, in a precise sense described herein. In particular, the concept applies to countable families, and thus *sequences* of functions.

In functional analysis and related areas of mathematics, **locally convex topological vector spaces** (**LCTVS**) or **locally convex spaces** are examples of topological vector spaces (TVS) that generalize normed spaces. They can be defined as topological vector spaces whose topology is generated by translations of balanced, absorbent, convex sets. Alternatively they can be defined as a vector space with a family of seminorms, and a topology can be defined in terms of that family. Although in general such spaces are not necessarily normable, the existence of a convex local base for the zero vector is strong enough for the Hahn–Banach theorem to hold, yielding a sufficiently rich theory of continuous linear functionals.

In topology, a **second-countable space**, also called a **completely separable space**, is a topological space whose topology has a countable base. More explicitly, a topological space is second-countable if there exists some countable collection of open subsets of such that any open subset of can be written as a union of elements of some subfamily of . A second-countable space is said to satisfy the **second axiom of countability**. Like other countability axioms, the property of being second-countable restricts the number of open sets that a space can have.

In topology and related areas of mathematics, a **topological property** or **topological invariant** is a property of a topological space which is invariant under homeomorphisms. That is, a property of spaces is a topological property if whenever a space *X* possesses that property every space homeomorphic to *X* possesses that property. Informally, a topological property is a property of the space that can be expressed using open sets.

In mathematics, **set-theoretic topology** is a subject that combines set theory and general topology. It focuses on topological questions that are independent of Zermelo–Fraenkel set theory (ZFC).

In general topology, a subset of a topological space is **perfect** if it is closed and has no isolated points. Equivalently: the set is perfect if , where denotes the set of all limit points of , also known as the derived set of .

In mathematics, properties that hold for "typical" examples are called **generic properties**. For instance, a generic property of a class of functions is one that is true of "almost all" of those functions, as in the statements, "A generic polynomial does not have a root at zero," or "A generic square matrix is invertible." As another example, a generic property of a space is a property that holds at "almost all" points of the space, as in the statement, "If *f* : *M* → *N* is a smooth function between smooth manifolds, then a generic point of *N* is not a critical value of *f*."

- ↑ Steen, L. A.; Seebach, J. A. (1995),
*Counterexamples in Topology*, Dover, ISBN 0-486-68735-X - ↑ Kleiber, Martin; Pervin, William J. (1969). "A generalized Banach-Mazur theorem".
*Bull. Austral. Math. Soc*.**1**(2): 169–173. doi: 10.1017/S0004972700041411 .

- Nicolas Bourbaki (1989) [1971].
*General Topology, Chapters 1–4*. Elements of Mathematics. Springer-Verlag. ISBN 3-540-64241-2. - Steen, Lynn Arthur; Seebach, J. Arthur Jr. (1995) [1978],
*Counterexamples in Topology*(Dover reprint of 1978 ed.), Berlin, New York: Springer-Verlag, ISBN 978-0-486-68735-3, MR 0507446

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