In topology and related areas of mathematics, a **product space** is the Cartesian product of a family of topological spaces equipped with a natural topology called the **product topology**. This topology differs from another, perhaps more obvious, topology called the box topology, which can also be given to a product space and which agrees with the product topology when the product is over only finitely many spaces. However, the product topology is "correct" in that it makes the product space a categorical product of its factors, whereas the box topology is too fine; in that sense the product topology is the natural topology on the Cartesian product.

Throughout, will be some non-empty index set and for every index will be a topological space. Let

be the Cartesian product of the sets and denote the ** canonical projections ** by The

The open sets in the product topology are unions (finite or infinite) of sets of the form where each is open in and for only finitely many In particular, for a finite product (in particular, for the product of two topological spaces), the set of all Cartesian products between one basis element from each gives a basis for the product topology of That is, for a finite product, the set of all where is an element of the (chosen) basis of is a basis for the product topology of

The product topology on is the topology generated by sets of the form where and is an open subset of In other words, the sets

form a subbase for the topology on A subset of is open if and only if it is a (possibly infinite) union of intersections of finitely many sets of the form The are sometimes called open cylinders, and their intersections are cylinder sets.

The product of the topologies of each forms a basis for what is called the box topology on In general, the box topology is finer than the product topology, but for finite products they coincide.

If the real line is endowed with its standard topology then the product topology on the product of copies of equal to ordinary Euclidean topology on

The Cantor set is homeomorphic to the product of countably many copies of the discrete space and the space of irrational numbers is homeomorphic to the product of countably many copies of the natural numbers, where again each copy carries the discrete topology.

Several additional examples are given in the article on the initial topology.

The product space together with the canonical projections, can be characterized by the following universal property: If is a topological space, and for every is a continuous map, then there exists *precisely one* continuous map such that for each the following diagram commutes:

This shows that the product space is a product in the category of topological spaces. It follows from the above universal property that a map is continuous if and only if is continuous for all In many cases it is easier to check that the component functions are continuous. Checking whether a map is continuous is usually more difficult; one tries to use the fact that the are continuous in some way.

In addition to being continuous, the canonical projections are open maps. This means that any open subset of the product space remains open when projected down to the The converse is not true: if is a subspace of the product space whose projections down to all the are open, then need not be open in (consider for instance ) The canonical projections are not generally closed maps (consider for example the closed set whose projections onto both axes are ).

Suppose is a product of arbitrary subsets, where for every If all are *non-empty* then is a closed subset of the product space if and only if every is a closed subset of More generally, the closure of the product of arbitrary subsets in the product space is equal to the product of the closures:^{ [1] }

Any product of Hausdorff spaces is again a Hausdorff space.

Tychonoff's theorem, which is equivalent to the axiom of choice, states any product of compact spaces is a compact space. A specialization of Tychonoff's theorem that requires only the ultrafilter lemma (and not the full strength of the axiom of choice) states that that any product of compact Hausdorff spaces is a compact space.

If is fixed then the set

is a dense subset of the product space .^{ [1] }

- Separation

- Every product of T
_{0}spaces is T_{0} - Every product of T
_{1}spaces is T_{1} - Every product of Hausdorff spaces is Hausdorff
^{ [2] } - Every product of regular spaces is regular
- Every product of Tychonoff spaces is Tychonoff
- A product of normal spaces
*need not*be normal

- Compactness

- Every product of compact spaces is compact (Tychonoff's theorem)
- A product of locally compact spaces
*need not*be locally compact. However, an arbitrary product of locally compact spaces where all but finitely many are compact*is*locally compact (This condition is sufficient and necessary).

- Connectedness

- Every product of connected (resp. path-connected) spaces is connected (resp. path-connected)
- Every product of hereditarily disconnected spaces is hereditarily disconnected.

- Metric spaces

- Countable products of metric spaces are metrizable space

One of many ways to express the axiom of choice is to say that it is equivalent to the statement that the Cartesian product of a collection of non-empty sets is non-empty.^{ [3] } The proof that this is equivalent to the statement of the axiom in terms of choice functions is immediate: one needs only to pick an element from each set to find a representative in the product. Conversely, a representative of the product is a set which contains exactly one element from each component.

The axiom of choice occurs again in the study of (topological) product spaces; for example, Tychonoff's theorem on compact sets is a more complex and subtle example of a statement that is equivalent to the axiom of choice,^{ [4] } and shows why the product topology may be considered the more useful topology to put on a Cartesian product.

- Disjoint union (topology)
- Final topology – finest topology making some functions continuous
- Initial topology – coarsest topology making certain functions continuous - Sometimes called the projective limit topology
- Inverse limit
- Pointwise convergence – notion of convergence in mathematics
- Quotient space (topology)
- Subspace (topology)
- Weak topology – Topology where convergence of points is defined by the convergence of their image under continuous linear functionals

- 1 2 Bourbaki 1989, pp. 43-50.
- ↑ "Product topology preserves the Hausdorff property".
*PlanetMath*. - ↑ Pervin, William J. (1964),
*Foundations of General Topology*, Academic Press, p. 33 - ↑ Hocking, John G.; Young, Gail S. (1988) [1961],
*Topology*, Dover, p. 28, ISBN 978-0-486-65676-2

In mathematics, one can often define a **direct product** of objects already known, giving a new one. This generalizes the Cartesian product of the underlying sets, together with a suitably defined structure on the product set. More abstractly, one talks about the product in category theory, which formalizes these notions.

In topology and related branches of mathematics, a **Hausdorff space**, **separated space** or **T _{2} space** is a topological space where for any two distinct points there exist neighbourhoods of each which are disjoint from each other. Of the many separation axioms that can be imposed on a topological space, the "Hausdorff condition" (T

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.

In topology and related branches of mathematics, a **topological space** may be defined as a set of points, along with a set of neighbourhoods for each point, satisfying a set of axioms relating points and neighbourhoods. The definition of a topological space relies only upon set theory and is the most general notion of a mathematical space that allows for the definition of concepts such as continuity, connectedness, and convergence. Other spaces, such as manifolds and metric spaces, are specializations of topological spaces with extra structures or constraints. Being so general, topological spaces are a central unifying notion and appear in virtually every branch of modern mathematics. The branch of mathematics that studies topological spaces in their own right is called point-set topology or general topology.

In topology and related branches of mathematics, **Tychonoff spaces** and **completely regular spaces** are kinds of topological spaces. These conditions are examples of separation axioms.

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 the mathematical field of topology, a **uniform space** is a set with a **uniform structure**. Uniform spaces are topological spaces with additional structure that is used to define uniform properties such as completeness, uniform continuity and uniform convergence. Uniform spaces generalize metric spaces and topological groups, but the concept is designed to formulate the weakest axioms needed for most proofs in analysis.

In mathematics, a **topological group** is a group G together with a topology on G such that both the group's binary operation and the function mapping group elements to their respective inverses are continuous functions with respect to the topology. A topological group is a mathematical object with both an algebraic structure and a topological structure. Thus, one may perform algebraic operations, because of the group structure, and one may talk about continuous functions, because of the topology.

In topology and related branches of mathematics, a **normal space** is a topological space *X* that satisfies **Axiom T _{4}**: every two disjoint closed sets of

In the mathematical discipline of general topology, **Stone–Čech compactification** is a technique for constructing a universal map from a topological space *X* to a compact Hausdorff space *βX*. The Stone–Čech compactification *βX* of a topological space *X* is the largest, most general compact Hausdorff space "generated" by *X*, in the sense that any continuous map from *X* to a compact Hausdorff space factors through *βX*. If *X* is a Tychonoff space then the map from *X* to its image in *βX* is a homeomorphism, so *X* can be thought of as a (dense) subspace of *βX*; every other compact Hausdorff space that densely contains *X* is a quotient of *βX*. For general topological spaces *X*, the map from *X* to *βX* need not be injective.

In the mathematical field of topology, the **Alexandroff extension** is a way to extend a noncompact topological space by adjoining a single point in such a way that the resulting space is compact. It is named after the Russian mathematician Pavel Alexandroff. More precisely, let *X* be a topological space. Then the Alexandroff extension of *X* is a certain compact space *X** together with an open embedding *c* : *X* → *X** such that the complement of *X* in *X** consists of a single point, typically denoted ∞. The map *c* is a Hausdorff compactification if and only if *X* is a locally compact, noncompact Hausdorff space. For such spaces the Alexandroff extension is called the **one-point compactification** or **Alexandroff compactification**. The advantages of the Alexandroff compactification lie in its simple, often geometrically meaningful structure and the fact that it is in a precise sense minimal among all compactifications; the disadvantage lies in the fact that it only gives a Hausdorff compactification on the class of locally compact, noncompact Hausdorff spaces, unlike the Stone–Čech compactification which exists for any topological space, a much larger class of spaces.

In mathematics, **Tychonoff's theorem** states that the product of any collection of compact topological spaces is compact with respect to the product topology. The theorem is named after Andrey Nikolayevich Tikhonov, who proved it first in 1930 for powers of the closed unit interval and in 1935 stated the full theorem along with the remark that its proof was the same as for the special case. The earliest known published proof is contained in a 1937 paper of Eduard Čech.

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 topology, a **subbase** for a topological space X with topology T is a subcollection B of T that generates T, in the sense that T is the smallest topology containing B. A slightly different definition is used by some authors, and there are other useful equivalent formulations of the definition; these are discussed below.

In mathematics, a **Radon measure**, named after Johann Radon, is a measure on the σ-algebra of Borel sets of a Hausdorff topological space *X* that is finite on all compact sets, outer regular on all Borel sets, and inner regular on open sets. These conditions guarantee that the measure is "compatible" with the topology of the space, and most measures used in mathematical analysis and in number theory are indeed Radon measures.

In functional analysis and related branches of mathematics, the **Banach–Alaoglu theorem** states that the closed unit ball of the dual space of a normed vector space is compact in the weak* topology. A common proof identifies the unit ball with the weak-* topology as a closed subset of a product of compact sets with the product topology. As a consequence of Tychonoff's theorem, this product, and hence the unit ball within, is compact.

In mathematics, the **compact-open topology** is a topology defined on the set of continuous maps between two topological spaces. The compact-open topology is one of the commonly used topologies on function spaces, and is applied in homotopy theory and functional analysis. It was introduced by Ralph Fox in 1945.

In topology, a **compactly generated space** is a topological space whose topology is coherent with the family of all compact subspaces. Specifically, a topological space *X* is compactly generated if it satisfies the following condition:

In topology, the cartesian product of topological spaces can be given several different topologies. One of the more obvious choices is the **box topology**, where a base is given by the Cartesian products of open sets in the component spaces. Another possibility is the product topology, where a base is given by the Cartesian products of open sets in the component spaces, only finitely many of which can be not equal to the entire component space.

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.

- Bourbaki, Nicolas (1989) [1966].
*General Topology: Chapters 1–4*[*Topologie Générale*]. Éléments de mathématique. Berlin New York: Springer Science & Business Media. ISBN 978-3-540-64241-1. OCLC 18588129. - Willard, Stephen (1970).
*General Topology*. Reading, Mass.: Addison-Wesley Pub. Co. ISBN 0486434796 . Retrieved 13 February 2013.

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