# Product topology

In topology, the cartesian product of topological spaces is turned into a topological space in the following way. Let*I*be a (possibly infinite) index set and suppose

*X*is a topological space for every

_{i}*i*in

*I*. Set

*X*= Π

*X*, the cartesian product of the sets

_{i}*X*. For every

_{i}*i*in

*I*, we have a

**canonical projection**

*p*:

_{i}*X*

`->`

*X*. The

_{i}**product topology**on

*X*is defined to be the coarsest topology (i.e. the topology with the fewest open sets) which turns all the maps

*p*into continuous maps.

_{i}
Explicitly, the topology on *X* can be described as follows. A subset of *X* is open if and only if it is a union of (possibly infinitely many) intersections of finitely many sets of the form *p _{i}*

^{-1}(

*O*), where

*i*in

*I*and

*O*is an open subset of

*X*. This implies that, in general, not all products of open sets need to be open in

_{i}*X*.

We can describe a basis for the product topology in a simple way using the bases of the constituting spaces *X _{i}*. Suppose that for each

*i*in

*I*we choose a set

*Y*which is either the whole space

_{i}*X*or a basis element in that space, and let

_{i}*B*be the product of the

*Y*. Then, as long as

_{i}*X*=

_{i}*Y*, that is, we choose the entire space, for all but finitely many

_{i}*i*in

*I*,

*B*will be a basis element of the product space, and a complete basis is generated in this way. In particular, this means that a finite product of spaces

*X*has a simple basis given by products of bases in the

*X*.

_{i}## Examples

If one starts with the standard topology on the real line **R** and defines a topology on the product of *n* copies of **R** in this fashion, one obtains the ordinary Euclidean topology on **R**^{n}.

The Cantor set is homeomorphic to the product of countably many copies of the discrete space {0,1} 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.

## Properties

The product topology is also called the *topology of pointwise convergence* because of the following fact: a sequence (or net) in \*X* converges if and only if all its projections to the spaces *X*_{i} converge. In particular, if one considers the space *X* = **R**^{I} of all real valued functionss on *I*, convergence in the product topology is the same as pointwise convergence of functions.

An important theorem about the product topology is Tychonoff's theorem: any product of compact spaces is compact. This is easy for finite products, but the statement is (surprisingly) also true for infinite products, when the proof requires the axiom of choice in some form.

The product space *X*, together with the canonical projections, can be characterized by the following universal property: If *Y* is a topological space, and for every *i* in *I*, *f _{i}* :

*Y*

`->`

*X*is a continuous map, then there exists

_{i}*precisely one*continuous map

*f*:

*Y*

`->`

*X*such that

*p*o

_{i}*f*=

*f*for all

_{i}*i*in

*I*. This shows that the product space is a product in the sense of category theory.

To check whether a given map *f* : *Y* `->` *X* is continuous, one can use the following handy criterion: *f* is continuous if and only if *p _{i}* o

*f*is continuous for all

*i*in

*I*. In other words, if we write

*f*as a tuple of its components,

*f*=(

*f*)

_{i}_{i in I}, then

*f*is continuous if and only if each of the

*f*is. Checking whether a map

_{i}*g*:

*X*

`->`

*Z*is continuous is usually more difficult; one tries to use the fact that the

*p*are continuous in some way.

_{i}## Relation to other topological notions

- Separation
- Every product of T
_{0}spacess is T_{0} - Every product of T
_{1}spacess is T_{1} - Every product of Hausdorff spaces is Hausdorff
- Every product of Regular spaces is Regular
- Every product of Tychonoff spaces is Tychonoff
- A product of normal spaces
*need not*be normal

- Every product of T
- Compactness
- Every product of compact spaces is compact (Tychonoff's theorem)
- A product of locally compact spaces
*need not*be locally compact

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

*Please add more results like these*