Product topology

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.

Definition

Let I be a (possibly infinite) index set and suppose X_i is a topological space for every i in I. Set X = Π X_i, the cartesian product of the sets X_i. For every i in I, we have a canonical projection p_i : X → X_i. The product topology on X is defined to be the coarsest topology (i.e. the topology with the fewest open sets) for which all the projections p_i are continuous.

Explicitly, the product topology on X can be described as the topology generated by sets of the form p_i⁻¹(U), where i in I and U is an open subset of X_i. In other words, the sets {p_i⁻¹(U)} form a subbase for the topology on X. 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(U).

We can describe a basis for the product topology using bases of the constituting spaces X_i. Suppose that for each i in I we choose a set Y_i which is either the whole space X_i or a basis set of that space, in such a way that X_i = Y_i for all but finitely many i in I. Let B be the cartesian product of the sets Y_i. The collection of all sets B that can be constructed in this fashion is a basis of the product space. In particular, this means that a product of finitely many spaces has a basis given by the products of base elements of the X_i.

If the index set is finite (in particular, for a product of two topological spaces) then the product topology admits a simpler description. In this case product of the topologies of each X_i forms a basis for the topology on X. In general, the product of the topologies of each X_i forms a basis for what is called the box topology on X. In general, the box topology is finer than the product topology, but for finite products they coincide.

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 Rⁿ.

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 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_i is a continuous map, then there exists precisely one continuous map f : Y → X such that the following diagram commutes:

This shows that the product space is a product in the category of topological spaces. If follows from the above universal property that a map f : Y → X is continuous iff f_i = p_i o f is continuous for all i in I. In many cases it is often easier to check that the component functions f_i are continuous. Checking whether a map g : X→ Z is continuous is usually more difficult; one tries to use the fact that the p_i are continuous in some way.

In addition to being continuous, the canonical projections p_i : X → X_i are open maps. This means that any open subset of the product space remains open when projected down to the X_i. The converse is not true: if W is a subspace of the product space whose projections down to all the X_i are open, then W need not be open in X. (Consider for instance W = R² \ (0,1)².) The canonical projections are not generally closed maps.

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 functions 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.

Relation to other topological notions

• Separation
  • Every product of T₀ spaces is T₀
  • Every product of T₁ spaces is T₁
  • 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
• 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.

A map that "locally looks like" a canonical projection F × U → U is called a fiber bundle.

See also

• box topology
• disjoint union (topology)
• quotient space
• subspace (topology)