# Alexandroff extension

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 for the Russian mathematician Pavel Alexandrov.

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.

## Example: inverse stereographic projection

A geometrically appealing example of one-point compactification is given by the inverse stereographic projection. Recall that the stereographic projection S gives an explicit homeomorphism from the unit sphere minus the north pole (0,0,1) to the Euclidean plane. The inverse stereographic projection is an open, dense embedding into a compact Hausdorff space obtained by adjoining the additional point . Under the stereographic projection latitudinal circles get mapped to planar circles . It follows that the deleted neighborhood basis of given by the punctured spherical caps corresponds to the complements of closed planar disks . More qualitatively, a neighborhood basis at is furnished by the sets as K ranges through the compact subsets of . This example already contains the key concepts of the general case.

## Motivation

Let be an embedding from a topological space X to a compact Hausdorff topological space Y, with dense image and one-point remainder . Then c(X) is open in a compact Hausdorff space so is locally compact Hausdorff, hence its homeomorphic preimage X is also locally compact Hausdorff. Moreover, if X were compact then c(X) would be closed in Y and hence not dense. Thus a space can only admit a one-point compactification if it is locally compact, noncompact and Hausdorff. Moreover, in such a one-point compactification the image of a neighborhood basis for x in X gives a neighborhood basis for c(x) in c(X), and—because a subset of a compact Hausdorff space is compact if and only if it is closed—the open neighborhoods of must be all sets obtained by adjoining to the image under c of a subset of X with compact complement.

## The Alexandroff extension

Put , and topologize by taking as open sets all the open subsets U of X together with all sets where C is closed and compact in X. Here, denotes setminus. Note that is an open neighborhood of , and thus, any open cover of will contain all except a compact subset of , implying that is compact.(Kelley 1975, p. 150)

The inclusion map is called the Alexandroff extension of X (Willard, 19A).

The above properties all follow from the above discussion:

• The map c is continuous and open: it embeds X as an open subset of .
• The space is compact.
• The image c(X) is dense in , if X is noncompact.
• The space is Hausdorff if and only if X is Hausdorff and locally compact.

## The one-point compactification

In particular, the Alexandroff extension is a compactification of X if and only if X is Hausdorff, noncompact and locally compact. In this case it is called the one-point compactification or Alexandroff compactification of X. Recall from the above discussion that any compactification with one point remainder is necessarily (isomorphic to) the Alexandroff compactification.

Let X be any noncompact Tychonoff space. Under the natural partial ordering on the set of equivalence classes of compactifications, any minimal element is equivalent to the Alexandroff extension (Engelking, Theorem 3.5.12). It follows that a noncompact Tychonoff space admits a minimal compactification if and only if it is locally compact.

## Further examples

### Compactifications of discrete spaces

• The one-point compactification of the set of positive integers is homeomorphic to the space consisting of K = {0} U {1/n | n is a positive integer.} with the order topology.
• A sequence in a topological space converges to a point in , if and only if the map given by for in and is continuous. Here has the discrete topology.
• Polyadic spaces are defined as topological spaces that are the continuous image of the power of a one-point compactification of a discrete, locally compact Hausdorff space.

### Compactifications of continuous spaces

• The one-point compactification of n-dimensional Euclidean space Rn is homeomorphic to the n-sphere Sn. As above, the map can be given explicitly as an n-dimensional inverse stereographic projection.
• The one-point compactification of the product of copies of the half-closed interval [0,1), that is, of , is (homeomorphic to) .
• Since the closure of a connected subset is connected, the Alexandroff extension of a noncompact connected space is connected. However a one-point compactification may "connect" a disconnected space: for instance the one-point compactification of the disjoint union of copies of the interval (0,1) is a wedge of circles.
• Given compact Hausdorf and any closed subset of , the one-point compactification of is , where the forward slash denotes the quotient space.[1]
• If and are locally compact Hausdorf, then where is the smash product. Recall that the definition of the smash product: where is the wedge sum, and again, / denotes the quotient space.[1]

### Compactification of function spaces

• The space of continuous functions on a locally compact Hausdorff space is locally compact but can be made compact if and only if we include the single point for all .

### As a functor

The Alexandroff extension can be viewed as a functor from the category of topological spaces with proper continuous maps as morphisms to the category whose objects are continuous maps and for which the morphisms from to are pairs of continuous maps such that . In particular, homeomorphic spaces have isomorphic Alexandroff extensions.