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 openset in the discrete topology.
Definitions
Given a set X:
the discrete topology on X is defined by letting every subset of X be open, and X is a discrete topological space if it is equipped with its discrete topology;
the discrete uniformity on X is defined by letting every superset of the diagonal in X × X be an entourage, and X is a discrete uniform space if it is equipped with its discrete uniformity.
a set S is discrete in a metric space, for, if for every, there exists some such that for all ; such a set consists of isolated points. A set S is uniformly discrete in the metric space, for, if there exists ε > 0 such that for any two distinct, > ε.
A metric space is said to be uniformly discrete if there exists a "packing radius" such that, for any, one has either or. The topology underlying a metric space can be discrete, without the metric being uniformly discrete: for example the usual metric on the set of real numbers. Let X =, consider this set using the usual metric on the real numbers. Then, X is a discrete space, since for each point 1/2n, we can surround it with the interval, where ɛ = 1/2 = 1/2n+2. The intersection ∩ is just the singleton. Since the intersection of two open sets is open, and singletons are open, it follows that X is a discrete space. However, X cannot be uniformly discrete. To see why, suppose there exists an r>0 such that d>r whenever x≠y. It suffices to show that there are at least two points x and y in X that are closer to each other than r. Since the distance between adjacent points 1/2n and 1/2n+1 is 1/2n+1, we need to find an n that satisfies this inequality: Since there is always an n bigger than any given real number, it follows that there will always be at least two points in X that are closer to each other than any positive r, therefore X is not uniformly discrete. ...
Properties
The underlying uniformity on a discrete metric space is the discrete uniformity, and the underlying topology on a discrete uniform space is the discrete topology. Thus, the different notions of discrete space are compatible with one another. On the other hand, the underlying topology of a non-discrete uniform or metric space can be discrete; an example is the metric space X := . This is not the discrete metric; also, this space is not complete and hence not discrete as a uniform space. Nevertheless, it is discrete as a topological space. We say that X is topologically discrete but not uniformly discrete or metrically discrete. Additionally:
A discrete space is separable if and only if it is countable.
Any function from a discrete topological space to another topological space is continuous, and any function from a discrete uniform space to another uniform space is uniformly continuous. That is, the discrete space X is free on the set X in the category of topological spaces and continuous maps or in the category of uniform spaces and uniformly continuous maps. These facts are examples of a much broader phenomenon, in which discrete structures are usually free on sets. With metric spaces, things are more complicated, because there are several categories of metric spaces, depending on what is chosen for the morphisms. Certainly the discrete metric space is free when the morphisms are all uniformly continuous maps or all continuous maps, but this says nothing interesting about the metric structure, only the uniform or topological structure. Categories more relevant to the metric structure can be found by limiting the morphisms to Lipschitz continuous maps or to short maps; however, these categories don't have free objects. However, the discrete metric space is free in the category of bounded metric spaces and Lipschitz continuous maps, and it is free in the category of metric spaces bounded by 1 and short maps. That is, any function from a discrete metric space to another bounded metric space is Lipschitz continuous, and any function from a discrete metric space to another metric space bounded by 1 is short. Going the other direction, a function f from a topological space Y to a discrete space X is continuous if and only if it is locally constant in the sense that every point in Y has a neighborhood on which f is constant.
Uses
A discrete structure is often used as the "default structure" on a set that doesn't carry any other natural topology, uniformity, or metric; discrete structures can often be used as "extreme" examples to test particular suppositions. For example, any group can be considered as a topological group by giving it the discrete topology, implying that theorems about topological groups apply to all groups. Indeed, analysts may refer to the ordinary, non-topological groups studied by algebraists as "discrete groups". In some cases, this can be usefully applied, for example in combination with Pontryagin duality. A 0-dimensional manifold is nothing but a discrete topological space. We can therefore view any discrete group as a 0-dimensional Lie group. A product of countably infinite copies of the discrete space of natural numbers is homeomorphic to the space of irrational numbers, with the homeomorphism given by the continued fraction expansion. A product of countably infinite copies of the discrete space 2 | is homeomorphic to the Cantor set; and in fact uniformly homeomorphic to the Cantor set if we use the product uniformity on the product. Such a homeomorphism is given by using ternary notation of numbers. In the foundations of mathematics, the study of compactness properties of products of is central to the topological approach to the ultrafilter principle, which is a weak form of choice.
Indiscrete spaces
In some ways, the opposite of the discrete topology is the trivial topology, which has the fewest possible open sets. Where the discrete topology is initial or free, the indiscrete topology is final or cofree: every function from a topological space to an indiscrete space is continuous, etc.
