Discrete Space - Properties

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 := {1/n : n = 1,2,3,...} (with metric inherited from the real line and given by d(x,y) = |x − y|). Obviously, 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:

• The topological dimension of a discrete space is equal to 0.
• A topological space is discrete if and only if its singletons are open, which is the case if and only if it doesn't contain any accumulation points.
• The singletons form a basis for the discrete topology.
• A uniform space X is discrete if and only if the diagonal {(x,x) : x is in X} is an entourage.
• Every discrete topological space satisfies each of the separation axioms; in particular, every discrete space is Hausdorff, that is, separated.
• A discrete space is compact if and only if it is finite.
• Every discrete uniform or metric space is complete.
• Combining the above two facts, every discrete uniform or metric space is totally bounded if and only if it is finite.
• Every discrete metric space is bounded.
• Every discrete space is first-countable; it is moreover second-countable if and only if it is countable.
• Every discrete space with at least two points is totally disconnected.
• Every non-empty discrete space is second category.
• Any two discrete spaces with the same cardinality are homeomorphic.
• Every discrete space is metrizable (by the discrete metric).
• A finite space is metrizable only if it is discrete.
• If X is a topological space and Y is a set carrying the discrete topology, then X is evenly covered by X × Y (the projection map is the desired covering)
• The subspace topology on the integers as a subspace of the real line is the discrete topology.

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 (on more than one element). 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.