Separated sets
In topology and related branches of mathematics, separated sets are pairs of subsets of a given topological space that are related to each other in a certain way: roughly speaking, neither overlapping nor touching. The notion of when two sets are separated or not is important both to the notion of connected spaces as well as to the separation axioms for topological spaces.
Separated sets should not be confused with separated spaces, which are somewhat related but different. Separable spaces are again a completely different topological concept.
Definitions
There are various ways in which two subsets and of a topological space can be considered to be separated. A most basic way in which two sets can be separated is if they are disjoint, that is, if their intersection is the empty set. This property has nothing to do with topology as such, but only set theory. Each of the following properties is stricter than disjointness, incorporating some topological information.The properties below are presented in increasing order of specificity, each being a stronger notion than the preceding one.
The sets and are ' in if each is disjoint from the other's closure:
This property is known as the. Since every set is contained in its closure, two separated sets automatically must be disjoint. The closures themselves do not have to be disjoint from each other; for example, the intervals and are separated in the real line even though the point 1 belongs to both of their closures. A more general example is that in any metric space, two open balls and are separated whenever The property of being separated can also be expressed in terms of derived set : and are separated when they are disjoint and each is disjoint from the other's derived set, that is,
The sets and are ' if there are neighbourhoods of and of such that and are disjoint. For the example of and you could take and Note that if any two sets are separated by neighbourhoods, then certainly they are separated. If and are open and disjoint, then they must be separated by neighbourhoods; just take and For this reason, separatedness is often used with closed sets.
The sets and are ' if there is a closed neighbourhood of and a closed neighbourhood of such that and are disjoint. Our examples, and are [|separated by closed neighbourhoods]. You could make either or closed by including the point 1 in it, but you cannot make them both closed while keeping them disjoint. Note that if any two sets are separated by closed neighbourhoods, then certainly they are separated by neighbourhoods.
The sets and are ' if there exists a continuous function from the space to the real line such that and, that is, members of map to 0 and members of map to 1. In our example, and are not separated by a function, because there is no way to continuously define at the point 1. If two sets are separated by a continuous function, then they are also separated by closed neighbourhoods; the neighbourhoods can be given in terms of the preimage of as and where is any positive real number less than
The sets and are if there exists a continuous function such that and Note that if any two sets are precisely separated by a function, then they are separated by a function. Since and are closed in only closed sets are capable of being precisely separated by a function, but just because two sets are closed and separated by a function does not mean that they are automatically precisely separated by a function.
Relation to separation axioms and separated spaces
The separation axioms are various conditions that are sometimes imposed upon topological spaces, many of which can be described in terms of the various types of separated sets. As an example we will define the T2 axiom, which is the condition imposed on separated spaces. Specifically, a topological space is separated if, given any two distinct points x and y, the singleton sets and are separated by neighbourhoods.Separated spaces are usually called Hausdorff spaces or T2 spaces.