Defining an Empty Set Metric Space: Understanding Properties of d

In summary, There is no problem with defining a metric space on the empty set, as all the required properties can still be satisfied. It is not necessary to explicitly state that the set is non-empty in the axioms for a metric space. This convention allows for easier application of theorems, such as the fact that every non-empty subspace of a metric space is also a metric space. A metric space is a function from one set to another, satisfying certain conditions. In the case of the empty set, it can be viewed as a function from the empty set to any other set, vacuously satisfying all the conditions.
  • #1
zooxanthellae
157
1
Can we define a metric space [itex](\emptyset, d)[/itex]? The metric is the part that confuses me, since it seems like all of the required properties of d are satisfied since they are "not not satisfied", but I'm not sure.

Thank you!
 
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  • #2
Yes, I don't see the problem with that. However, I think you would probably want to include "non-empty" in the axioms for a metric space, it's just that you wouldn't usually bother because you don't gain anything of interest by looking at an empty metric space.
 
  • #3
It's a perfectly fine metic space and it's a good thing to keep that convention. So we don't always have to make awkward exceptions to theorems, such as "every non-empty subspace of a metric space is a metric space."
 
  • #4
OK, thanks to you both.
 
  • #5
zooxanthellae said:
Can we define a metric space [itex](\emptyset, d)[/itex]? The metric is the part that confuses me, since it seems like all of the required properties of d are satisfied since they are "not not satisfied", but I'm not sure.

Thank you!

how are they not satisfied? every condition for a metric holds for every element of Ø, no matter how we define the metric (although if you must have a definition, use d(x,x) = 0, d(x,y) = 1, for all x,y not in Ø).
 
  • #6
... this all comes to vacuous truths again, as discussed elsewhere.
 
  • #7
What is a metric?

It's a function from X cross X to ℝ, plus some conditions.

If X is the empty set, it's a function from the empty set to ℝ.

What's a function from a set S to a set T?

Formally, it's a subset of S cross T, satisfying some condition.

So, the empty set can be viewed as a function from the empty set to any other set. The empty function. So, that's your metric and it vacuously satisfies all the conditions.
 
  • #8
Deveno said:
how are they not satisfied? every condition for a metric holds for every element of Ø, no matter how we define the metric (although if you must have a definition, use d(x,x) = 0, d(x,y) = 1, for all x,y not in Ø).

I wrote "not not satisfied", which is a slightly stupider way of saying "vacuously true" as Jamma and homeomorphic have clarified.
 

1. What is an empty set metric space?

An empty set metric space is a mathematical concept in which the set of objects, or elements, is empty. This means that there are no objects in the set, and therefore, no distances can be defined between any elements. In other words, the distance between any two elements is undefined or infinite.

2. What is the significance of an empty set metric space?

An empty set metric space is important in mathematics and sciences because it allows for the study of properties and concepts without the restriction of having actual objects or elements. This concept is also used in topology to define open and closed sets.

3. How is an empty set metric space represented mathematically?

An empty set metric space is represented as a metric space with no elements, denoted as (X, d), where X is the set of elements and d is the distance function. In this case, X is the empty set and d is undefined.

4. Can an empty set metric space have a metric?

No, an empty set metric space cannot have a metric because a metric is a function that defines the distance between any two elements in a set. In an empty set, there are no elements, and therefore, no distances can be defined.

5. What are some real-world examples of an empty set metric space?

An empty set metric space may seem like a theoretical concept, but it can be applied to real-world situations. For example, in a library with no books, the set of books can be considered an empty set metric space. Similarly, in a classroom with no students, the set of students can be seen as an empty set metric space.

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