[Math] Boundedness in a topological space

general-topologymetric-spacestopological-vector-spaces

  1. I was wondering if there is a concept of boundedness for subsets of
    a topological space?
  2. If yes to 1, is it this one from Wiki

    Elements of a Bornology B on a set X are called bounded sets and the pair (X,
    B) is called a bornological set.

    For any topological space X, the set of subsets of X with compact
    closure is a Bornology.

  3. If yes to 2, does it coincide with boundedness in a metric space and in a
    topological vector space? How is it related to total boundedness in
    a uniform space?

Thanks and regards!

Best Answer

In general, there is no notion of boundedness on a topological space.

Exercise: given a metric space $(X,d)$, show that $D(x,y):=\mbox{min}(1,d(x,y))$ defines a second metric on $X$ which is equivalent to the first one; that is, a subset $U$ of $X$ is open with respect to the first metric if and only if it is open with respect to the second one.

Note that with the second metric, every set is bounded, but the topologies are the same. What this shows is that, in metric spaces, where the notion of boundedness is well-defined, one can show that it is in fact independent of the topology. Boundedness is a property which arises from the metric.

The concept of bornology (although I am not familiar with it) allows you to study boundedness by adding extra structure to your topological space, and what you get is a bornological space.