# What is a topology? Intuition.

Hi! I'm trying to get some intuition for the notion of the topology of a set. The definition of a topology ##\tau## on a set ##X## is that ##\tau## satisfies the following:

- ##X## and ##Ø## are both elements of ##\tau##.
- Any union of sets in ##\tau## are also in ##\tau##.
- Any finite intersections of sets in ##\tau## are also in ##\tau##.

Alright, so one associates the set X with another set ##\tau## and the two together define a topological space if ##\tau## satisfies certain properties. Topology, as I understand it, is the study of the most qualitative features of a space. Have i understood it properly if i say that the role of the topology ##\tau## is to distribute the elements in ##X## in a certain way?

For example in the excluded point topology on the real line, the point ##p = 0##, is given by

$$\tau_E = \{\mathbb{R}\}\cup\{S \subseteq \mathbb{R} : 0 \notin S\}.$$

So while the point zero is in the set of reals it is not in the topology and therefore not in the topological space. Thus the topology has distributed the set of points in a certain way.

So how do you like intuitively to think about a topology and it's role?

Last edited:

dx
Homework Helper
Gold Member
There is an intuitive way to think about what the open sets, or the elements of the topology represent. An open set is essentially a set which 'contains all points sufficiently close to its members.'

From that idea, it is easy to see why the empty set and the space itself must be open. The space itself contains all points, so it certainly contains all the points 'sufficiently close to its members.' The empty set contains no points, and therefore there is no point in the empty set whose neighbors are also not contained in it.

Once you get the intuitive idea of what an open set is, you can use that concept to construct other concepts. For example, an 'isolated point' x does not have any neighbors, therefore the set {x} is open. You can then define an 'isolated point' as a point x such that {x} is open.

WannabeNewton
It's just a type of generalization of a metric space. We lose the notion of distance, that's all. A lot of definitions from metric spaces carry over, just recast in terms of open sets instead of metrics (some things don't e.g. Cauchy sequences).

rubi
The book "Modern differential geometry for physicists" by Chris Isham explains very well where the axioms of topology come from. Don't worry about the name; it's quite mathy.

For example in the excluded point topology on the real line, the point ##p = 0##, is given by

$$\tau_E = \{\mathbb{R}\}\cup\{S \subseteq \mathbb{R} : 0 \notin S\}.$$

So while the point zero is in the set of reals it is not in the topology and therefore not in the topological space. Thus the topology has distributed the set of points in a certain way.

So how do you like intuitively to think about a topology and it's role?

Notice that the topology is a set who's elements are sets, namely open subsets of the reals, and is not itself a subset of the reals, but of its power set.

I don't know what you mean by "the point p=0 is given by"

HallsofIvy