# What exactly is topology?

I wanted to ask what exactly is in it---as a subject what is studied in this field.

I have seen videos wherein they oversimplify tell us that in topology, you squeeze and stretch but don't cut, or how a donut and a mug is equivalent. (I think it must be like simplifying calculus and saying that it's just fancy addition.)

How does it help in mathematics, because it is studied as a full fledged course, it must have its perks and used too.... [ Also considering that there are so many tags under this topic on SE ] ??

(I am still in high school so I don't have a lot of information, but I consider myself to be a curious math enthusiast)

• This is definitely a good question to be asking yourself; the downvotes are perhaps because some people don't think the question is a good fit for this forum. One point is that a topological space has the minimum structure necessary in order to define and study continuous functions. Omitting unnecessary assumptions often leads to a deeper or simpler understanding, and also avoids the need to repeat the same proofs in all the different contexts where continuous functions are encountered. – littleO Jun 26 '19 at 20:19
• When I was your age I read a book called "Experiments in topology" by Stephen Barr and I was hooked for life. – Cheerful Parsnip Jun 26 '19 at 23:41
• OP appears to know something about calculus, so they might know what continuous functions are. A lot of high school students who like math know about continuous functions. In any case, hopefully OP will just ignore any answers that aren't written at a level that is helpful for OP. – littleO Jun 26 '19 at 23:52
• The book en.wikipedia.org/wiki/What_Is_Mathematics%3F is aimed at HS students and, among other things, explains at HS level what is topology. In my view, it is still one of the best intro books of this type. – Moishe Kohan Jun 27 '19 at 0:39
• The wikipedia entry Topology is an excellent answer to your question. – J.-E. Pin Jun 27 '19 at 4:52

Most basically, topology is about open sets.

It may sound silly, but, as it turns out, for instance, point-set-topology is considered an indispensable tool for any working mathematician.

While every mathematician should know the basics, general topology is a fascinating subject in its own right. It goes up and up, just like the homotopy groups I consider one of its neatest topics. There are algebraic topology and differential topology, to name a couple advanced variants.

Sometimes topology is referred to as "rubber sheet geometry". Two spaces that can be bent or stretched, without tearing, into one another are considered the same, or "isomorphic" (actually "homeomorphic").

Geometry and topology have various connections and overlaps. For instance, Thurston's Geometrization Conjecture, for which he was awarded the Fields medal in 1982.

Or, the Poincaré conjecture, which Perelman got the same award for more recently (Smale did it in lower dimensions, I think, in the late $$60$$'s, and also got the award. But a story going around Berkeley was that there was an error in Smale's solution, and that Stallings had done it correctly.

In a nutshell, it was one of the biggest unsolved problems for a long time, and said that "there are no homology spheres". Homology is another important notion in topology/geometry.

As a topologist and knot theorist, I want to give a more picture oriented answer. I think of a good classic problem in geometric topology is the classification of surfaces. It tells you exactly what every 2-dimensional manifold (or surface) is, and how to tell them apart. You don't really need to know what that is, the pictures are better at relaying the idea. All these images come from this page on wikipedia about genus $$g$$ surfaces. Of the four pictures, there are only three surfaces. Just count the "holes."

In some sense, it is easier to tell what surface you have by something called the Euler Characteristic. I will let you look through that at your leisure.

The next thing I wanted to share, was the idea of knot theory, which falls under the umbrella of topology. Unlike with the surfaces, it is in general very hard to tell if two knots are the same or not. Think about an extension cord plugged, tied up and then plugged into itself. Can you untangle it without unplugging it? Here is a table of the first few knots.

A really nice program by Robert Scharein is called KnotPlot and it lets you see how knots can be deformed and wiggled about to change how they look and (hopefully) simplified. You can download it for free and play around. One little demo they have is to guess the knot they present, unknot or trefoil. This isn't easy! In this case, it happens to be the trefoil, which is $$3_1$$ in the table. Then try to imagine if we had a knot with 100 crossings. It is basically impossible to be sure what you are looking at! Knot theory is all about finding ways of helping us answer these questions, without actually having to let KnotPlot wiggle them around and simplify them for us. I hope this helps give you an idea of what some topologists think about.

Topology, in the sense and meaning you are referring to, can be thought of as study of some continuous processes and what is and what is not changed by them.

For example, the cube and the ball are in some senses equivalent and in some of them are not. They can be thought of as equivalent with respect to dimension (you can turn some ball into any cube and some cube into any ball continuously and the dimension is not changed by that process).

However, you can stretch some small cube into as big as you want cube so volume is changed with some continuous processes.

The "what is changed" by continuous processes and "what is not" is important in topology.

Also, the continuous processes are also important themselves.

As an easy example, it is not possible to continuously turn one ball into two balls that do not touch and do not intersect each other, and discontinuous processes are, largely, not the part of standard topology.

To understand what topology is about, maybe it is useful to look at a few concrete examples.

For example, take the set $$\mathbb R$$ of real numbers, the two-dimensional set $$\mathbb R^2$$, and the set of non-integer numbers, $$\mathbb R\setminus\mathbb Z$$.

Now if we look at them just with the eye of set theory, they are pretty much the same: All three have the same number of elements, and you can easily take bijections between any of them.

However we know that those three sets are very different to each other. The set of non-integer numbers has infinitely many "gaps" in it, and $$\mathbb R^2$$ is of a completely different dimension.

If you look closer, the reason why pure set theory doesn't “see” the difference is that set theory looks at the elements “in isolation”. But the three sets differ in how those points are related to each other. That is, there's more structure in there than just the sets.

Now you might think the extra structure might be the distance, or as it is called in mathematics, the metric, and you wouldn't be entirely wrong, because the metric indeed fixes those things. But in a sense, it already tells us more than we need. For example in the case of $$\mathbb R\setminus\mathbb Z$$, the metric tells us that the half-integer numbers are very special because they lie exactly in the middle of the gaps. But for the character of those points that is completely irrelevant; the surrounding of a point slightly left or slightly right of that doesn't really look different.

Another related concept is that of a continuous function, that is, intuitively, a function where a sufficient small error in the input doesn't make a big difference in the output. Again, the metric actually gives us too much information, as we are (at this point) not really interested in how much it changes, just that this change can be made arbitrary small.

Topology now is the minimal additional structure you have to add to a set to make sense of all these concepts. The fundamental additional concept is the open set, which roughly speaking is a set where a sufficiently small error will not get you out of the set. This gives some basic notion of “nearness” without the need to quantify that nearness.