# Why the axioms for a topological space are those axioms?

This question might have even been asked here before, I don't really know, so sorry if it's duplicate. I've started to study topological spaces and I've found the axioms for such spaces kind of hard to motivate. Well, the ideia of a metric space is much easier to motivate: "the concept of distance is context dependent, so we want a general idea of what distance is and a general idea of a set on which we can measure distances". The axioms then for a metric are very intuitive, easy to motivate and everything else.

Then we start studying properties of subsets of metric spaces. We define open balls as a way to make precise the notion of "the set of all points that are sufficiently close to a central point" and we define open sets to make precise "sets such that for every point, other points sufficiently close are also in the set", which can also be thought as sets such that each point can be oscilated a little bit from it's position and the point will stay on the set.

After that we can define lots of things: limit points, closed sets, dense sets, perfect sets, compactness and so on. We also see that all of those notions can be made precise mentioning the open sets alone: the metric is not really necessary to talk about those things, as soon as we can define what open sets are. So this is enough motivation to define a structure on which we have open sets.

The answer to this problem is to define a topology on a set $X$ as a set $\mathcal{T} \subset \mathcal{P}(X)$ such that $\mathcal{T}$ is closed under arbitrary unions, finite intersections and such that $X,\emptyset \in \mathcal{T}$. If, $X$ is a metric space, and we let $\mathcal{T}$ be the set of open sets as they are defined using balls, then the three properties are satisfied.

My only question is: why those properties capture completely the idea of an open set? I mean, amongst all properties of open sets, why do we select those three? I've always heard that topology is meant to study qualitatively global properties of forms, that's the way we start in $\mathbb{R}^n$ and the way we generalize to metric spaces: we introduce tools that allows us to define carefully some of these properties and we work out definitions. It doesn't seem clear the connection of this motivation for topology and the actual definition.

I've seem a similar question on MathOverflow, and there was one answer trying to motivate this in terms of rulers, but I really didn't get the idea. Can someone give a little help with this?

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My answer here may be of interest. –  Asaf Karagila Jul 24 at 15:59
It may be worth pointing out that Hausdorff (Grundzüge der Mengenlehre) developed topological concepts from the notion of neighbourhood. It took some time until the overall most convenient way to define topological concepts was agreed to be the one stating the axioms of a topology. Sometimes, the neighbourhood view is more convenient, and it is probably more intuitive to understand its axioms. But its axioms are not as concise. –  Daniel Fischer Jul 24 at 16:21
possible duplicate of Motivation behind topology –  Zev Chonoles Jul 24 at 21:47
I think that this is the link to the MO question mentioned in this question mathoverflow.net/questions/19152/… –  Baby Dragon Jul 24 at 22:25
One of the facts this question brings out is that texts generally do not consider it necessary to explain the motivation, history and context of the theory they expose, so students have to ask it separately. Yet the analysis of why is one of the ways mathematics progresses. There is a danger of "They ask for bread and we give them stones." –  Ronnie Brown Jul 25 at 9:58

The notion of open may best be comprised by "If $x$ is in the open set, then all points with $y\approx x$ should please also be in the set".

This makes $\emptyset$ and the space $X$ itself oopen automatically - either because there is no $x$ to ckeck or no $y$ that could complain.

And still without specifying further what $\approx$ really means, it follows immediately that an arbitrary union of open sets is open again.

We could stop here, but these axioms alone don't make a good structure yet. Thus it is motivated to have a look at the "other" set operation, intersection. If we have two open sets (with possibly different interpretations of $\approx$, say $\approx_1$ and $\approx_2$) it seems to be a nice feature to assume that any two points that are witnessed in two ways as being no too different are no too different and vice versa, i.e. that the conjunction of $\approx_1$ and $\approx_2$ should make a valid $\approx$, so the intersection of two open sets should be considered open again. And there we are. One might consider some strengthening, such as arbitrary intersections, but as already the case of metric spaces shows, this will more often then not boil down to considereing th epower set of $X$ (or possibly with some points identified), so quite boring.

In hindsight, it is just the case that these axioms very nicely give us practically all the nice properties of metric spaces plus applicability to some interesting structures that do not allow a metric

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The heart of it is that, while your intuition is that "continuity" is about distances - you first learn continuity in terms of $\epsilon-\delta$ proofs - it turns out that continuity is really only about open sets. Whether a map $f:X\to Y$ between metric spaces is continuous is entirely determined by the open sets of $X$ and $Y$. If you have two different metrics for $X$ which determine the same open sets, the set of continuous functions on $X$ are the same.

Since topology is the study of continuity, it makes sense to only care about open sets, then, not metrics.

The original definitions for "topology" had more rules about the open sets that made topologies "more like" metric spaces. But as mathematicians started playing with these things, they found that it made sense to ask questions about continuity in the cases where these rules were broken, too. So the definition was broadened to give the widest meaning.

A very basic example might be left-continuity. A function $f:\mathbb R\to Y$ with $Y$ a metric space is called left-continuous if $\lim_{x\to a-} f(x)=f(a)$ for all $a\in\mathbb R$.

It turns out, there is a topology on the real line, call it $\tau_{L}$, under which a function $f$ is left-continuous if and only if it is (topologically) continuous as a function from $(\mathbb R,\tau_L)\to Y$. (Note: $\tau_{L}$ has a basis the intervals of the form $(x,y]$.) Now, $\tau_{L}$ is a point-set topology, but it is not one that comes from a metric. So the notion of "left-continuity" is an example of an idea that fits the point-set definition of continuity, but does not fit the $\epsilon-\delta$ notion of continuity - you essentially need to redefine it.

The fact that left-continuity and right-continuity together is the same as "normal" continuity is a statement about the three different topologies. Somehow, the usual real line topology is a combination of these two other topologies.

It turns out there is something really deep going on here. Somehow, the open neighborhoods of a point in a topology contains a lot of information - what we call "local information" - about behavior of "nice" functions at that point.

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I agree with Andreas.

The open set axioms for a topology are usually presented as the axioms for a topology, but in fact there are as number of equivalent ways of defining a topology, via neighbourhoods, open sets, closed sets, closure, and even interior or boundary. The neighbourhood axioms are surely the most intuitive to a beginner, and most clearly related to the usual notion of continuity in analysis. Indeed the definition: "$f$ is continuous at a point $x$ of its domain if for all neighbourhoods $N$ of $f(x)$ there is a neighbourhood $M$ of $x$ such that $f(M) \subseteq N$" is more direct and geometric than the $\epsilon$-$\delta$ definition, since the latter are just measures of the sizes of neighbourhoods, and so a step away from the geometric picture. Those numbers are of course a useful computational device, as numbers are.

Of course the open set axioms are very elegant and logically simpler than the neighbourhood axioms, and are is some case the best way of defining a topology, for example for identification spaces. But beginners should be encouraged to make their own judgement on the advantages and disadvantages of each definition of a topology.

Part of the need for such abstraction from the notion of metric space was the development of ideas of Riemann surfaces, and then the notion of manifold. The work of Hausdorff was a key in this development.

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Thanks for the answer @RonnieBrown. I've tried searching a little for the history of topology and many places motivate the study of topology saying that "topology is the study of shapes and continuous deformations of those shapes". This is really the most common idea everyone makes of topology. Can you tell where I can read about how the axioms about open sets relate to this notion of studying "shapes and continuous deformations of those shapes"? Thanks very much again for the help! –  user1620696 Jul 27 at 18:24
@user1620696: Your quotation confirms that topology is about continuity, in which a doughnut is equivalent to a teacup (with handle!). There is a picture of this on p. 43 of my book "Topology and Groupoids". The problem was to find the most "suitable" axioms allowing the notion of continuity, and also allowing spaces to be formed by "gluing". See also p.16 and Chapter 4. So one throws away distance and angle, which were features of Euclidean Geometry. Also do a web search on "History of Topology". –  Ronnie Brown Jul 30 at 15:45
I think the usual axioms for topology developed in several steps. First, people worked with Euclidean spaces and their subspaces, and the most important topological notions, like convergence and continuity, were developed in this context. When this is done, one can hardly help noticing that only the notion of distance, not the rest of Euclidean structure is needed. Furthermore, there are other sorts of "spaces" with reasonable notions of distance, leading, just as in Euclidean space, to reasonable and useful notions of convergence and continuity. In particular, one has the notions of uniform convergence of functions and (I believe somewhat later) $L_1$ and $L_2$ convergence. So people abstracted the relevant properties of distance and thus defined metric spaces. But there are some quite reasonable notions of convergence that don't fit into the context of metric spaces. Perhaps the most important one is pointwise convergence of functions (on uncountable sets like the reals). In other situations, it is possible to define a metric but the metric tends to look awkward compared with the notions of convergence and continuity that it supports. I'm thinking here of things like uniform convergence on compact sets (say for functions $\mathbb C\to\mathbb C$), or convergence of functions (on Euclidean space, say) together with their derivatives of all orders. Such situations lead naturally to the question whether one really needs a metric or whether convergence and continuity make sense in a broader (and perhaps simpler) context. The key observation here is that, when one defines convergence and continuity using a metric, the role of the metric is just to produce a notion of neighborhood. Once one knows what is meant by a neighborhood of a point, one can proceed without further reference to the metric. And neighborhoods in metric spaces have special properties that are never really needed for topological purposes. For example, each point has a nested sequence of neighborhoods, the balls of radius $1/n$ centered at the point, such that every neighborhood of the point includes one of these. It turns out that one doesn't really need either the nestedness or the countability of this sequence. So one can (and people like Hausdorff and Fréchet did) axiomatize what's really needed about neighborhoods. That essentially produced the modern notion of topological space (perhaps with a little additional information that one could regard as "really needed", like the Hausdorff separation axiom). The step from this "neighborhood" axiomatization to the currently standard "open set" axiomatization is, I believe, motivated only by a desire to make the axioms look as simple as possible. I would think of neighborhood systems as the "true" intuition underlying the notion of topological space, while the open sets provide a convenient way of describing and working with neighborhoods.