# 2.01 Topological spaces, continuous maps

## Video

Below the video you will find accompanying notes and some pre-class questions.

- Next video:
**Bases, metric and product topologies**. - Index of all lectures.

## Notes

*(0.00)* This is the first in a sequence of videos about topological
spaces, aimed at people who have already seen the theory of *metric
spaces*.

Recall the following lemma from the theory of metric spaces.

*(0.49)* If \(X,Y\) are metric spaces and \(F\colon X\to Y\) is a
map then \(F\) is continuous if and only if \(F^{-1}(U)\) is open
for all open sets \(U\subset Y\).

*(2.00)* The key point here is that *continuity of the map \(F\)* can
be formulated purely in terms of open sets, with no \(\epsilon\)s or
\(\delta\)s in sight. To go back to the usual definition of
continuity, the way the \(\epsilon\)s and \(\delta\)s reappear is in
the *definition of the open sets*: an open ball in a metric space is
specified by its centre and its radius (the radius will be the
\(\epsilon\) or \(\delta\)).

### Topological spaces

*(3.23)* We want to turn the previous lemma into a definition (that a
map is continuous if the preimages of open sets are open). For that,
we need to work in a context where the notion of an ``open set'' is
defined. The most general context where open sets make sense is that
of a *topological space*.

*(3.40)* A topology, \(T\), on a set \(X\) is a collection of
subsets of \(X\) satisfying some requirements (below). The sets in
\(T\) will be called the *open sets* of the topology, and the
requirements below are the bare minimum we need in order for these
to behave in more-or-less the way we expect from the theory of
metric spaces:

- \(\emptyset\in T\), \(X\in T\),
*(5.10)*Arbitrary unions of sets in \(T\) are still in \(T\), i.e. given a collection \(\{U_i\}_{i\in I}\subset T\) where \(I\) is an indexing set then \(\bigcup_{i\in I}U_i\in T\). Arbitrary means that \(I\) can be any set (infinite, uncountable, anything). This is because unions of open sets are*always*open.*(6.42)*Finite intersections of sets in \(T\) are still in \(T\), i.e. given a finite collection \(\{U_i\}_{i\in I}\subset T\) where \(I\) is a finite indexing set then \(\bigcap_{i\in I} U_i\in T\). Only finite intersections are assumed open because, for example, it is possible to find infinite collections of open sets in metric spaces (e.g. \(\mathbf{R}\)) such that the intersection fails to be open.

*(8.00)* A *topological space* is a set \(X\) equipped with a
topology \(T\) on \(X\).

*(8.42)* The same set \(X\) can have many different topologies.

*(9.05)* For any set \(X\), the *indiscrete topology* is the
smallest topology you could write down:
\(T=\{\emptyset,X\}\). Certainly these two subsets need to be
included in \(T\), and if you take intersections or unions of
\(\emptyset\) and \(X\) then you either get \(\emptyset\) or \(X\),
so it is a topology. But it is not a very useful topology: it has
very few open sets (we say it is *coarse*).

*(10.14)* For any set \(X\), the *discrete topology* is the largest
topology you could write down: \(T=\{\mbox{all subsets of }X\}\),
sometimes written \(T=PX\) (the *powerset of \(X\)*). This has lots
of open sets (we say it is *fine* or *refined*). You should think of
\(X\) equipped with the discrete topology as just being a disjoint
collection of points: any point is an open set.

### Continuous maps

*(12.00)* Given topological spaces \((X,S)\) and \((Y,T)\), a map
\(F\colon X\to Y\) is called *continuous* if \(F^{-1}(U)\in S\) for
all \(U\in T\) (i.e. if \(F^{-1}(U)\) is open in \(X\) for all open
sets \(U\subset Y\)).

*(13.10)* Given topological spaces \(X,Y,Z\) and continuous maps
\(F\colon X\to Y\) and \(G\colon Y\to Z\), the composition \(G\circ
F\colon X\to Z\) is continuous.

*(14.43)* Let \(U\subset Z\) be an open set. Its preimage under
\(G\circ F\) is \((G\circ F)^{-1}(U)=F^{-1}(G^{-1}(U))\). Since
\(G\) is continuous and \(U\) is open, \(G^{-1}(U)\) is open. Since
\(F\) is continuous and \(G^{-1}(U)\) is open, \(F^{-1(}G^{-1}(U))\)
is open. Therefore \((G\circ F)^{-1}(U)\) is open and \(G\circ F\)
is continuous.

In the next video, we will see more examples of topological spaces and ways to construct new topological spaces out of old ones.

## Pre-class questions

- Can you think of an infinite collection \(U_i\), \(i\in\mathbf{N}\), of open sets in \(\mathbf{R}\) such that \(\bigcap_{i\in\mathbf{N}}U_i\) is not open?
- I gave no proof for the first lemma (a map \(F\) between metric spaces is continuous if and only if \(F^{-1}(U)\) is open for every open set \(U\)) because I was assuming you have seen it before. Either by thinking for yourself or by looking at your old notes (from Analysis 4), remind yourself how to prove this.

## Navigation

- Next video:
**2.02 Bases, metric and product topologies**. - Index of all lectures.