We’re going to talk about an interesting merger of abstract algebra and topology, namely topological groups.
Now you might be wondering, why study topological algebraic structures? Topology and abstract algebra are hard enough! Well as a matter of fact they pop up all the time. Most notably, in calculus, you were actually working with the topological field . The topology of as well as its algebraic properties were used to prove statements about differentiation and integration. There’s no better place to start than with definitions, so let’s define a topological group and introduce some of its basic properties.
The Definition of A Topological Group and Some Consequences
Definition: A topological group is a group endowed with a topology such that the group operation and inverse map
are continuous maps. Here we give the product topology. By convention, if is finite it’s given the discrete topology.
Now a nice property of groups is that we can move between any two elements in the group by translation (i.e. multiplication). We would hope that if is open, then and are open for any . This is indeed true, and to see it start by fixing . Then the map is obviously continuous. Composing with the group operation, the map is continuous. This composition is said to be a left-translation or more precisely left-translation by . Since is a continuous inverse, left-translation is a homeomorphism. There is a symmetric argument for right-translation. So, the above tells us in particular that if is open, then so is and . We’ve neglected the inverse map up until now, but it has an analogous property as the above translation maps do. It’s a continuous involution, so is a homeomorphism. Therefore if is open, then the image of under the inverse map is open, and we call it the inverse of and denote it by .
Another topological property which is important for topological groups is homogeneity. If is a topological space, then is homogeneous if for any there is a homeomorphism such that . Intuitively, this says that the topology of looks the same locally any any point . If we look at a topological group , then it’s homogeneous because if , then the left-translation map sends to . A consequence of this is that if we have a local basis at the identity , then it determines a local basis everywhere and therefore a basis for the entire topology. Moreover, homogeneity says that it suffices to verify local topological properties of only at the identity . Indeed, if we have such a property at we can then translate this property everywhere in . We will see an example of this below.
Symmetric Neighborhoods and The Equivalence of and
An interesting property of topological groups is that the and (Hausdorff) separation axioms are equivalent! In the following we will prove this statement, but we will need a small technical proposition first which is incredibly useful beyond its introduction here. This proposition is about symmetric neighborhoods. Firstly, a set is called symmetric if . In other words, the set is closed under inverses. Now for the proposition:
Proposition: Let be a topological group.
i. Every neighborhood of the identity contains a neighborhood of the identity such that .
ii. Every neighborhood of the identity contains a symmetric neighborhood of the identity.
In particular, every neighborhood of the identity contains a symmetric neighborhood of the identity such that .
proof: Let us first prove i. We may assume is open because if not, we take its interior. The restriction of the group operation is continuous, so is an open set containing of the form for some open neighborhoods and of . If we set , then is a neighborhood of such that . To prove ii note that and are both neighborhoods of the identity. Then is as well and by definition is symmetric. This is the desired open set. To prove the last statement, apply i and then ii.
Note that by our homogeneity discussion above, the proposition holds for any , not just the identity. Now that we have the technical proposition, let’s state and prove the equivalence of and .
Theorem: Let be a topological group. Then is if and only if it’s .
proof: The forward implication is clear since every space is . To prove the inverse implication, let . Since is we can find a neighborhood about the identity disjoint from , and we may assume is open by replacing it with its interior. By the proposition, there is a symmetric open neighborhood , about the identity, contained in , and such that . Again, we may assume is open by replacing it with its interior. We claim the open sets and are disjoint. If they are not, we have for some which implies , a contradiction. Hence is .
It’s a very easy exercise, that I leave to you, to show these equivalent statements are further equivalent to the conditions of the identity being closed and every singleton of being closed. It’s also a fun exercise to show that the general linear group over the reals is a topological group with respect to matrix multiplication and the Euclidean topology (here you view matrices as dimensional vectors in ), and that its special linear group is a closed subgroup. I leave this to you as well if you want a slightly more difficult exercise. That’s all, thank you for reading!
Fourier Analysis on Number Fields – Dinakar Ramakrishnan, Robert J. Valenza