How Likely is a Finite Group to be Hausdorff?
This is the third in a series of posts on some fun little toy problems of mine. Today’s question is one I came across while trying to come up with an example of a topological group which is not Hausdorff. Anyone familiar with topological groups knows that such examples are scarce in the literature. This is not without reason: any group is actually Hausdorff, so non-Hausdorff topological groups are particularly pathological. But how scarce really are the non-Hausdorff topological groups? In other words, how likely is a topological group to be Hausdorff?
We begin by noting that, as stated, our question is ill-posed. First, we should clarify that we are working with a fixed group and considering all possible topologies which turn it into a topological group — which we call “-topologies”. That is to say, our question is actually: given and some random -topology, how likely is it to be Housdorff? However — and this is more important — the meaning of “some random topology” is still unclear.
To make the question precise, we focus on the case where is finite. This is because a finite set can only be endowed with a finite number of distinct topologies — indeed, the power set is finite. Counting how many topologies there are in terms of the size of is a famous open problem and these numbers have only being computed for very small . Here is a plot of (the logarithm of) the first 19 of these numbers:
What is relevant to us is that this implies a finite group admits only a finite number of -topologies. We may thus consider a random variable with uniform distribution and ask: can we compute ? This is the question we will attempt to answer in this article. Since any discrete group is Hausdorff, we certainly know
What is perhaps surprising is that is precisely . In other words, the odds of being Hausdorff are as slim as they could possibly be! Indeed, it is a well known fact from the theory of finite topological spaces that any a finite Hausdorff space is discrete, so the only finite Hausdorff groups are the discrete ones. All it’s left is to compute . To do so, we investigate the closure of under a given -topology .
Since the translations by elements of are all homeomorphism, for all . It is then easy to see that
for any . Similarly, since the inversion is a homeomorphism, we find for any . Finally, for all and . This goes to show is a normal subgroup of .
There is thus a natural map
between the space of -topologies and the set of normal subgroups of . That’s all well and good, but how does any of this helps us to count -topologies? Well, the point is that, perhaps surprisingly, this map is actually a bijection.
To see that our map is injective, we remark that the collection of subsets of which are closed under a given -topology is actually determined by . Indeed,
for any which is closed under . Conversely, since is finite, any union of -cosets is closed under — a finite union of closed sets is closed. All in all, the subsets which are closed under are precisely the unions of -cosets.
This last equation also goes to show that our map is surjective: given some , the set is a natural candidate for a -topology such that . It is clear that is a topology and . With enough effort, one can also show that if and are the group multiplication and inversion, respectively, then
which goes to show that is indeed a -topology.
Finally, and thus
We conclude this article with a table of the values of for some noteworthy finite groups , which were computed with the help of the Group Explorer library. I would like to thank my dear friend Eduardo Sodré for his help with the problem. I really hope the ride was as interesting to you as it was us both! 😁
for | |
for even | |
for odd |