Construction of Riemann surfaces as quotients

There is a theorem in Chapter 4 Section 5 of Schlag’s complex analysis text. I went through it a month ago, but only half understood it, and it is my hope that passing through it again, this time with writeup, will finally shed light, after having studied in detail some typical examples of such Riemann surfaces, especially tori, the conformal equivalence classes of which can be represented by the fundamental region of the modular group, which arise from quotienting out by lattices on the complex plane, as well as Fuchsian groups.

In the text, the theorem is stated as follows.

Theorem 4.12.  Let \Omega \subset \mathbb{C}_{\infty} and G < \mathrm{Aut}(\mathbb{C}_{\infty}) with the property that

  • g(\Omega) \subset \Omega for all g \in G,
  • for all g \in G, g \neq \mathrm{id}, all fixed points of g in \mathbb{C}_{\infty} lie outside of \Omega,
  • for all K \subset \Omega compact, the cardinality of \{g \in G | g(K) \cap K \neq \phi\} is finite.

Under these assumptions, the natural projection \pi : \Omega \to \Omega / G is a covering map which turns \Omega/G canonically onto a Riemann surface.

The properties essentially say that the we have a Fuchsian group G acting on \Omega \subset \mathbb{C}_{\infty} without fixed points, excepting the identity. To show that quotient space is a Riemann surface, we need to construct charts. For this, notice that without fixed points, there is for all z \in \Omega, a small pre-compact open neighborhood of z denoted by K_z \subset \Omega, so that

g(\overline{K_z} \cap \overline{K_z}) = \emptyset \qquad \forall g \in G, g \neq \mathrm{id}.

So, in K_z no two elements are twice represented, which mean the projection \pi : K_z \to K_z is the identity, and therefore we can use the K_zs as charts. The gs as Mobius transformations are open maps which take the K_zs to open sets. In other words, \pi^{-1}(K_z) = \bigcup_{g \in G} g^{-1}(K_z) with pairwise disjoint open sets g^{-1}(K_z). From this, the K_zs are open sets in the quotient topology. In this scheme, the gs are the transition maps.

Finally, we verify that this topology is Hausdorff. Suppose \pi(z_1) \neq \pi(z_2) and define for all n \geq 1,

A_n = \left\{z \in \Omega | |z-z_1| < \frac{r}{n}\right\} \subset \Omega

B_n = \left\{z \in \Omega | |z-z_2| < \frac{r}{n}\right\} \subset \Omega

where r > 0 is sufficiently small. Define K = \overline{A_1} \cup \overline{B_1} and suppose that \pi(A_n) \cap \pi(B_n) \neq \emptyset for all n \geq 1. Then for some a_n \in A_n and g_n \in G we have

g_n(a_n) \in B_n \qquad \forall n \geq 1.

Since g_n(K) \cap K has finite cardinality, there are only finitely many possibilities for g_n and one of them therefore occurs infinitely often. Pass to the limit n \to \infty and we have g(z_1) = z_2 or \pi(z_1) = \pi(z_2), a contradiction.

 

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Nostalgia

I was just looking at some baseball statistics, starting from Alfonso Soriano, prompted by my receiving mail from another of the same surname. I remember I was a keen baseball fan in grade school, and would watch almost every single game. I didn’t like studying at all, and I was even the kid who didn’t do his homework. In third grade, there was this animal project, where we had to write some report on some Australian animal, and I was the only kid who didn’t complete it by the deadline (in fact, I barely did anything). I got a low grade on it at the end for finishing like two weeks after, and I was super embarrassed about that. Surely, my parents weren’t very happy with me. Amazing how even to this day I still know the names of many of the best players from back in that day. Ichiro, Barry Bonds (steroids), Mark McGuire, Derek Jeter, Alex Rodriguez, David Ortiz, etc.

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Русская практика

I wrote the following over a month ago. I was quite pleased, because it was the first time I actually spoke Russian to the point of being able to carry on a passable conversation. Of course, English words were interspersed here and there, but it wasn’t too bad. I was excited enough afterwards that I wrote this piece, which almost certainly has some errors, which I expect to pick out over time. Not to toot my own horn too much, but this is not a bad result after a little over a year of reading and Facebook pinging in it off and on, when I feel too lazy to do anything more productive. Russian learning wise, it was also awesome to meet online this guy, an undergraduate at MIT in physics Индийского присхождения, a child prodigy who taught himself Russian in high school, who also spent a summer in St. Petersburg if I remember correctly. He is obviously much better at it than I am, but I expect to catch up soon. It will only become easier and easier over time. Maybe I can even write some music with lyrics in it, eventually, who knows.

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Back to blogging

Some might have noticed that over the last some number of weeks, I privatized this blog, for reasons that one can guess. I’ve been busy, learning math. Some cool stuff about Riemann surfaces. Maybe not long after, I can understand Teichmüller theory, for which Riemann surfaces is somewhat of a precursor. Maybe not too long after that, I can even understand Calabi-Yau and Kähler–Einstein metrics. I’m more convinced now that I’m not bad at math at all, though I’m not yet back in school for real, and as for that, I don’t find most graduate students in math, who I’ve had more contact with mathematically lately, terribly inspiring. The level of interestingness of most people, even in supposedly intellectual places, is, frankly, rather disappointing.

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A revisit of the drama behind the Poincaré

I recall back in 2008, when I first cared enough to learn about mathematicians, I read a fair bit of the media articles on the proof of the Poincaré conjecture. At that time, I was clueless about math, and these mathematicians seemed to me like these otherworldly geniuses. I do remember thinking once to myself that maybe it would be kind of cool to part of that world. Except at that time, I was way too dumb, and maybe I still am. However, now I actually have some idea of what math research is about, unlike back then, when my conception of math and mathematicians was more of a naive popular one.

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Luboš Motl, and some thoughts on monopolies

I had the pleasure of reading some blog posts of Luboš Motl on present day academia. I first learned of him when I was a clueless undergrad. He seemed like this insanely smart theoretical physicist. Of course back then I was dumb and in awe of everything, so what else could I think? I know that he pissed off so many people that he was forced from resign from his tenure track position at Harvard physics in string theory. His academic work I am of course nowhere close to qualified to comment on, but people have said it’s first rate, and I’ll take their word. I even thought the guy was crazy. My very smart friend, in some online interaction with him, was scoffed off with: “You don’t understand vectors!” That guy later characterized the hypothetical combination of Luboš and this other guy I know, a PhD student in string theory, who is quite academically elitist and also so in terms of expecting good values and a fair degree of cultural/historical knowledge, as “a match made in heaven.” I also recall a commenter on Steve Hsu’s blog remark that Luboš has Aspergers syndrome or something like that. Anyhow, this time when reading the blog of Luboš, I no longer felt a sense of awe but rather a strong sense of clarity and reasonability in his thinking. He can be quite abrasive in some other contexts maybe, such as in his campaign against the climate change advocates (oh, on that I recently learned Freeman Dyson is also on the same side as Luboš on this one), but I believe it arises purely out of positive intentions on his part for the future of humanity, which many view as on a course of decline.

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Variants of the Schwarz lemma

Take some self map on the unit disk \mathbb{D}, f. If f(0) = 0, g(z) = f(z) / z has a removable singularity at 0. On |z| = r, |g(z)| \leq 1 / r, and with the maximum principle on r \to 1, we derive |f(z)| \leq |z| everywhere. In particular, if |f(z)| = |z| anywhere, constancy by the maximum principle tells us that f(z) = \lambda z, where |\lambda| = 1. g with the removable singularity removed has g(0) = f'(0), so again, by the maximum principle, |f'(0)| = 1 means g is a constant of modulus 1. Moreover, if f is not an automorphism, we cannot have |f(z)| = |z| anywhere, so in that case, |f'(0)| < 1.

Cauchy’s integral formula in complex analysis

I took a graduate course in complex analysis a while ago as an undergraduate. However, I did not actually understand it well at all, to which is a testament that much of the knowledge vanished very quickly. It pleases me though now following some intellectual maturation, after relearning certain theorems, they seem to stick more permanently, with the main ideas behind the proof more easily understandably clear than mind-disorienting, the latter of which was experienced by me too much in my early days. Shall I say it that before I must have been on drugs of something, because the way about which I approached certain things was frankly quite weird, and in retrospect, I was in many ways an animal-like creature trapped within the confines of an addled consciousness oblivious and uninhibited. Almost certainly never again will I experience anything like that. Now, I can only mentally rationalize the conscious experience of a mentally inferior creature but such cannot be experienced for real. It is almost like how an evangelical cannot imagine what it is like not to believe in God, and even goes as far as to contempt the pagan. Exaltation, exhilaration was concomitant with the leap of consciousness till it not long after established its normalcy.

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