# The Most Obvious Secret in Mathematics

Yes, I agree. The title for this post is a little pretentious. It's certainly possible that there are other mathematical secrets that are more obvious than this one, but hey, I got your attention, right? Good. Because I'd like to tell you about an overarching theme in mathematics - a mathematical mantra, if you will. A technique that mathematicians use all the time to, well, do math.

I'm calling it a 'secret' because until recently, I've rarely (if ever?) heard it stated explicitlyThis suggests to me that it's one of those things that folks assume you'll just eventually pick up. Hopefully. Like some sort of unspoken rule of mathematics. But a few weeks ago while chatting with my advisor*, I finally heard this unspoken rule uttered! Explicitly. Repeatedly, in fact. And at that time I realized it needs to be ushered further into the spotlight. Today's post, then, is my invitation to you to come listen in on that conversation.

So enough with the chit-chat! What's the secret? Here 'tis:

## A mathematical object is determined by its relationships to other objects.

Practically speaking, this suggests that

## an often fruitful way to discover properties of an object is NOT to investigate the object itself, but rather to study the collection of maps to or from the object.

Or to be a little less formal,

## you can learn a lot about an object by studying its interactions with other things.

By "object" I mean things like sets or groups or measurable spaces or vector spaces or topological spaces or.... And by "maps" I mean the appropriate version of 'function':  functions, group homomorphisms, measurable functions, linear transformations, continuous functions, and so on.

So now do you see why I'm calling this an obvious secret? We students have been using this technique - though perhaps unknowingly - since we were mathematical infants! We've known about functions since grade school. We've labored over properties of real-valued functions and their (anti)derivatives throughout Calculus. We became well-acquainted with linear transformations and their corresponding matrices in linear algebra. We battled with homomorphisms during the first week of undergrad abstract algebra. We finally learned the real definition of a continuous map in point-set topology. The list goes on and on.

See how pervasive this idea is? It's obvious!

And that's my point.

Because have you ever stopped to really think about it?

At first glance, perhaps it seems a little odd that "best" way to study an object is to divert your attention away from the object and focus on something else. But we do this all the time. Take people-watching, for instance. You can learn a lot about a person simply by looking at how they relate to the folks around them. And the same is true in mathematics.

I've hinted at this theme briefly in a previous post, but I'd like to list a few examples to further convince you. Keep in mind, though, that this is a philosophy that permeates throughout all of mathematics. So what I'm sharing below is peanuts compared to what's out there. But I hope it's enough to illustrate the idea.

## In analysis...

One word: sequences! Recall (or observe) that a sequence $\{x_n\}=\{x_1,x_2,\ldots\}$ is - yes, a long list of numbers but ultimately - a function $\phi:\mathbb{N}\to\mathbb{R}$, where $x_n=\phi(n)$. By using sequences to 'probe' the real line $\mathbb{R}$, we learn that $\mathbb{R}$ has no "holes" - if you point your infinitesimally small finger anywhere on the real line, you'll always land on a real number. This property of $\mathbb{R}$ is called completeness, and it is investigated by special types of sequences called Cauchy sequences. Another good example is curvature. Need to measure how much a curve or surface is bending in space? Then you'll want to think about second derivatives which, assuming the curve/surface is "nice enough," are themselves continuous functions to $\mathbb{R}$!**

## In group theory...

By looking at homomorphisms from arbitrary groups to special types of groups called symmetric groups, we discover that the raison d'être of a group is to shuffle things around! This is captured in Cayley's Theorem, a major result in group theory, which says that every group is isomorphic to a group of permutations or, less formally, a group is to math what a verb is to language. In fact, this (not the theorem, per se, but the idea) is historically how groups were first understood and is precisely what motivated Galois to lay down the foundations of the discipline of mathematics that bears his name. You might recall that we've chatted previously about the verb-like behavior of groups in this non-technical introduction to Galois theory.

## In topology...

Want to know if your topolgical space $X$ is connected? Just check that any continuous map from it to $\{0,1\}$ is constant! Want to determine how many 'holes' $X$ has? Study continuous functions from the circle, $S^1$, into it! This leads to the fundamental group, $\pi_1$. Want to know many higher dimensional 'holes' there are? Look at continuous functions from the $n$-sphere into it! This leads to the higher homotopy groups, $\pi_n$. Want to know what the topology on any given space is? Simply look at the collection of continuous functions from it to a little two-point space! In fact, this last example is really quite paradigmatic, and I'd like to elaborate a bit more. So stay tuned for next time!

In the mean time, how many examples of today's not-so-secret secret can you think of? I'd love to hear 'em. Let me know in the comments below!

*Yes! I have an advisor now! And since my written qualifying exams are out of the way, the next thing on my to-do list is passing the oral qual. I've also picked up a teaching assignment this year. For both of these reasons, blogging has been - and may continue to be - a little bit slow. But although my posts may become less frequent, I'm hoping the content will be richer. I'm almost positive they'll be more topology/category-flavored, too.

** This is really a statement about differential geometry rather than analysis, for it generalizes nicely for things called manifolds. In fact, the whole premise behind differential geometry is a great example of today's theme. The idea is that globally, a manifold $M$ may be so complicated and wonky that we don't have many tools to probe it with. But - following the old adage, How do you eat an elephant? One bite at a time. - the impossible becomes possible if we just consider $M$ little patches at a time. Why? Because locally manifolds look exactly like Euclidean space, $\mathbb{R}^n$. (Take the earth, for example. Even though it's round, it looks flat locally.) And since we have tons of tools at our disposal in $\mathbb{R}^n$ (like calculus!), we can apply them to the little patches of our manifold too.

# Resources for Intro-Level Graduate Courses

In recent months, several of you have asked me to recommend resources for various subjects in mathematics. Well, folks, here it is! I've finally rounded up a collection of books, PDFs, videos, and websites that I found helpful while studying for my intro-level graduate courses. Now before we jump in, let me preface the list by saying

it's FAR from comprehensive!

Most of the items below are accessible at the advanced undergraduate/early graduate level, so if you're looking for more advanced resources, you might be disappointed. Also, if you read my ramble about qualifying exams you'll know that my quals were in algebra, real analysis, and topology. So naturally my list is a little biased. But, hey, I figured a biased list is better than no list at all! In particular, I have very few items under differential geometry. That's simply because I haven't taken the course. But I am taking it this year, so I'll update this post as the months go on. (Bear with me, friends!)

And I've undoubtedly omitted several great resources either because I forgot to include them or I'm simply unaware of their existence. For this reason, I strongly encourage you to share your recommendations in the comments below! What books/links/etc. do you think are gems? Let me know! I'd love to hear.

Lastly, you'll see that I've included 'my favorite' books below. These aren't necessarily the most popular or 'the best.' They are simply the ones I enjoyed reading because of their accessibility and intuitive explanations.

And now, without further ado, I present to you The List!

## Books

### My favorite

• Basic Abstract Algebra by P. B. Bhattacharya, S. K. Jain and S. R. Nagpaul (I especially found the discussion on modules over a PID, Smith normal form, and rational canonical form helpful. See chapters 20-21.)

• Shameless plug: I once wrote a non-technical introduction to Galois theory. It's one of the blog's more popular posts!
• Keith Conrad has a goldmine of expositions (on pretty much everything) on his website. The reading is fairly easy, and there are plenty of worked-out examples.
• Here is a helpful collection of videos by Matthew Salome on Galois theory (field extensions, minimal polynomials, Galois groups, field automorphisms, etc.)
• CUNY's own MathDoctorBob also has a nice collection of videos on everything from the definition of a group to Sylow's Theorems to polynomial rings to Galois theory!
• What's the big deal with conjugation in group theory? I really like the accepted answer on this StackExchange post.

## Books

### My favorite:

• Real Analysis by N. L. Carothers (This is really a fantastic book (see my review here)! I found the discussion on absolute continuity especially illuminating. See chapter 20.)

## Books

### My favorite

• The Shape of Space by Jeffrey R. Weeks. (Filled with pictures and easy-to-read explanations, this book is a great resource for those first learning algebraic topology. I've written a review about it here!)

## Books

### My favorite

• Visual Complex Analysis by Tristan Needham (The intended audience for this book is undergraduate students in math, physics, and engineering, but it's so wonderfully written that I'm compelled to recommend it. I've also written a review about it here.)

• Shameless plug: Computations got you in the blues? Here's a motivational post for why we care about the automorphisms of the unit disc (upper half plane), complex plane, and Riemann sphere.
• Here's a video lecture series on advanced complex analysis: part 1 (Rouche's theorem, the open mapping theorem, Schwarz's Lemma, the Riemann mapping theorem, etc.) and part 2 (the Casorati-Weierstrass theorem, Picard's theorems, the Arzelà-Ascoli theorem, Montel's theorem etc.)

• coming soon!

# A Ramble About Qualifying Exams

Today I'm talking about about qualifying exams! But no, I won't be dishing out advice on preparing for these exams. Tons of excellent advice is readily available online, so I'm not sure I can contribute much that isn't already out there. However, it's that very web-search that has prompted me to write this post. You see, before I started graduate school I had heard of these rites-of-passage called the qualifying exams. And to be frank, I thought they sounded terrifying....

# Automorphisms of the Riemann Sphere

This is the last in a four-part series in which we prove that the automorphisms of the unit disc, upper half plane, complex plane, and Riemann sphere each take on a different form. Today our focus is on the Riemann sphere.

# Automorphisms of the Complex Plane

This is part three of a four-part series in which we prove that the automorphisms of the unit disc, upper half plane, complex plane, and Riemann sphere each take on a different form. Today our focus is on the complex plane.

# Automorphisms of the Upper Half Plane

This is part two of a four-part series in which we prove that the automorphisms of the unit disc, upper half plane, complex plane, and Riemann sphere each take on a different form. Today our focus is on the upper half plane.

# Automorphisms of the Unit Disc

This is part one of a four-part series in which we prove that the automorphisms of the unit disc, upper half plane, complex plane, and Riemann sphere each take on a different form. Today our focus is on the unit disc.

# Three Important Riemann Surfaces

In this post we ramble on about Riemann surfaces, the uniformization theorem, universal covers, and two secret (or not-so-secret!) techniques that mathematicians use to study a given space. Our intent is to provide motivation for an upcoming mini-series on the automorphisms of the unit disc, upper half plane, complex plane, and Riemann sphere.

# Good Reads: The Princeton Companion to Mathematics

Next up on Good Reads: The Princeton Companion to Mathematics, edited by Fields medalist Timothy GowersThis book is an exceptional resource! With over 1,000 pages of mathematics explained by the experts for the layperson, it's like an encyclopedia for math, but so much more. Have you heard about category theory but aren't sure what it is? There's a chapter for that! Seen the recent headlines about the abc conjecture but don't know what it's about? There's a chapter for that! Need a crash course in general relativity and Einstein's equations, or the P vs. NP conjecture, or C*-algebras, or the Riemann zeta function, or Calabi-Yau manifolds? There are chapters for all of those and more

# Clever Homotopy Equivalences

You know the routine. You come across a topological space X and you need to find its fundamental group. Unfortunately, X is an unfamiliar space and it's too difficult to look at explicit loops and relations. So what do you do? You look for another space Y that is homotopy equivalent to X and whose fundamental group is much easier to compute. And voila! Since X and Y are homotopy equivalent, you know that the fundamental group of X is isomorphic to the fundamental group of Y. Mission accomplished.

Below is a list of some homotopy equivalences which I think are pretty clever and useful to keep in your back pocket for, say, a qualifying exam or some other pressing topological occasion.

# Snippets of Mathematical Candor

A while ago I wrote a post in response to a great Slate article reminding us that math - like writing - isn't something that anyone is good at without (at least a little!) effort. As the article's author put it, "no one is born knowing the axiom of completeness." Since then, I've come across a few other snippets of mathematical candor that I found both helpful and encouraging. And since final/qualifying exam season is right around the corner, I've decided to share them here on the blog for a little morale-boosting.