## Book review: Beautiful Symmetry

[Disclosure of Material Connection: MIT Press kindly provided me with a free review copy of this book. I was not required to write a positive review. The opinions expressed are my own.]

Beautiful Symmetry: A Coloring Book about Math
Alex Berke
The MIT Press, 2020

Alex Berke’s new book, Beautiful Symmetry, is an introduction to basic concepts of group theory (which I’ve written about before) through symmetries of geometric designs. But it’s not the kind of book in which you just read definitions and theorems! First of all, it is actually a coloring book: the whole book is printed in black and white on thick matte paper, and the reader is invited to color geometric designs in various ways (more on this later). Second, it also comes with a web page of interactive animations! So the book actually comes with two different modes in which to interactively experience the concepts of group theory. This is fantastic, and exactly the kind of thing you absolutely need to really build a good intuition for groups.

The book is not, nor does it claim to be, a comprehensive introduction to group theory; it focuses exclusively on groups that arise as physical symmetries in two dimensions. It first motivates and introduces the definitions of groups and subgroups, using 2D point groups (cyclic groups $C_n$ and dihedral groups $D_n$) and then going on to catalogue all frieze and wallpaper groups (all the possible types of symmetry in 2D), which I very much enjoyed learning about. I had heard of them before but never really learned much about them.

One thing I really like is the way Berke characterizes subgroups by means of breaking symmetry via coloring; I had never really thought about subgroups in this way before. For example, consider a simple octagon:

An octagon has the symmetry group $D_8$, meaning that it has rotational symmetry (by $1/8$ of a turn, or any multiple thereof) and also reflection symmetry (there are $8$ different mirrors across which we could reflect it).

However, if we color it like this, we break some of the symmetry:

$1/8$ turns would no longer leave the colored octagon looking the same (it would switch the blue and white triangles). We can now only do $1/4$ turns, and there are only $4$ mirrors, so it has $D_4$ symmetry, the same as a square. In particular, the fact that we can color something with $D_8$ symmetry in such a way that it turns into $D_4$ symmetry tells us that $D_4$ is a subgroup of $D_8$. Likewise we could color it so it only has $D_2$ symmetry (we can rotate by $1/2$ turn, or reflect across two different mirrors; left image below) or $D_1$ symmetry (there is only a single mirror and no turns; right below). Hence $D_2$ and $D_1$ are also subgroups of $D_8$.

Along different lines, we could color it like this, so we can still turn it by $1/8$ but we can no longer reflect it across any mirrors (the reflections now switch blue and white):

This symmetry group (8 rotations only) is called $C_8$; we have learned that $C_8$ is a subgroup of $D_8$. Likewise we could color it in one of the ways below:

yielding the subgroups $C_4$, $C_2$, and $C_1$. Note $D_1$ and $C_2$ are abstractly the same: both feature a single symmetry which is its own inverse (a mirror reflection in the case of $D_1$ and a $180^\circ$ rotation in the case of $C_2$), although geometrically they are two different kinds of symmetry. $C_1$ is also known as the “trivial group”: the colored octagon on the right has no symmetry.

Anyway, I really like this way of thinking about subgroups as “breaking” some symmetry and seeing what symmetry is left. If you like coloring, and/or you’d like to learn a bit about group theory, or read a nice presentation and explanation of all the frieze and wallpaper groups, you should definitely check it out!

Posted in books, review | Tagged , , , , , | 1 Comment

## Hypercube offsets

In my previous posts, each drawing consisted of two offset copies of the previous drawing. For example, here are the drawings for $n=3$ and $n=4$:

You can see how the $n=4$ drawing contains an exact copy of the $n=3$ drawing, plus another copy with the fourth red element added to every set. The second copy is obviously offset by one unit in the vertical direction, because every set gained one element and hence moved up one row. But how far is the second copy offset horizontally? Notice that it is placed in such a way that its second row from the bottom fits snugly alongside the third row from the bottom of the original copy.

In this case we can see from counting that the second copy is offset five units to the right of the original copy. But how do we compute this number in general?

The particular pattern I used is that for even $n$, the second copy of the $(n-1)$-drawing goes to the right of the first copy; for odd $n$ it goes to the left. This leads to offsets like the following:

Can you see the pattern? Can you explain why we get this pattern?

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## Post without words #31

Posted in posts without words | Tagged , , , , , , , | 2 Comments

## A few words about PWW #30

A few things about the images in my previous post that you may or may not have noticed:

• As several commenters figured out, the $n$th diagram (starting with $n = 1$) is showing every possible subset a set of $n$ items. Two subsets are connected by an edge when they differ by exactly one element.
• All subsets with the same number of elements are aligned horizontally.
• Each diagram is made of two copies of the previous diagram—one verbatim, and one with a new extra element added to every subset, with edges connecting corresponding subsets in the two copies. Do you see why this makes sense? (Hint: if we want to list all subsets of a set, we can pick a particular element and break them into two groups, one consisting of subsets which contain that element and one consisting of subsets which don’t.)
• As commenter Denis pointed out, each diagram is a hypercube: the first one is a line (a 1-dimensional “cube”), the second is a square, the third is a cube, then a 4D hypercube, and so on. (On my own computer I rendered them up to $n=8$ but it gets very hard to see what’s going on after $5$.)
• Each subset can also be seen as corresponding to a bitstring specifying which elements are in the set. A dot corresponds to a 1, and an empty slot to a 0. So another way to think of this is the graph of all bitstrings of length $n$, where two bitstrings are connected by an edge if they differ in exactly one bit.
• Thinking of it as bitstrings makes it clearer why we get hypercubes: each bit corresponds to a dimension. So for example for the 3D case you could think of the three bits as corresponding to back/front, left/right, and down/up.
• I drew something similar to this many years ago, in Post without words #2. The big difference is that it recently occurred to me how to lay out the nodes recursively to highlight the hypercube structure, so they don’t all just smoosh together on each line.
• There was actually some interesting math involved in figuring out the horizontal offsets to use for the subset nodes; perhaps I’ll write about that in another post!
Posted in posts without words | Tagged , , , , , , , | 4 Comments

## Post without words #30

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## The First Six Books of the Elements of Euclid, by Oliver Byrne (Taschen)

It’s also surprisingly inexpensive—only \$20! You can get a copy through Taschen’s website here.

In a similar vein, the publisher Kronecker Wallis decided to finish what Byrne started, creating a beautifully designed, artistic version of all 13 books of Euclid. (Byrne only did the first six books; I am actually not sure whether because that’s all he intended to do, or because that’s all he got around to.) Someday I would love to own a copy, but it costs 200€ (!) so I think I’m going to wait a bit…

Posted in books, geometry, pictures | Tagged , , , , , , | 3 Comments

## do go no to

dodo
do go on
do no harm
do to others
go do likewise
go-go music
go no further
goto considered harmful
no can do
it’s a no go
a big no-no
say no to drugs
what to do
here or to go
to no avail
I don’t think we’re in Kansas anymore, Toto

English is strange.

Posted in counting, humor | Tagged , , | Comments Off on do go no to

## Book review: Tales of Impossibility

[Disclosure of Material Connection: Princeton Press kindly provided me with a free review copy of this book. I was not required to write a positive review. The opinions expressed are my own.]

Tales of Impossibility: The 2000-Year Quest to Solve the Mathematical Problems of Antiquity
David S. Richeson
Princeton University Press, 2019

Let me get right to the point: this was hands-down my favorite math book that I read this year. If you don’t already have a copy, you should stop reading this post right now and go buy one! Go on, you’ll thank me. Need more convincing? Read on.

The book is focused around the four “problems of antiquity”: squaring the circle (i.e. constructing a square with the same area as a given circle), angle trisection, doubling the cube (constructing the side length of a cube double the volume of a given cube), and constructing regular $n$-gons. The “problem” of each is to carry out the required construction using only a compass and straightedge (a set of tools that is probably familiar to most readers from some point in their mathematical education). As Richeson so ably relates, these problems inspired all sorts of advances in mathematics over thousands of years—even though (because?) all were eventually proved impossible in general: Wantzel (angle trisection, doubling the cube, regular $n$-gons) and Lindemann (squaring the circle) gave the final, definitive proofs, but both built on top of a great deal of mathematics that came before them. Each new player in the story added layer upon layer of understanding over thousands of years.

First and foremost, I am amazed at the incredible amount of historical and mathematical background research that Richeson obviously did for this book, and the way he intertwines mathematics and history into a compelling story. Stereotypically, a book of mathematical history runs a double risk of being dry: too much unmotivated historical or mathematical detail can put anyone to sleep. Richeson deftly avoids this trap, and his book exudes human warmth. But it doesn’t skimp on details either; I learned a great deal of both history and mathematics. In many cases (such as with many of the purely geometric arguments) proofs are included in full detail. In other cases (such as in the discussion of irreducible polynomials), some mathematical details are omitted. Richeson has a good nose for sniffing out the most elegant way to present a proof, and also for knowing when to omit things that would bog down the story too much.

Alternating with the “regular” chapters, Richeson includes a number of “tangents”, each one a short, fascinating glimpse into some topic which is related to the previous chapter but isn’t strictly necessary for driving the story forward (e.g. toothpick constructions, Crockett Johnson, origami, the Indiana pi bill, computing digits of pi, the tau vs pi debate, etc.). Even though none of them are strictly necessary, taken as a whole these “tangent” chapters do a lot to round out the story and give a fuller sense of the many explorations inspired by the problems of antiquity.

In addition to the many mathematical and historical details I learned from the book, I also took away a more fundamental insight. I had always thought of “compass and straightedge” constructions as being rather arbitrary: these are the tools the Greeks happened to choose, and so now we are stuck in a rut of thinking about geometrical constructions using these tools—or so I thought. However, it turns out that they are not quite so arbitrary after all: there are many different sets of tools that lead to exactly the same set of constructible things (there is even some interesting history here, as mathematicians figured out what it should even mean to say that you can “construct the same things” with different tools, leading to definitions of constructible points and constructible numbers). For example, toothpicks, a straightedge and “rusty” compass, a straightedge and a single circle, a compass by itself, or a “thick” straightedge by itself (with two given starting points), all can perform exactly the same set of constructions as a traditional straightedge and compass. And as we learned in later centuries, the constructible numbers have a nice algebraic characterization as well: a point $(x,y)$ is constructible with straightedge and compass if and only if $x$ and $y$ can be described using the four arithmetic operations and square roots. In other words, the set of constructible points seems to be a robust set that can be described in many equivalent ways; it is a more fundamental notion than the arbitrary-sounding description in terms of compass and straightedge would seem to imply. I don’t think I would have been able to understand this without someone like Richeson to do a lot of research and then put all the details together into a coherent story.

[It reminds me of a similar phenomenon with computation: for example, the description of a Turing machine seems rather arbitrary, and in some ways it is, but it turns out that many different models of computation (Turing machines, multi-tape Turing machines, lambda calculus, Post canonical systems, RAM machines…) all yield the same set of computable functions, and so the arbitrary-seeming choice is actually describing something more fundamental.]

In the same way, I thought the problems of antiquity themselves were somewhat arbitrary; but they were famous because they are hard, and it turns out they were hard precisely because they were really getting at the heart of some fundamentally deep ideas. So the fact that they inspired so much rich mathematics is no mere accident of history. One gets the sense that if we ever encounter intelligent life elsewhere in the universe, we may find that they struggled with the same mathematical problems—in very different forms, to be sure, but recognizably the same on a fundamental level.

Anyway, I’ve written more than enough at this point, and I think you get the idea: I thoroughly enjoyed this book, learned a lot from it, and highly recommend it!

Posted in books, review | | 4 Comments

## A new counting system

0 = t__ough
1 = t_rough
2 = th_ough
3 = through

So, for example, $458 =$ trough through tough though though.

English is so strange.

Posted in arithmetic, counting | Tagged , , , | 1 Comment

## A simple proof of the quadratic formula

If you’re reading this blog you have probably memorized (or used to have memorized) the quadratic formula, which can be used to solve quadratic equations of the form

$ax^2 + bx + c = 0.$

But do you know how to derive the formula? Usually the derivation is presented via completing the square and it involves some somewhat messy algebra (not to mention the idea of “completing the square” itself).

My colleague Gabe Ferrer recently brought to my attention a remarkable new paper by Po-Shen Loh, A Simple Proof of the Quadratic Formula. This paper is remarkable for several reasons: first of all, it’s remarkable that anyone could discover anything new about the quadratic formula; it’s also remarkable for a research mathematician to publish something about elementary mathematics. (But Po-Shen Loh is not your average research mathematician either; he does lots of really cool work making mathematics more accessible for all kinds of learners.) I’m going to explain the basic idea but I highly recommend actually reading the paper, which not only explains the ideas but also does a great job putting everything in proper historical context. Loh has also made a whole web page dedicated to explaining the ideas, with a video, worked examples, etc.; it’s definitely worth taking a look!

# The Setup

Suppose we have a quadratic equation we want to solve,

$x^2 + bx + c = 0.$

To make things simpler, we’ll assume that $x^2$ has a coefficient of $1$. (If we have a quadratic equation with some other coefficient $ax^2$, we can always divide everything by $a$ first.)

Now imagine we knew how to factor the quadratic. Then we could rewrite the equation into the form

$(x - r)(x - s) = 0$

which would imply that $x = r$ and $x = s$ are the two solutions. If we multiply out the above factorization (using, you know, “FOIL”), we get

$x^2 - (r+s)x + rs = 0$

which means we’re looking for values $r$ and $s$ whose product is $c$ and whose sum is $-b$.

So far, so good; everyone learns this much in high school algebra. The way one usually goes about factoring quadratic polynomials is to make informed guesses for values of $r$ and $s$ and check whether their sum and product give the right coefficients.

# The Insight

The key insight at this point, however, is that we don’t actually have to guess! Starting from $r + s = -b$, let’s divide both sides by $2$:

$\displaystyle \frac{r+s}{2} = -\frac{b}{2}$

The left-hand side is the average of $r$ and $s$, which lies halfway in between them on the number line. Let’s use $z$ to denote the distance from $r$ to $-b/2$. Since $-b/2$ is halfway in between $r$ and $s$, $z$ must also be the distance from $-b/2$ to $s$. So we can write $r$ and $s$ in the form

$r,s = -b/2 \pm z$

Now, we know their product has to be $c$, and multiplying them is particularly easy because we get a difference of squares:

$\displaystyle c = rs = \left(-\frac{b}{2} + z \right) \left(-\frac{b}{2} - z \right) = \left(-\frac{b}{2} \right)^2 - z^2$

Now solving for $z$ is easy; just move $z^2$ to one side of the equation by itself and take the square root:

$\displaystyle z = \pm \sqrt{\frac{b^2}{4} - c}$

That means the solutions are

$\displaystyle r,s = -\frac{b}{2} \pm z = -\frac{b}{2} \pm \sqrt{\frac{b^2}{4} - c}.$

If you like, you can use the same method starting from $ax^2 + bx + c = 0$ to derive the usual quadratic formula including an arbitrary value of $a$, although the required algebra gets a bit messier.

# Using it in practice

One particularly nice thing about this derivation is that it corresponds to a simple algorithm for solving an arbitrary quadratic equation $x^2 + bx + c = 0$, so there’s no need to memorize a formula at all:

1. Note that the two solutions must add up to $-b$, so their average is half of $-b$, and hence they can be written as $-b/2 \pm z$.
2. Write down the equation $(-b/2 + z)(-b/2 - z) = b^2/4 - z^2 = c$, and solve for $z$.
3. The solutions are $-b/2 + z$ and $-b/2 - z$.

Of course if you need to solve something of the form $ax^2 + bx + c = 0$, you can add an extra step to divide through by $a$ first.

And that’s it! I really hope this new method will make its way into classrooms around the world; Loh makes the argument (and I agree) that it really is much easier for early algebra learners to grasp. And again, I really encourage you to go look at Loh’s web page to read more, especially about the historical context: at what point in human history could someone have come up with this idea? And why didn’t they? (Or if they did, why did we forget?) All this and more are in the original paper, which is a really fascinating and accessible read.

Posted in algebra, proof | Tagged , , , , | 3 Comments