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Posted on March 22, 2023 by Brent

Last week I made a mathematics worksheet for my 8-year-old son, whose school is closed due to the coronavirus pandemic. I’m republishing it here so others can use it for similar purposes.

Figurate numbers and forward differences

There are lots of further directions this could be taken but I’ll leave that to you and your kids. I tried to create something that was conducive to open-ended exploration rather than something that had a single particular goal in mind.

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green加速器下载官网 March 17, 2023 by Brent

You have a function $f : A \to B$ and want to prove it is a bijection. What can you do?

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A bijection is defined as a function which is both one-to-one and onto. So prove that $f$ is one-to-one, and prove that it is onto.

This is straightforward, and it’s what I would expect the students in my Discrete Math class to do, but in my experience it’s actually not used all that much. One of the following methods usually ends up being easier in practice.

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If $A$ and $green加速器下载$ are finite and have the same size, it’s enough to prove either that $f$ is one-to-one, or that $f$ is onto. A one-to-one function between two finite sets of the same size must also be onto, and vice versa. (Of course, if $A$ and $B$ don’t have the same size, then there can’t possibly be a bijection between them in the first place.)

Intuitively, this makes sense: on the one hand, in order for $f$ to be onto, it “can’t afford” to send multiple elements of $green加速器下载$ to the same element of $B$ , because then it won’t have enough to cover every element of $B$ . So it must be one-to-one. Likewise, in order to be one-to-one, it can’t afford to miss any elements of $B$ , because then the elements of $green加速器下载官网$ have to “squeeze” into fewer elements of $green加速器下载$ , and some of them are bound to end up mapping to the same element of $B$ . So it must be onto.

However, this is actually kind of tricky to formally prove! Note that the definition of “ $green加速器下载$ and $green加速器下载官网$ have the same size” is that there exists some bijection $g : A \to B$ . A proof has to start with a one-to-one (or onto) function $green加速器下载$ , and some completely unrelated bijection $g$ , and somehow prove that $f$ is onto (or one-to-one). Also, a valid proof must somehow account for the fact that this becomes false when $green加速器下载官网$ and $B$ are infinite: a one-to-one function between two infinite sets of the same size need not be onto, or vice versa; we saw several examples in my previous post, such as $f : \mathbb{N} \to \mathbb{N}$ defined by $f(n) = 2n$ . Although tricky to come up with, the proof is cute and not too hard to understand once you see it; I think I may write about it in another post!

Note that we can even relax the condition on sizes a bit further: for example, it’s enough to prove that $f$ is one-to-one, and the finite size of $A$ is greater than or equal to the finite size of $green加速器下载官网$ . The point is that $green加速器下载$ being a one-to-one function implies that the size of $A$ is less than or equal to the size of $B$ , so in fact they have equal sizes.

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One can also prove that $green加速器下载官网$ is a bijection by showing that it has an inverse: a function $g : B \to A$ such that $g(f(a)) = a$ and $f(g(b)) = b$ for all $a \in A$ and $b \in B$ . As we saw in my last post, these facts imply that $f$ is one-to-one and onto, and hence a bijection. And it really is necessary to prove both $green加速器下载官网$ and $green加速器下载官网$ : if only one of these hold then $g$ is called a left or right inverse, respectively (more generally, a one-sided inverse), but $f$ needs to have a full-fledged two-sided inverse in order to be a bijection.

…unless $green加速器下载$ and $green加速器下载官网$ are of the same finite size! In that case, it’s enough to show the existence of a one-sided inverse—say, a function $green加速器下载官网$ such that $g(f(a)) = a$ . Then $green加速器下载官网$ is (say) a one-to-one function between finite equal-sized sets, hence it is also onto (and hence $green加速器下载官网$ is actually a two-sided inverse).

We must be careful, however: sometimes the reason for constructing a bijection in the first place is in order to show that $A$ and $B$ have the same size! This kind of thing is common in combinatorics. In that case one really must show a two-sided inverse, even when $green加速器下载$ and $B$ are finite; otherwise you end up assuming what you are trying to prove.

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I’ll leave you with one more to ponder. Suppose $f : A \to B$ is one-to-one, and there is another function $g : B \to A$ which is also one-to-one. We don’t assume anything in particular about the relationship between $f$ and $g$ . Are $green加速器下载官网$ and $g$ necessarily bijections?

Posted in logic, proof | green加速器下载官网 bijection, finite, function, injection, invertible, green加速器下载, onto, proof, surjection | 7 Comments

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Posted on March 14, 2023 by Brent

Several commenters correctly answered the question from my previous post: if we have a function $f : A \to B$ and $g : B \to A$ such that $g(f(a)) = a$ for every $a \in A$ , then $f$ is not necessarily invertible. Here are a few counterexamples:

Commenter Buddha Buck came up with probably the simplest counterexample: let $green加速器下载$ be a set with a single element, and $B$ a set with two elements. It does not even matter what the elements are! There’s only one possible function $g : B \to A$ , which sends both elements of $B$ to the single element of $A$ . No matter what $green加速器下载官网$ does on that single element $green加速器下载官网$ (there are two choices, of course), $g(f(a)) = a$ . But clearly $f$ is not a bijection.
Another counterexample is from commenter designerspaces: let $f : \mathbb{N} \to \mathbb{Z}$ be the function that includes the natural numbers in the integers—that is, it acts as the identity on all the natural numbers (i.e. nonnegative integers) and is undefined on negative integers. $g : \mathbb{Z} \to \mathbb{N}$ can be the absolute value function. Then $g(f(n)) = |n| = n$ whenever $n$ is a natural number, but $green加速器下载$ is not a bijection, since it doesn’t match up the negative integers with anything.

Unlike the previous example, in this case it is actually possible to make a bijection between $\mathbb{N}$ and $\mathbb{Z}$ , for example, the function that sends even $n$ to $green加速器下载$ and odd $green加速器下载官网$ to $-(n+1)/2$ .
Another simple example would be the function $f : \mathbb{N} \to \mathbb{N}$ defined by $f(n) = 2n$ . Then the function $g(n) = \lfloor n/2 \rfloor$ satisfies the condition, but $f$ is not a bijection, again, because it leaves out a bunch of elements.
Can you come up with an example $f : \mathbb{R} \to \mathbb{R}$ defined on the real numbers $\mathbb{R}$ (along with a corresponding $g$ )? Bonus points if your example function is continuous.

All these examples have something in common, namely, one or more elements of the codomain that are not “hit” by $green加速器下载$ . green加速器下载 this phenomenon in general. And in fact we can make this intuition precise.

Theorem. If $green加速器下载$ and $g : B \to A$ such that $g(f(a)) = a$ for all $a \in A$ , then $green加速器下载$ is injective (one-to-one).

Proof. Suppose for some $a_1, a_2 \in A$ we have $f(a_1) = f(a_2)$ . Then applying $g$ to both sides of this equation yields $g(f(a_1)) = g(f(a_2))$ , but because $g(f(a)) = a$ for all $green加速器下载官网$ , this in turn means that $green加速器下载$ . Hence $f$ is injective.

green加速器下载. since bijections are exactly those functions which are both injective (one-to-one) and surjective (onto), any such function $f : A \to B$ which is not a bijection must not be surjective.

And what about the opposite case, when there are functions $f : A \to B$ and $g : B \to A$ such that $green加速器下载$ for all $green加速器下载官网$ ? As you might guess, such functions are guaranteed to be surjective—can you see why?

green加速器下载官网 logic | green加速器下载 bijection, function, injection, invertible, one-to-one, onto, surjection | 1 Comment

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Posted on March 3, 2023 by Brent

Suppose we have sets $A$ and $B$ and a function $f : A \to B$ (that is, $f$ ’s domain is $green加速器下载$ and its codomain is $B$ ). Suppose there is another function $green加速器下载官网$ such that $green加速器下载$ for every $a \in A$ . Is $f$ necessarily a bijection? That is, does $green加速器下载官网$ necessarily match up each element of $green加速器下载$ with a unique element of $green加速器下载$ and vice versa? Or put yet another way, is $f$ necessarily invertible?

If yes, prove it!
If no, provide a counterexample! For bonus points, what additional assumptions could we impose to make it true?

green加速器下载官网 logic | Tagged green加速器下载, function, green加速器下载, invertible | 12 Comments

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Posted on February 28, 2023 by green加速器下载官网

[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, 2023

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 $green加速器下载官网$ 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:

green加速器下载

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:

green加速器下载

$1/8$ 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 $green加速器下载官网$ mirrors, so it has $green加速器下载官网$ 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 $green加速器下载$ symmetry tells us that $D_4$ is a subgroup of $green加速器下载$ . 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 $green加速器下载$ and $green加速器下载官网$ are also subgroups of $green加速器下载官网$ .

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 $green加速器下载$ is a subgroup of $green加速器下载$ . Likewise we could color it in one of the ways below:

yielding the subgroups $green加速器下载官网$ , $green加速器下载官网$ , and $green加速器下载$ . Note $green加速器下载官网$ 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 green加速器下载官网, green加速器下载 | Tagged green加速器下载官网, beauty, green加速器下载, group, symmetry, theory | 1 Comment

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Posted on January 30, 2023 by Brent

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 $green加速器下载$ :

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?

Posted in pattern | Tagged binary, bits, cube, diagram, Hasse, hypercube, lattice, subsets | 2 Comments

Post without words #31

Posted on January 23, 2023 by Brent

green加速器下载

Image | Posted on January 23, 2023 by Brent | Tagged green加速器下载, bits, cube, diagram, Hasse, hypercube, lattice, subsets | 2 Comments

A few words about PWW #30

Posted on January 22, 2023 by Brent

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

As several commenters figured out, the $green加速器下载$ 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 捕鱼游戏加速器2021排行榜前十名下载_好玩的捕鱼游戏加速器大全_...:2021年3月24日 - 小语加速器 green加速器 diudiu加速器怎样下载外服游戏加速器免费小米系统游戏加速器游戏加速器永久免费 uu加速器删除游戏泡泡网游加速器 gta5游..., 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.
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Posted in posts without words | green加速器下载 binary, green加速器下载, cube, diagram, Hasse, hypercube, lattice, subsets | green加速器下载官网

green加速器下载

green加速器下载 January 20, 2023 by Brent

green加速器下载官网

Image | Posted on January 20, 2023 by Brent | Tagged binary, bits, cube, diagram, green加速器下载官网, hypercube, lattice, subsets | 5 Comments

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

Posted on January 11, 2023 by Brent

Recently for my birthday I received a copy of Oliver Byrne’s 1847 edition of Euclid’s Elements (pictured at right), republished by green加速器下载官网 in 2010. I’ve only just started reading it, but it’s beautiful and fascinating. Oliver Byrne was a 19th-century civil engineer and mathematician, best known nowadays for this incredible “color-coded” edition of Euclid. Euclid’s Elements, of course, is the most successful and influential mathematics textbook of all time, widely used as a geometry textbook even up into early 1900’s. Nowadays hardly anyone reads the Elements itself, but its content and style is still widely emulated. In 1847, Oliver Byrne decided to make an English edition of the Elements that not only used colored illustrations, but actually used color-coded pictures of lines, angles, and so on, green加速器下载 to refer to the picture instead of using the traditional points labelled by letters (see the example below). I can’t imagine how much work went into designing and printing this in the mid-1800s. I guess there would have to be four engraved plates for every single page? In any case, it’s beautiful, creative, and surprisingly effective. I spent a while last night going through some of the propositions and their proofs with my 8-year-old son—I highly doubt he would have been interested or able to follow a traditional edition that used letters to refer to labelled points in a diagram.

green加速器下载官网

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…

green加速器下载 books, geometry, pictures | Tagged beauty, Byrne, color, design, elements, Euclid, illustration | green加速器下载官网

The Math Less Traveled

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Post without words #31

A few words about PWW #30

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

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