Welcome to another Mathologer video. Today we’ll take a well-earned break

from all the heavy-duty algebra of the last couple of videos. Today it’s all

going to be super visual and super accessible, promise. Okay, to start with

let me first get you hooked. Three colors: red, yellow, blue. Put down a row of ten

hexagons and color them randomly. There that’s one possible coloring. Draw a row

of nine hexagons underneath. The color of one of these new hexagons depends on the

colors of the two hexagons above it. See whether you can guess what the rule is.

Yellow and red add to blue. Red and yellow add to … also blue. Yellow and blue

add to red. Easy, right? I’m sure you’ve guessed

already that whenever we are adding two different colors the result is the third

color, and so on. Blue plus yellow is red. Yellow plus red is blue. Okay, what if we

have to add a color to itself? Well, red plus red is red. That’s the most natural

rule. Agreed? Agreed! Draw a row of eight hexagons underneath

what we’ve got so far and, using the same rules, just keep

filling in colors and drawing rows until it all ends, like this. So you end up with

an equilateral triangle composed of hexagons. Important observation:

once the colors of the top row have been chosen, all the other colors are pinned

down. Here a couple of examples of triangles resulting from different

choices of colors in the first row. There and that one and that one and that one

and that one. Very pretty. Before we take a closer look, let’s play

a speed game. In a moment I’ll randomly color this

first row. Then I’ll count down from 5 and ask you to decide what the resulting

color of the bottom hexagons is. Get it right and you win, hmm, a lifetime

subscription of Mathologer videos. Yes, okay, it’s free anyway but you win

bragging rights. What are you going to do? Of course, you can just guess for a one

and three chance. On the other hand, since the first row will determine all the

colors of the hexagons below, you can just calculate the color of the bottom

hexagon. IF you’re really really quick (or if you pause the video 🙂 Anyway, time to go.

Ready? Well, ready or not here we go! Okay, make your guess. What’s the color of

the bottom hexagon. Five, four, three, two one. And the answer is … yellow. Did you get

it without pausing the video or just guessing? Sounds impossible? Well it turns

out that there is a super surprising shortcut for this calculation. It turns

out that the color of the bottom corner of our triangle is simply the sum of the

two colors in the top corners. There, yellow plus yellow at the top is yellow at the

bottom. Yellow + blue at the top is equal to red at the bottom. Blue plus red is

yellow. Blue plus blue gives blue. It really works. Pretty amazing, isn’t it?

Choosing different colors in the top row gives very different colors of our

triangle and yet the colors of the top two corners alone determine the color of

the bottom corner. Weird, hmm? Well, it gets weirder. What if instead of ten hexagons

at the top we started with nine hexagons Well, then the short cut doesn’t work

anymore. For example, stripping a slanted column

off of the width 10 triangle over there gives us a width 9 triangle and here the

shortcut clearly doesn’t work: blue plus blue should be blue not yellow, stripping

off another row gives a width 8 triangle. Again the shortcut doesn’t work. Keep on

going. Hmmmm, In this case, blue plus blue is blue, as

expected. However, if instead we had stripped

things like this we get a width seven triangle that

doesn’t work. Width six doesn’t work, and neither

does width five. Okay so why is 10 special? And are there any other special numbers.

To hunt for clues let’s have a closer look at this larger triangle. Notice all

the smaller solid color triangles here and there? It’s raining little triangles.

Yeah raining raining raining. Maybe just this reminds you of something.

Hmm, can you think of a famous mathematical supermodel dressed in

little triangles? 🙂 No? Okay have a look at this special super symmetric example. In

this case the first row is entirely yellow except for a single red hexagon

in the middle. I first encountered the strange shortcut phenomenon in an

article by mathematicians Erhard Behrends and Steve Humble in the Mathematical

Intelligencer. It reminded me of the famous Sierpinski triangle fractal in

which it is also raining little triangles. Beautiful pattern isn’t it. And

I was also reminded of the pattern on a giant snail shell that I bought a couple

of years ago in a butcher shop here in Melbourne. Butcher shop? Yep you can find

the strangest things in Australian butcher shops. And, of course, the whole

summing two above gives the one below business should be very very familiar.

Yep our good old friend Pascal’s triangle is based on the same sort of

growth principle. Steve Humble one of the authors of the article I

mentioned earlier created the three color game as a mathematical outreach

activity in 2002 and discovered the shortcut while demonstrating it to kids.

Today I’d like you to imagine that you are a mathematician who, just like Steve, has

stumbled across this mathematical gem. so As a true mathematician you are now

cursed to not be able to sleep until you’ve come up with an explanation for the

shortcut and how it relates to the snail pattern and to Pascal’s triangle and

Sierpinski’s triangle. Well, let’s find out together, shall we? In

five easy chapters. Okay time to investigate. Why does the

shortcut work for width ten triangles. And, are there any other special numbers? Well

we already saw that nine, eight, seven, six and five are not special. And that’s

where we stopped. Hmm, I wonder why? 🙂 Let’s have a look at four: blue + yellow is red.

Probably also just a fluke. Right? Wrong! If you keep experimenting with widths four

triangles you’ll find that the shortcut always works. Four is also special. Okay,

let’s say it’s been a long night and we’re all pretty brain-deaded. In this

state can we still show that 4 is special? Yes, no problem. There are four

hexagons in the first row and three possible colors. This translates into

three to the power of four that’s 81 different ways to color the first row.

this means that they are exactly 81 with four triangles and even if I’m half dead

I can quickly throw together a computer program that will in an instant list all

these triangles and check that our shortcut works for all of them. Actually

if you’re not quite so brain-deaded then we can use symmetry and permuting

colors to reduce the number of triangles we need to check. Ao a little challenge

for you: How many essentially different widths four triangles are there? Let us

know in the comments. Of course, checking all those triangles doesn’t tell us why four

is special, we still haven’t proved that ten is special and we haven’t checked many

other widths. So it’s time to go deeper. With my computer program I can also

quickly make up larger widths triangles and check that among all the numbers

from 1 to 100 the only numbers that appear to be special are 2, 4, 10, 28

and 82 – hmm – for 2, 4, 10, 28, 82… tricky! Nope, not tricky at all. Did you

spot the pattern? These numbers are all one up from a power of 3. 4 that’s 3 plus

1. 10 3 squared plus 1,28 3 cubed plus 1, 82 3 to the power 4 plus 1. And let’s not

forget 2 at the beginning. 2 is 3 to the power of 0 plus 1. The plot thickens and

based of what we’ve got so far we conjecture that exactly the numbers one

up from powers of three are special. Now let me show you something absolutely

beautiful a proof that all these numbers are indeed special. this one is really

good, promise. Let’s begin where we began by showing that width 10 is special. So let’s

start with any old width 10 triangle. Now focus on these hexagons here. Then these

three hexagons are the corners of this widths 4 triangle and since width 4

triangles are special, the top two corners must add to the corner at the

bottom. The same is true here and here and here and here and here. And now, well

I probably don’t even have to say it, right? Can you see what’s happening. we

just showed that over there any two adjacent colors add to the color below

and what this means is that these highlighted heagons combine into a

widths four triangle. And, therefore, because four is special the top two

corners add to the bottom corner. But since the top corners of the width four

triangle are also the top corners of the original widths ten triangle, it

follows that the top two corners of a width 10 triangle add to the bottom

corner. Always! In other words, 10 is special.

Ta da. An argument like this makes my day. So on a scale from one to ten how

beautiful an argument is this? Yep we are doing polling now 🙂 Let me know in the comments

what you think. To prove that twenty eight and indeed all the other powers of

three plus one are special, we just repeat this argument over and over. Here

JUST the quick animated first iteration of the argument that shows that twenty

eight is special, using the fact that four and ten are special. So nice. Qnd a little challenge for you:

Can you think of a second way to argue? Hint: switch the roles played by four and

ten. Anyway, I suspect that at this point most people

would declare the mystery solved. TIme for a cat video? Well, that’s not

what we do here on Mathologer, right? Is it really just a big coincidence that

widths four is special or is there a deeper reason? Qlso we know that two four ten

etc special and we suspect there are no others, but how can we be certain? And

what’s up with all these similar phenomena? Sirpinski and snail shells and whatnot?

Are there any beautiful connections? Ready to go deeper? The basic rule of two adjacent colors

in one riw summing to give the color immediately underneath just cries out

for us to have a look at the tip of Pascal’s triangle. Right? Any two numbers

in this famous number triangle add to the number right below. Here four plus

six that’s ten, five plus one that’s six. Okay, this addition process is very

similar to our coloring scheme but there are also obvious differences: in Pascal’s

triangle there are no hexagons, we’re adding numbers instead of colors and

everything starts from the tip rather than from a row of numbers. Hmm, okay, you

want hexagons? Well here are a couple. What else? You want the whole thing to

start from a row and not from the tip? Not a problem at all. So we can think of

Pascal’s triangle growing from an infinite row of zeros with a single one thrown in somewhere in

the middle. And that should also remind you of the Sierpinskish color triangle we

saw earlier which starts from a row of yellows with one red exception in the

middle. Interesting, huh? Anyway to keep things uncluttered and to escape from

this frame here, I’ll hide all the zeroes anyway and we return to the familiar

picture of Pascal’s triangle. Okay, just remember that the zeroes are

still there but hidden. Now what about colors? Well, the first idea that

comes to mind is to color the hexagons according to one of the natural ways to

split the integers into a finite number of classes. For example, coloring all the

odd numbers dark and the even numbers white, we get this. That looks promising

and it’s even more promising when we zoom out to reveal a larger slice of our

triangle. Whoa pretty impressive, huh? In fact, if you

keep zooming out, our ever more detailed even odd triangles will converge to the

famous Sierpinski triangle fractal. Anyway, it’s most definitely raining

little triangles in there, just like in our three color

triangles. Another way to interpret our odd-even coloring is to say that we

color according to what remainder a number has on division by two. After

division by two the possible remainders are zero and one. For odd numbers the

remainder is one and we color dark. For even numbers the remainder is zero and

we color in white. What we then get is Pascal’s triangle using what is called

base two modular arithmetic. Fancy words but it’s just even and odd. So wherever

you see one and zero next to each other there will be a 1 underneath, 0 plus 1

equals 1. That translates to even plus odd equals

odd. Similarly 0 plus 0 equals 0 and then there’s the slightly weird 1 plus 1

equals 0 which amounts to odd plus odd equals

even. Okay we found a natural numberish way of

coloring with two different colors. What happens now if we play our game with the

even-odd rule. Are width ten triangles still special? Hmm, here a few random width ten

triangles you get this way. There’s one, there’s another one, there’s another one,

there’s another one, there’s another one and … Well, is 10 still special? No,

definitely not. If it were, black and white on top should

give black at the bottom. Right? 1 plus 0 equals 1 not the 0 we got there. But don’t

despair, there are also special widths for our even/odd coloring. However

instead of 1 plus powers of 3, this time it’s 1 plus can you guess it … powers of 2? It is easy to prove that all these

numbers are special using the same collapsing argument as for our red

yellow, blue triangles. This lovely and simple construction also suggests a very

natural model that can be used to at least partly explain the formation of

snail shell patterns. A snail shell grows in thin layers that are added onto its

lip. In our mathematical model these layers are the horizontal rows of

hexagons. Then the color of the hexagons and each new layer is determined by the

simple rules for adding odd and even numbers. This is all very neat and is

based upon a biological mechanism that can be observed where the

characteristics of existing cells determine the characteristics of newly

formed adjacent cells. So Nature has already found a lovely

application of the games we are playing here. Do any of you apply similar this

plus that mathematics in anything you do in your working life? Let us know in the

comments. Okay, so this is what Pascal’s triangle

looks like when you color it according to the remainder after division by two.

For our original game we use three colors. So let’s see what happens when we

use remainder on division by three. Of course, now the possible remainders are 0

1 and 2 and let’s respectively assign them the colors yellow red and blue. And this

is what you get. There, very pretty again. Now compare

this triangle to the nicely symmetric triangle that resulted from our original

three color game. Yep so close 🙂 Not quite the same but

we’re definitely onto something. Okay let’s have a close look at the growth

rules for our remainder three Pascal triangle. So with Pascal to add a and b we simply

go a + b mod 3. Now in terms of colors this doesn’t quite correspond to

what happens in our original color game. For example, in our original color game

red plus red equals red, but Pascal gives the answer as blue. However, there’s a

super simple algebraic adjustment to get perfect agreement. Can you see what we

need to do? Well, first notice that at least the middle

rules always work. But to make things work completely

these 2s need to be replaced by 1s and these 1s need to be replaced by

2s and there’s a really simple tweak to do that. Just go – (a+b) here.

Let’s check that this tweak really has the desired effect. Okay – 0

well that’s 0. Nothing changes here which is great. What about the top rules? Well

now we have minus – 2. But in mod 3 arithmetic – 2 is the same as -2 + 3 which equals 1. Got it? And on the bottom row we now get – 1

which is equal to – 1 + 3=2. This really works!

So we’ve done it. This algebraic rule has captured a mathematical soul of our

original three color game. As a first application of this clever insight, let me

show you how you can see pretty much at a glance why four was special in the

original game. Remember, originally we showed this by listing all possible 81

widths four triangles and checking that the shortcut works for all of them. We

can do much better than that using just a little algebra. We start with four

random colors a, b, c, d and then we can simply calculate the remaining colors

like this… But, of course, in mod 3 arithmetic three

times anything is just the same as zero and so …. Fantastic, b and c just cancel out

and the number at the bottom only depends upon the numbers a and d in the

top corners, in the right way. So our simple calculation confirms, once again,

that 4 is special for our original setup. But actually, this calculation tells us

something about Pascal’s triangle as well. Instead of our original three color

game, take a look at the Pascal 3 color game. Then the algebra is exactly the

same, except all the minuses here turn into pluses. And again the three times

somethings in the bottom hexagon disappear. So for this Pascal 3 color

game as well the number 4 and therefore all those other numbers 10, 28, etc. are

special. What about the odd-even, black-white mod 2 Pascal game. In the case of this

game, we only need to worry about this smaller part of the triangle. Why? Because

2 times anything is even and so it reduces to 0 mod 2 and so the 2b term

in pink zaps to 0. This proves with 3 is special

for the mod 2 game and then by our collapsing argument we can see that all powers of 2

plus 1 are special. Of course, we can play the same games

using any number of colors. With m colors we can then use the sum modulo m or its

negative as our generation rule and then the same algebra and our contraction

argument can pin down the corresponding special numbers. For this we need to

identify those entries here for which all the middle coefficients are

multiples of m. As we’ve already seen for m is equal to 2 the first time this

happens is here and as we’ve also already seen for m is equal to 3 the

first time this happens is here. And now a challenge for you, tiny little

challenge. Figure out the smallest non-trivial special numbers for m is

equal to 4, 5, 6, etc. until you get sick of it. Ok, a hint: just focusing on the

blue entries can you see another Pascal triangle? If

you’re keen to learn absolutely everything else there is to know about

these games check out the Intelligencer article by Erhard Behrends and Steve

Humble that I mentioned earlier. In particular, in this article you can find a

proof that the special numbers we’ve spotted are really the only special

numbers. This proof is based on the 1909 article by the Indian mathematician Balak

Ran in the Journal of the Indian Mathematical Club. How on earth did they

find that one? So, using the special numbers that result

from our additional rules, we can fashion ourselves some nifty keys with which to

explain those striking self-similar patterns. Let’s have a closer look at the

triangle over there. A reminder, we got this triangle using our original three

color addition rules starting from a row of yellows with a single red hexagon

right in the middle. Okay so the whole thing starts with this row of hexagons

at the top. Since yellow plus yellow equals yellow

it’s clear that we’ll get those two huge yellow oceans on the left and right

right. Now what about the mystery white region in the middle. Let’s figure out

what yellows we get there. In order to do that I’ll make myself one template each

for the special numbers 4, 10, 28 and so on. I now place the template so that the

two upper corners sit on yellows in the outer regions. Whenever this is done, we

know that the bottom corner of the template must also be yellow. Let me

show you. All make sense, right? But just to pinpoint

exactly where the self-similarity of the pattern comes from, think about the

yellows being built in this order: When we place one of the templates so that

it’s top two corners lie in the left and right oceans of the yellow, the template

generates one of the centered yellow triangles, like this. This takes care of

all these centered triangles, there, all all of those. But now focus on this off-center

region here. What’s happening here? Well we already have large lakes of

yellow on the left and right plus a starting row all yellows with one

hexagon of a different color in the middle. So within this frame we can now

generate centered triangles exactly as before, there, and so on. This really gives

a very good intuitive feel for where this self-similar pattern comes from,

doesn’t it? And it’s not that hard to turn this intuition into rigorous proofs

for the fractal nature of these patterns. There’s one very surprising feature of

the original three-color game that I did not mention yet. When you rotate one of

the triangles colored like this you get another triangle of the same type. So

what this means is that this new triangle also grows from its first row.

This feature is shared by all the other negative mod m colorings, but not by the

straight Pascal mod m colorings, except mod 2. Can you think of the simple

explanation for this phenomenon? Anyway, to finish off, let me just show you an

animated introduction to the most natural three-dimensional counterpart of

Steve’s three color game that also shares the 3d counterpart of the

rotation feature. And

that’s all for today.

## Comments

Author

So, looks like rotating the triangle by 120 or 240 degrees will still have it work.

Author

While I definitely didn't know for sure, I did assume the top two corners would determine the bottom. So I won bragging rights! Now to watch the rest of the video to learn why.

Author

64- , 6+4=10

Author

28 , 2+8=10

Author

I suspected that it was the top two corners that had to do with the bottom colour because I noticed the pattern in the previous random triangles and I'm so ecstatic that I got it right

Author

I guessed that it was the top corners somewhat arbitrarily.

Author

27 different triangles —-81/3

Author

Is it just me, or is the thumbnail Tommy Pickles's mother?

Author

Very interesting!

By the way you have very cool t-shirts, where to buy?

Regards

Author

Where is 27 in i can't even

Author

It's fascinating

Author

It makes more sense to think of it as times 3 minus 2 based on his own explanation. to get 4 hexagons in the top row, you duplicate the original span twice to the right – BUT – because it inclusive, you lose one each time. 4×3-2 = 10. 10×3-2 = 28 etc.

Author

but can you odd?

Author

I wonder about the number of ways for generating Sierpinski triangle. Recently, I did it using chaos game. 🙂

Author

It looks like there are 10 size 4 triangles? If you exclude symmetries and treat colors as interchangeable (i.e. RRRR and YYYY are basically the same configuration)

OOOO

OOOX

OOXO

OOXX

OXOX

XOOX

OOXT

OXOT

XOOT

OXTO

Author

really love how much production quality goes into this (all the animation programming, etc)!

Author

The sliding magenta triangles at 26:04 made my jaw drop. The concept, while amazing as are most automata, wasn't hyper-noteworthy, but the idea of these little protein-like slider triangles infixing colors upon corner matching was just a revolutionary conceptualization for me. Now I want to reconsider all sorts of automata and the Wolfram/Cook Turing completeness in light of little slider machines.

Author

Guessed yellow correctly with a method I was really proud of and I was gonna explain it, but I just realized the method doesn't work, I was just lucky.

Author

The color game version of this is basically a cellular automaton, so if you like this you should look at Rule 110: https://en.wikipedia.org/wiki/Rule_110?wprov=sfla1

Lots of cool but very different raining triangles.

Author

In general, when the 1st row has n number of circles, we can count the color of the most bottom circle by using Lucas Theorem. Here is the paper( in Chinese).

http://web.math.sinica.edu.tw/math_media/d413/41304.pdf

Author

16:38 is that a coincidence that every random example you displayed is a piece of a Sierpinski's triangle? I mean they seem like cut off parts of a huge Sierpinski's triangle

Author

Great video! From the beginning of the video I waited for cellular automaton to be mentioned. Still waiting 🙂

Author

These animations are so cool wonder how you made them.

Author

Great video. Want to see more videos on origami. Or about the "Golden ratio" for example with such a "Golden sword".

1 – 1(one way)

2=1+1 (2w)

3=1+1+1=1+2=2+1 (3w, prime numbers)

4=1+1+1+1=1+1+2=2+1+1=1+2+1=2+2 (5w, cquare number)

5=1+1+1+1+1=… (8w)

…

9=…=2+2+2+1+1+1 (..w, cquare number)

9-4=2+2+2+1+1+1-2-2=2+1+1+1=5 (prime number).

Author

Do those funny ‘quantum colour alterations’ have anything to do with this math?

Author

The colour rule also works for width 1. The cell at the foot of the side 1 triangle always is identical to both ends if the top row, and that satisfies the rule.

That presumably means that zero is a power of three 😉

Author

Just after 12 min asks if the problem is solved. Answer, no: it can't be you are less than half way through the video…

Author

if we calculate next color as

c = -a – b mod n

the colors satisfies equation

a + b + c = 0 mod n

thats make things simetric and for every three neihbour cells following the rule can will follow it rotated

Author

I guessed

Author

Sorry, Mathologer, I couldn't fully concentrate on the video because I was distracted by your really odd looking T-shirt.

Author

Smug: Guessed the rule, so got the right answer. It's one of those existential arguments; an easy rule must exist, else he would not have asked the question. So what's the easiest simplest rule you can think of that depends on something about the top row?

Author

20:27 I thought it was 2a + 2b. Oh well, the result is still the same, isn't it?

Author

yellow

Author

Why do I keep getting unsubscribed from this channel ? ! 🧐🇬🇧🤔🤓😤😤😤

Author

PEOPLE, HIS SHIRT HAS AN

EVENNUMBER OF ODDS!01 03 05 07 09 11 13 15 17 19 21 23

49 47 45 43 41 39 37 35 33 31 29 25

Author

22:50 pffft, no Hamlet joke?

Author

Do you think there's similarly nice/visual intuition as to why binomial coefficients show up in pascal's triangle? That's always been something a little mysterious to me. I suppose induction is probably the easiest way to prove it but I'm more curious about the deeper reasons.

Great video by the way. Keep up the good work!

Author

26 people accidently clicked on dislike while try to press like :

Author

Strong induction!

Author

Hey I guessed the rules for the bottom corner before they were mentioned! Let's go yellow

Author

Yay got it. Thought it was related to the number of hexagons wide it was. But assumed as you wanted a quick reply it was the easy solution of… follow the basic logic.

Author

In an even number of starting hexagon's "n" , if you start from the n+1 row and guess the last color

Author

27 is missing on your T-shirt…

Author

It takes me a week just to mentally prepare for watching this

edit – should have watched first 10 seconds x)

Author

Hi Mathologer, the chords in your intro sound familiar. Do they come from a full song?

Author

I love this sort of thing. Excellent and even insightful video despite starting from such a simple rule, a deep dive into the "why" of things, as much of the best math starts.

Am I the only one who feels an odd moment of terror when I realize that sierpinski's triangle is going to show up again though? I've seen it in so many surprising places and yet can't understand why, so my guess is that my brain fears it as some intelligent unknown, like some lovecraftian entity lmao.

Author

Pascal's triangle is so cool! There are all kinds of interesting patterns in it. This color thing is about the best I've seen.

Author

Never mind all that, what happened to 27 on his t-shirt? Favorite number since the Simpsons episode

Author

there is a connection between these triangles and generator functions and Fibonacci… maybe a continuation video about this one day?

Author

For a triangle of width 4, there are only 6 triangles that need to be checked:

❤❤❤❤

❤❤💛💙

❤💛❤💛

❤💛💛❤

❤💛💛💙

❤💛💙❤

Author

I rate the argument 10/10 💖

Author

Yeah… red and yellow is blue xD and yeah, blue and yellow is red. Well what xD

Author

You would probably think that the worse possible odds for guessing the right color of the final triangle would be one in three, but I actually managed to lower it to zero in three by forgetting what the valid choices were and guessing green at the last second… Hahaha

Author

I got it! I chose the yellow!

Author

7 excitement

Author

"Did you get it? Without pausing the video?"

Yes!

"Or just guessing?"

Oh…

Author

Shirt idea: "I CAN'T EVEN" with prime numbers. Small print next to 2: "YOLO"

Author

I win!

🙂

Author

Funny, my rule for 20:30 was to multiply a+b by 2, but I suppose there are infinitely many combinations in a modular system

Author

What better way to begin a lazy Sunday morning than with some simple and beautiful mathematics presented so eloquently?

Author

i paused the video and went little back to check the student combinations.. i was able to figure out the trick.. yellow it was.. is my dog to brag… 🐱 also i could figure out (-a-b) mod 3… lol.. great video sir…

Author

I wasn't wrong because i didn't guess. Do i not not get bragging rights?

Author

PLEASE please PLEASE do a COMBINATORIAL proof of the Lagrange Inversion Formula (LIF). I need to understand this for my own research!! PLEASE show how the LIF is connected to a certain class of noded trees (trees with a single distinguished node). Thank you!

Author

The snail shell thing is amazing.

Author

i was watching at 2x speed and still guessed (just) in time.

(mostly because there were so many yellow cells)

Author

Over last summer, I programmed this in processing (KA was faster than Repl): https://www.khanacademy.org/computer-programming/triangles-1d-cellular-automata/6678150943440896. It seems to correspond to rule [2,1,1,0,2] in the program. Hmmm…

EDIT: Found that this was one of the "notable rules", named "WHAT THE" in the comments. The other rule outlined later, with modular arithmetic, was actually what inspired me to make this program in the first place. The rule was [1,2,2,0,1], called "The original pascal" in the comments. Other rules can be used as well, but when I saw that thumbnail, I IMMEDIATELY opened up that program to see what would be discussed.

Author

As soon as you showed 4 and 10 I was thinking,

what about 55?Author

9:10 Why isn’t width 1 special? 🙂

Author

Q-Bert?

Author

~ 27:00 Suppose the number of hexagons in the top rope is even?

Author

Anytime you repeatedly apply an algorithm, you're bound to find some sort of fractal pattern.

Author

I think it's possible that the building blocks of our universe works exactly like that 3d pyramid… With a set of rules that determines each particle's state based on its neighbours' states (Well maybe higher than 3d). It not only determines the solid appearance/chemical composition of that space but also what "traffic" of other charges/particles are allowed through, ie it creates pathways (like roads to drive on in the space in-between) and travelling particles bounce off the angled walls similar to light travelling through fiver optic cables. It's a continual feedback system where travelling particles affect the state of the static ones and visa versa

Author

Well, that must have been a lot of work to animate. I'll try to find an application of this in my work, as you suggest.

Author

so I'm about 2 minutes in, and it seems like the bottom color will always be what the top rows edges (far left and far right) add up to. for example, far left is yellow, far right is blue. bottom will be red. I've also noticed how if you rotate the entire triangle so that the left or right side is now the top, all of the colors still add up correctly.

edit: welp looks like me idea worked at least for the 5 second challenge 😀

edit 2: hey my idea was right 😀 😀

Author

7.5/10 too little algebra

Author

I'm 34 and I just learned about Pascal's triangle yesterday.. Very interesting indeed.

Author

82 out of 10

Author

At last a Mathologer video that makes me feel special.

Author

This video was really awesome and accessible. Really fun!

Author

1:42

it is 1/3 i hope

Author

28 and 82 are also two of the seven magic nuclear numbers.

Author

Can you prove that the 4 triangle is special, because it can be broken into to 2 triangles in the same way that 10 is special, because it can be broken into 4 triangles?

Author

3:20 …wait so my gut instinct was RIGHT??? When I saw all the examples I thought "maybe the corners are related!" and it worked somehow???

Author

14 unique triangle patterns? 9:00

(78/6 + 3/3, since monocolor triangles don't have two variants based on swapping the other two colors)

Author

3D one is 4th color if all different, less dominant color if two colors, and same color if one color. (Yep got it right woo)

Author

Mistake @ 8:58: bottom row has 2 copies of:

Blue, Blue, Red, Yellow

and

Blue, Blue, Red, Blue

The ones missing are: Blue, Blue, Yellow, Yellow, and Blue, Blue, Yellow, Blue

The trick still works for these 2, of course.

Great video 🙂

Author

I believe you only need to check 10 of the 81 4-rows:

0000, 0001, 0010, 0011, 0012, 0101, 0102, 0110, 0112, 0120

Everything else is a version of this – recolored and/or flipped horizontally.

Author

Yay, induction!

Author

The protons have 3 quarks playing pythagora's theorem.

The protons have 3 laws of robotics.

Author

Hey dude. I dont even have a massive interest in maths, i just really like your voice lol. Keep up the vids

Author

That argument is at least a '7'. Very cute.

Author

How about color game in 3 dimensions? Something like crystallography nightmare? Hard to build movie about that mechanism, how it works in 4D? It should be game for Gods.

Author

I guessed yellow because there was mostly yellow in the top row

Author

The equation c≡-(a+b) (mod 3) can be rearranged to a+b+c≡0 (mod 3), which shows why the triangles can be rotated.

For the 3D tetrahedral rules (28:52), the four colours can be represented numerically with four ordered pairs in mod 2.

Red = (0,0)

Yellow = (0,1)

Green = (1,0)

Blue = (1,1)

The colouring rule can now be modelled by the equation

d≡-(a+b+c) (mod 2)

or equivalently,

a+b+c+d≡(0,0) (mod 2)

Again, this second form makes it clear that the rotational symmetry arises from the interchangability of the variables.

Example:

Say you have red, blue, and yellow. Inserting them into the equation, you get:

-(red+blue+yellow)

= -((0,0)+(1,1)+(0,1))

= (-1,-2)

≡ (1,0) (mod 2)

= green

Try out some more examples and convince yourself of why this representation works. Also, try to demonstrate that the special sizes for the tetrahedrons come in the form 2^n + 1.

Author

I'd say… 81/9.

Author

Amazing video ! Though I do lament there not being a reference to cellular automata, of which these problems are examples. Take your seashell: https://en.wikipedia.org/wiki/Rule_30

To answer one or two questions:

– why does a rotated triangle/tetrahedron/n-simplex that behaves like described in the video still work with the given rule ? Well when you take a unit "generation" (like a small triangle of 3 hexagons, or a small tetrahedron of 4 spheres), the rule chosen (same colors to same color; all colors but one to the remaining color; two of the same and one unique to a copy of the third sphere's color) leads to all rotation of a "unit generation" to still work with the rule. It follows then that the overall n-simplex can also be rotated and the rule be kept, as the overall n-simplex is just a juxtaposition of "unit generations". This might not be the case for other cellular-automata-like color-based rules of this type.

-why does modulo 2 arithmetic behave well with both addition and subtraction, and mod > 2 arithmetic only behave well with subtraction ? That is because the space of mod 2 arithmetic (the "two-hour clock", called GF2 or Z/2Z) is the only field (space with +, -, *, and /) where 1 = -1. A fun consequence is that addition in GF2 is the logical XOR operator, and multiplication is the logical AND operator.

Thanks again for this great video !

Author

29:33 honestly the rotation part surprised me less than the fact it's 2^n+1 on a 4 number base :') What would the 2^n+1 case be on a 4-dimensional game? Given the 2-dimensional one being 2 and the 3-dimensional one being 4, I'm going to take a guess at 8 as in a 2^(k-1) progression? Maybe the game breaks entirely in 4 dimensions? I also noticed you obviously needed an extra rule to define the extra scenario you have 2 matching colors and 1 off-color so I'm assuming a 4th dimension adds yet another or more rules.

This one raises more questions than the usual algebra video's xD

Author

9:29 (https://youtu.be/9JN5f7_3YmQ) … 4, that's 3 plus 1. 10 3 squared plus 1, 28 3 cubed plus 1, 82 3 to the power of 4 plus 1…

Could we instead use the expression 3 tesseracted? ps what's a five dimensional cube called?

Author

Well… I got it right

Author

"5"

Okay so those left two add to red

"4"

Those right two add to blue

"3"

Hold up so both corners are yellow

"2"

The corners are opposites. Not sure if that means anything but I'm running out of time now so I'll go all in

"1"

Maybe the corners are all the same colour. Is it yellow?

"It was yellow! Did you get it?"

Wait what… How did I…

"It's the sum of the two corners"

How did that line of reasoning work? I basically guessed!