Why do all shapes lie in the Polyhedron Plane?

Check out AMP or suggest it to someone who could benefit. And here is the puzzle: bit.ly/sum-amp
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I'll be doing An Evening of Unnecessary Detail at the Bristol Beacon, UK on Tuesday 12 March 2024. Come and celebrate Almost Pi Day with me! And celebrate Ages Until Tau Day with Steve Mould.
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Here is Grant's old, old 3blue1brown video. It's so good. • Euler's Formula and Gr...
Check out Ben's GeoGebra file we used to simulate the PolyPlane: www.geogebra.org/m/h9gpaawf
Polyplane will be at the UCLA campus in Los Angeles, February 01 to 28, 2024. You can also still send in a polyhedron from anywhere in the world or enquire about hosting it yourself. polyplane.org/
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Thanks to Henry Segerman for helping out as well. You should watch their awesome videos: / @henryseg
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CORRECTIONS
- None yet, let me know if you spot anything!
Filming by Matt Parker
Editing by Alex Genn-Bash
Written and performed by Matt Parker
GeoGebra graphics and addition material by Ben Sparks
Edge complaining by Grant Sanderson
Yes they bent the rules by Henry Segerman
Produced by Nicole Jacobus
Music by Howard Carter
Design by Simon Wright and Adam Robinson
MATT PARKER: Stand-up Mathematician
Website: standupmaths.com/
US book: www.penguinrandomhouse.com/bo...
UK book: mathsgear.co.uk/collections/b...
Oh look at this: forms.gle/hWes6BgPji3L8RfH6

[@CivilWarWeekByWeek]

I have a correction at this point Ben Sparks isn't kindly making geogebra programs he is clearly in your basement forced to code against his will

[@SparksMaths]

Help me...

[@CivilWarWeekByWeek]

@@SparksMaths I would but I do need a Geogebra file to help me in Matrix Algebra so I think I'll just make a deal with Matt to get it

[@standupmaths]

How did you get on the wifi?

[@hexisplus9104]

​@@standupmathsplease do this for higher dimensions. This explains so much. The projection from 3d to 2D explains the 4D to 3D projections I have seen and was beautiful.

[@SamuQu]

at 13:37 shouldn't it be ax+by=c so you can get every line that crosses the origin?

[@johnchessant3012]

16:42 just to spell out what Grant is saying here, if you calculate V - E + F for a polyhedron that has a hole in it (e.g. if you approximated the surface of a torus with plane faces), then you won't get 2. Instead, you'll get 2 - 2g, where g is the number of holes. So this is a way to formalize the notion of "holes" (since you can just count them via vertices, edges, faces) and prove that the number of holes is invariant with respect to continuous deformations.

[@prdoyle]

I love that this video includes Grant being extra Grant and Henry being extra Henry.

[@internetuser8922]

The United States education system uses "y = mx + b" for the equation of lines. Also, big fan of the "technically correct if you're a topologist" entries.

[@carolinecowley427]

that must get confusing when they get to quadratics do they do quadratics in the US?

[@iout]

@@carolinecowley427 We do, and it's really not that confusing at all. Variables get reused all over the place, it's not any weirder here than when it happens elsewhere. We just don't think about it.

[@SiberCatLP]

@@carolinecowley427 The distinction I was taught was that "b" was the y-intercept, while "B" was the coefficient of the term with exponent 1. Since they were different looking, "They're different Bs, so they're different values" was easy to accept.

[@sphaera2520]

@@carolinecowley427it’s no more confusing than when c shows up in a new equation.

[@mina86]

@@sphaera2520, with a, b and c there’s clear pattern. Meanwhile, if you’re using m and b for linea, what’s the clear pattern for going to higher order polynomials?

[@coltonchinn2615]

7:13 TIL that “way way way more faces” is equivalent to “two more faces”

[@douglaswolfen7820]

I noticed that too

[@adalson9200]

+2

[@jh-ec7si]

It is if you're a cube

[@jamespalmer9033]

Ask any good programmer and they'll tell you there's no such thing as two - the only numbers are zero, one and infinity. Two is just a special case of infinity. 😁

[@dembro27]

Indeed, it seems that "way" has a value of 0.666666 (repeating, of course).

[@madspacepig]

6:30 Funny that you demonstrated a simulation of the polyhedra being projected onto a plane, when in fact, due to the nature of them being rendered on a computer, and displayed on a flat screen, they were already being projected onto a plane, just by us looking at them.

[@T3sl4]

Projectiception!

[@n0tthemessiah]

Got'em!

[@ballparkjebusite]

How high were you?

[@gONSOTE]

yeah but, ironically, by the nature of those 2 different types of projections, the projection of the screen couldn’t be used for making a planar graph

[@iwikal]

@@gONSOTE Are you sure? They seem quite similar to me. How are they different?

[@John73John]

1:40 I mean.. 3 Blue 1 Brown was just sitting right there...

[@k0pstl939]

That was why. Grant Sanderson was in the chat of that livestream. 7:30

[@raptor4916]

Its a real Parker Name...

[@zyxwvut4740]

7:25 ?

[@iout]

I've not seen the livestream, but I'm pretty sure that's why they did it that way. And I can say with a relatively high certainty that the conversation went like this: "Make a tetrahedron with 3 blue and 1 brown face!" "That's a great idea. But wait, we don't have any brown tiles. We'll use gold, it's close enough." *makes the tetrahedron* "Here we have it. 3 Blue, 1 Gold"

[@plackt]

So… it’s a Parker reference.

[@nathanielpranger7370]

From my experience in the Netherlands we use "y = ax + b". Nice and clear that we use the first two available letters for unknown parameters, so I thought everyone did. Then I saw you use "m" and I just felt sorry for 14-year-olds learning Newton for the first time.

[@biscuit715]

I learnt both in the UK, (m,c, and a,b). I don't actually remember when but ax+b turned up later, possibly at uni, and I wouldn't go for it naturally. I do prefer it though.

[@charlesclaudel3958]

In france we also use ax+b and for polynomials you just add new letters in alphabetic order e.g ax²+bx+c or ax³+bx²+cx+d. I logical and it old itself up when integrating and derivating.

[@januszkobayashi1361]

In Poland it's the same

[@Tvillingklippan]

I think kx+m is standard notation in Sweden

[@gekylafas]

y = αx + β in Greece

[@reddcube]

The dual line is easy to explain. One shape and its dual are reflections of each other along the line. That is because when making a dual shape, each Vertex becomes a Face, each Face becomes a Vertex, and each Edges just changes orientation. So reflections of the line is just swapping the V and F.

[@japanada11]

Why you get lines and not just planes: For any polyhedron with only triangular faces, you have the additional relation 3F=2E (each face touches three edges, and each edge touches two faces). The intersection of V-E+F=2 and 3F=2E gives a line that contains all polyhedra with triangular faces. It just so happened that the only polyhedra Matt used in his visualization were triangle-faced polyhedra and their duals (which satisfy 3V=2E, giving the other line). There are lots of polyhedra that don't lie on either line that just didn't get drawn - but the triangle-faced ones and their duals are definitely quite common! (In particular, every platonic solid or its dual is triangle-faced)

[@walterkipferl6729]

And, just to make one final point clear: The reason that the Triangular-faced objects satisfy 3V=2E while their duals satisfy 3F=2E is that the switch between duals swaps the number of vertecies and number of faces. This also explains why the tetrahedron and square pyramid (any pyramid really) is on the line of symmetry between the groups: That line is F=V, since mirroring at that line is how you swap number of faces and number of vertices. A pyramid is always on that line since pyramids are self-dual! So they must have identical face-count and vertex-count! This brings up the question: are there other self-dual polyhedra? I don't know and I really shouldn't get into the Geometry Wikipedia rabbithole at 3 in the morning.

[@mathcookie8224]

OK, so the two diverging lines are because of triangle shenanigans, but what about the center line? Are all duals, even non-triangular ones, at reflections of each other across that center line? And if there is a universal center line, what IS the center line? I would think it’s V=F, since duals swap faces and vertices; is that correct?

[@japanada11]

@@mathcookie8224 That's exactly right: the center line is the V=F line, and every dual is given by reflection across that line because dual corresponds to swapping the V and F coordinates.

[@japanada11]

@@walterkipferl6729 Good clarification! Also worth noting that the number of vertices on each face turns into the number of faces touching each vertex in the dual. So while one line contains all the "every face is a triangle" polyhedra (tetrahedron, octahedron, icosahedron, etc), its reflection contains all the "exactly three faces meet at each vertex" polyhedra (tetrahedron, cube, dodecahedron, etc). And yes, there are many other self-dual polyhedra that can be easily found in the geometry wikipedia rabbithole.

[@japanada11]

Also, there are polyhedra that satisfy V=F but are NOT self-dual. For example, you can start with a cube and draw two new edges coming out of one of the vertices. The result has 8 faces (4 squares and 4 triangles), 14 edges, and 8 vertices. The dual has 8 faces (5 triangles, two quadrilaterals, and a pentagon), 14 edges, and 8 vertices. These are clearly not the same, so you get two distinct polyhedra occupying the same point (8,14,8) on the V=F line.

[@GeekRedux]

Given how prevalent TI-80-something graphing calculators are in the US, I'm surprised we haven't seen a shift from y = mx + b to y = ax + b, since that's how those calculators have always presented it.

[@JohnDoe-ti2np]

In his memoir, mathematician Goro Shimura says that he once set an exam question for a student who was trying to transfer from another university, which went something like this: Find the equation of the line in the plane that passes through the points (1,5) and (1,2). He wanted to see if the student would blindly use the formula y = mx + c. The student fell into the trap and then complained about being tricked.

[@garr_inc]

m+c results in being both 2 and 5, which is impossible for the equation. But if you think about it a little, or even plot them, you see the obvious solution with m=oo.

[@jameshart2622]

@@garr_inc Or you use the generalized formula for a line ax+by+c=0. Yes, the constants are equivalent up to a non-zero scalar multiplier, but it's symmetric in the variables and can represent any line without infinities. It can also represent lines at infinity, which is nifty. See projective geometry.

[@garr_inc]

@@jameshart2622 I was more describing why the mindless mx+c fails than claiming how to solve the "unusual" problem. But thanks for the input!

[@Muhahahahaz]

Ah, yes… x = 1 This is exactly why I complained when Matt said that y = mx + c could represent “any” line 😅

[@Voshchronos]

Quite clever!

[@HunterJE]

I feel like the easiest shortcut to understanding the "why" of the symmetry of duals is that a dual is very much by definition what you get if you swap the things being counted by two of our three variables for one another (while keeping the thing counted by the third constant...)

[@zahirgizzi7009]

about 12:58: I studied in Germany (Leipzig to be precise) and we learned is as y = mx+n 😆

[@henryrroland]

Thought that it was y=b•x +a

[@apfel1appelmann]

In Bavaria we used y = mx + t

[@zahirgizzi7009]

LOL In germany schoolsystem is a mess. It is "länder"-specific, so in saxony you have other standards than in bavaria for example 😂 One other big thing i think are the axes. I heard in some regions at school they label the axes x1, x2 and x3. We always labeled them x, y and z (probably mathematicly x1,x2,x3 makes more sense but maybe it's easier to get confused too idk ‾\°°/‾)

[@apfel1appelmann]

@@zahirgizzi7009 in high school we used the x and y axis for 2D and x1, x2, and x3 for 3D. In university we used x, y, and z.

[@omgitguy]

Interesting. I learned it as y = ax + b. We then extended to y = ax² + bx + c. After that we started using indexes: y = aₙxⁿ + ...

[@The_Knife_Pie]

Sweden uses y =kx + m, though I think that’s just because k-value (Swedish: K-värde) sounds better in Swedish than a lot of alternatives I’ve seen here

[@magnuswibeck1279]

k for koefficient (coefficient in Swedish). But I never got m.

[@Anonymous-ow6jz]

@@magnuswibeck1279 in the US, we use m because it stands for mlope :)

[@erkinalp]

​@@Anonymous-ow6jz magnitude

[@pyramear5414]

I always thought it was y = mx + c, where m is short for "multiplier" and c is short for "constant".

[@mumiemonstret]

@@pyramear5414 "Constant" is spelled "konstant" in Swedish so here it really should be "y = kx + k". Guess it would be a bit crippling for our mathematicians...

[@sachacendra3187]

Here in Switzerland we used a multiplicity of letters for the line: ax+by+c=0 or y=ax+b or y=px+q or y=mx+h or y=px+h were all things i encountered in my education. I believe the goal was to teach us that the letters didn't really matter. Also, since Swiss education is very decentralised and each teacher can more or less choose the material they want to use i wouldn't be surprised if elsewhere in Switzerland they would use completely different letters.

[@Tasarran]

I work in 3D and programming, and I still go back and forth between 'vertices' and 'vertexes' all the time...

[@mytube001]

As long as you don't say "verticee" for the singular, as unfortunately some do...

[@WindsorMason]

​@@mytube001 vertisay

[@Tasarran]

@@mytube001 That's silly, everyone knows it is 'vertiss'

[@chrishillery]

​@mytube001 A friend of mine in Linear Algebra class persistently used the term "matricee" as the singular of "matrices".

[@georgebayliss3291]

England (UK), GCSE: y = mx+c A-level: Very rarely told to give in the y = mx +c format, most commonly we leave in the format y-y1 = m(x-x1) or ax+by+c

[@Qermaq]

Making a tetrahedron with 3 blue faces and 1 brown face is brilliant, I'll grant you that.

[@collin4555]

The emergence of those lines is a great example of mathematical beauty. But I do love those quirky 3D printed shapes, too.

[@walderlopes3372]

It's been a while but I think I learned as y = ax + b here in Brazil back in the 80's.

[@hallohoegaathet7182]

Same in the Netherlands.

[@waxis9153]

Same in Belgium.

[@alesecq2172]

Same in Czech republic

[@taavettiihantola561]

I think I learned y=kx+b in Finland.

[@walderlopes3372]

@@taavettiihantola561that's the most different one so far. nice.

[@crowman8905]

Very interesting way of interpreting and visualising Euler's polyhedron Formula

[@Tranbarsjuice]

In Sweden, where I studied, the linear equation was introduced as y=kx+m. As far as I know it is still taught that way.

[@TrimutiusToo]

I studied in Russia, and there they used: y = ax + b or sometimes y = kx + a

[@gmr7901]

я всегда встречал только y = kx + b

[@omp199]

Do Russian people use Latin letters for variables, then?

[@gmr7901]

@@omp199 yes, of course :D but all the math terminology basically translated into Russian, like "многочлен" instead of "polynomial"

[@TrimutiusToo]

@@omp199 yeah latin and greek like everyone else

[@zoerycroft4300]

im so proud of myself, i knew nothing about this before the video, never even thought about arranging any polyhedra or anything, and when you were saying "well,, what different ways can we arrange them" i said... "i bet the euler characteristic is what makes it a plane"

[@jimsilsby3841]

Same. The instant he mentioned vertices, edges and faces, I immediately thought, "It's going to be Euler, isn't it?" Thanks, Numberphile!

[@Like4Schnitzel]

In Austria (not Australia) we typically use f(x) = kx+d for linear functions. I assumed this was the same in Germany but as other comments have shown me it isn't! Very interesting

[@AbiGail-ok7fc]

I would have been tempted to submit my favourite shape: 7 triangles making up a torus, but that would have been disqualified as it has Euler characteristic 0. (7 vertices, 14 edges, 7 faces), and hence, not on the plane. I remember tinkering with an early version of Mathematica for hours to get an R^3 embedable 7-triangle torus. But as an ex-topologist, I do agree with the "off the scale" submissions. Two sides faces, vertices with just two edges, or multiple edges between pairs of vertices, nothing wrong with that. As for the proof of the Euler characteristic being a constant (for planar graphs), instead of starting with a spanning tree, you can start with just a single vertex (V = 1, E = 0, F = 1), then add edges one by one, in such a way the graph remains connected. Each edge either adds a new vertex (in which case, V := V + 1, E := E + 1), or connects two existing vertices, adding a face (in which case E: = E + 1, F := F + 1). In either case, V - E + F remains constant.

[@nosarcasm1]

In Germany we have Different Letters vor y=mx+b ==>(m,b). So we also use (m,n),(a,b),(p,q),(m,k). In A-levels it's common using m for the pitch. It depends on the teacher and also the schoolbooks they use.

[@koinkorillas1692]

Thank you for the legible and useful video description

[@Gunstick]

Spanning tree is a term well known by network engineers. There is a "spanning tree protocol" which ensures your network does not have any loops, independent on how you interconnect everything. The network switches just "figure it out" (if you have loops in your network, everything just breaks down (you can have something called "broadcast storm")

[@KerryHallPhD]

I love the pivot at 12:56 from dismissive frustration to a positive query :D Excellent video all around!

[@moimoi73000]

13:03 hey! I'm French and I learned with y=ax+b. Also, very interesting video thanks matt

[@greenkiwi7941]

13:00 In Hungary, in 5-6th grade, we learn it like "y=ax+b" but later, in high school (9th grade and up) we use "y=mx+c". We often use 'm' as slope, and 'c' as a constant, for moving the graph up and down.

[@henryrroland]

12:56 I was raised in Brazil, here we use y = a·x + b

[@hendrikd2113]

This doesn't make sense. Once you go up to other polynomias the system breaks down. "b*x + a" seems logical.

[@henryrroland]

@@hendrikd2113 It does... y = ax² +bx+c The order of the coefficients follows the alphabet

[@octopus44445]

I heard "Glen and Friends" and thought this was about to be a very unexpected collab.

[@belg4mit]

Needs more maple syrup, eh?

[@stephanep.joanisse7712]

Would you really be that surprised if you found out Glen did a bit of math(s) on the side? Cooking, flying planes, video/film making… just another hobby?

[@octopus44445]

@@stephanep.joanisse7712 good point well made.

[@LeoStaley]

13:00 America uses y=mx+b, but of course you knew that, which is why you brought it up

[@marksman1416]

Also in Canada

[@Rhynome]

c for constant b for bintercept

[@gcewing]

b for where it bonks into the y axis.

[@BrentDeJong]

Great video! at 10:29 the captions said "Spanish tree graph" instead of "spanning" 12:22 "oiless" lol

[@mox3909]

I just learned about spanning trees for the first time 2 weeks ago. I thought it was cool but couldn't understand how it would ever be useful. I'm amazed.

[@ZetaTwo]

Sweden: in elementary school it was definitely y=kx+m but then in later parts of high school and at university I think ax+b was pretty common to be consistent with polynomials of arbitrary degree (ax^2+bx+c, etc)

[@Sam_on_YouTube]

Hey, I was on that live stream! Good times, good times.

[@dysphoricpeach]

good to know that i can always cut a sandwich made of polyhedra plotted by number of faces, edges, and vertices, no matter how many ingredients i add, perfectly in two! also interesting how matt went with the 3 blue 1 brown tetrahedron instead of the parker cube (a 3d solid with parker square faces)

[@philipmurphy2]

Always appreciate a good education KZbin channel

[@artificercreator]

Oh nice! Thanks for the good stuff

[@MrKalerender]

y = mx + c for Australia, however I use y = zx + c for my physics classes as m is for mass, and we do a lot of topics where you are trying to solve for mass from a gradient of an experiment and students writing m = f(m) is problematic. Z doesn't get used (no 3d vectors at high school) in any equations in our formula book so that's our side step!

[@mop9542]

Agreed, 2000s high school planar mathematics was y=mx+c. When I got to unii the tutors always used to write z=ax+b. Their reasoning was that m is for mass, c is the speed of light and z is the vertical plane. I still use it z=ax+b now because I've ended up a place where I'm doing calcs with masses and vector-forces and need variables that represent what is actually being input/output.

[@OverkillSD]

Because all shapes are liars, Matt! Had a great time seeing you in LA, by the way! I reference that software engineer joke all the time now and it's glorious. I kind of wish I had that slideshow :)

[@frankharr9466]

Well, I'm glad you're having fun. Let us know if you're coming to the Boston area. That would be cool.

[@Yhnertful]

about 30 years ago Faroe Islands used "y=ax+b", both in Faroese language books and Danish language books.

[@RagingRats]

Even though I always use y=mx+b, using ‘a’ instead of ‘m’ makes a lot more sense

[@mrWade101]

Sweden uses "y = kx+m", where k stands for "koefficient" = coefficient.

[@sergiorestrepo6657]

Thank you Matt

[@Sinnistering]

USA (IN), formative education in the 2000s, we used y = mx + b

[@osmanbadroodin3215]

I want that fused megaminx puzzle in the background 😭😭 , it would be so cool in my collection , I should try to make one

[@DrR0BERT]

At 15:00 you said that ax+by+cz=d, that you only need three of the unknowns a, b, and c. This only applies to planes not passing through the origin.

[@landsgevaer]

To limit the range of value such that bigger ones fit, you could hang them at (√v,√e,√f) and get a nice hyperboloid curved surface shape.

[@Zeitoun-bs8cj]

In France it's y=ax+b

[@marcosl2871]

Brazil too.

[@davidjowett8195]

16:15 isn't it great to see someone so passionate and animate about a subject they care for? 😄

[@LouisEmery]

5:00 I remember there was such a rule that included an offset of 2 when I was young, probably discovered by greeks.

[@adamrowedotcom]

16:00 my artwork is above your hand (but in the background) - made my day to see it make a cameo since it was inspired by watching another of your videos!

[@alicederyn]

"You can just divide through by that constant" UNLESS it's zero! ax + by + cz = 0 is a separate case from ax + by + cz = 1!

[@martijn8554]

Glad I'm not the only one who noticed this!

[@JamesWanders]

Bad news, Matt. When you said we should go "marvel" at the display, the auto-caption wrote it as "Marvel" so your channel belongs to Disney now.

[@mkb6418]

I knew from the start it was Euler's formula. But I give credits for the visualization, now you never forget it.

[@redvinstone]

In Sweden we use y=kx+m for the line equation.

[@mittarimato8994]

That line at 19:42 reminds me of the elemt table and their isotopes. The further you are away from the line, the more likely it is going to be an unstable isotope.

[@OneTrueBadShoe]

I would love to see Ben make a geogebra model with unfolded polyhedron and polychoron nets.

[@newwaveinfantry8362]

My guess before watching the full video (around 4 minutes): All polyhedra, when squashed, are planar graphs, thus v - e + f = 2 applies and defines a plane.

[@jumpman8282]

During the tree graph proof I kept thinking to myself: "But what does this tree graph have to do with the polyhedron in the first place?" It wasn't until you filled in the missing edges that I, too, "connected the dots" :)

[@fabienrymland3191]

Hello and thank you. In France, we use y = ax+b or y = mx+p (the first one mainly linked with the function f(x) = ax+b.

[@fabienrymland3191]

we tend to write function as : f(x) = ax^n + bx^n-1 ...

[@michaelwoodhams7866]

Here's a nice related result: For a polyhedron (e.g. a cube), at each edge we can define an angular deficit, being 360 degrees minus the angles of all the polygon vertices which meet there. E.g. for the cube, each vertex has three squares, each of which have 90 degree angles. So the deficit is 360 - 3 x 90 = 90. Now calculate this deficit for every vertex of the polygon, and add them up. In the case of the cube, there are eight identical vertices, so the total deficit is 90 x 8 = 720 degrees. Consider a regular triangular prism. Now each vertex has two squares and a triangle, so the vertex deficit is 360 - 2 x 90 - 60 = 120. There are six vertices, and 6 x 120 = 720. For any polyhedron which obeys Euler's polyhedron formula (i.e. no holes) and has plane faces, the answer is always 720 degrees. I leave the proof as an exercise for the student, but leave the hint to use Euler's polyhedron formula. It isn't difficult. I'm pretty sure, but haven't proved, that this extends to continuous surfaces: at every point there is a curvature. Integrate the curvature over the surface, and you'll get 4 pi (720 degrees in radians.) (Assuming your surface is embedded in Euclidian space and is topologically a sphere.)

[@AlexandHuman]

This feels somehow connected to prime numbers. Especially at the end when the central splits off from the square base pyramid. It reminds me of how 2 and 3 branch off into being above and/or below a multiple of 6.

[@mihir2012]

This is going to have something to do with the Euler's formula relating vertices, edges and faces, isn't it?

[@lazergurka-smerlin6561]

That's exactly what I was thinking aswell

[@quantumgaming9180]

The equation of a plane is ax + by + cz + d = 0 Euler's formula can be thought of as a plane equation( where a,c = 1 and b = -1 and d = -2) if x,y,z repressent the vertices, edges and faces of a polyhedron. Which is exactly what Matt shows in the video

[@HunterJE]

The dual of the beachball is a sort of puffed up pillow with two nonagonal faces with nine edges, trying to picture the dual of the over-verticed tetrahedron...

[@alexpotts6520]

It would be an over-faced tetrahedron with the standard four vertices. Each triangular face would have multiple copies bulging above and below the plane of the three vertices it connects to. EDIT: turns out I wasn't quite right about this. Thank you Mr Goomba dude.

[@galoomba5559]

@@alexpotts6520 That's not true. There would actually be a bunch of thin 2-gonal faces between every pair of triangular faces.

[@douglaswolfen7820]

I'm honestly confused about how you even define a face in this context. I'm used to assuming that a face is a flat polygon, and the 4 "faces" of that jagged tetrahedron aren't flat

[@jameshart2622]

My math classes always used y=ax+b for lines, but I prefer ax+by+c=0 because that naturally includes all lines, including vertical ones.

[@illadiel6049]

Matt knows how much we all enjoy pre-pre-orders

[@dgthe3]

Canadian here (specifically Ontario, if it makes a difference) y=mx+b m means slope, because they said so. b means y intercept, because they said so. Super easy for children to intuit.

[@channelzoldleo6841]

In Hungary we use y=mx+b, where m stands for "meredekség"=steepness, and b stands for "idk, just learn it".

[@scottdebrestian9875]

The polyplane is very interesting! I'd love to see the polyhedron with -14 vertices, -20 edges and -4 faces!

[@tobiaskarlsson7565]

Since you asked for letters/symbols used in different countries: in Sweden, we use 'k' for slope/gradient and 'm' for intersect. So the line equation would be y=kx+m.

[@chaos.corner]

I've seen a lot of comments about how the line is defined in the US but a lot of people don't realize that it was recently changed to be 'y=mx+you know the thing'

[@skyscraperfan]

The fact there always is a circle free path through all vertices was new to me. It does not seem obvious, but can probably also be solved be induction.

[@cftug]

Me, seeing the thumbnail: This is gonna be about Euler's formula, isn't it? Matt, at 4:10 : Vertices, edges, and faces. Me: Called it!

[@josephmarrow5598]

I was at JMM, super cool to see this

[@0xTJ]

I'm Canadian, and it's `y=mx+b' or death!

[@Jar.in.a.Bottle]

This must have been the reason why Saruman said about Gandalf, "So, you have chosen death".

[@foxquirk]

In Sweden we use y = mx + k for the line equation, because we spell constant with a k (konstant)

[@phillipbrown4686]

I think it's cool to see that the regular polyhedra made out of triangles are on one side of the dual line and the others are on the other side.

[@GregorShapiro]

The triangular polyhedra are 'minimal' in the sense that each face cannot be split further by connecting vertices of a face.

[@GavrielFleischer]

ax+b from Hungary

[@KubaSzymanowski]

Same in Poland. Lengyel, magyar két jó barát

[@DeGuerre]

The way I think about it is V + F = E + C + 1, where C is the number of components. A blank plane has V=0, F=1, E=0, C=0. Adding a vertex adds 1 to V and 1 to C, which keeps the equation true. Adding an edge either connects two components or connects two vertices in the same component. In the first case, it adds 1 to E, and subtracts 1 from C. In the second case, it adds 1 to E and 1 to F. Either way, any addition keeps the equation true.

[@pierreabbat6157]

I heard "jewel" and was confused for several seconds, until you swapped vertices and faces and I figured out you said "dual".

[@theVtuberCh]

When you said “3 blue” I wanted to say “1 brown”

[@BrianFullerPDX1]

Master class in how not to get to the point even including footage of walking on the sidewalk for no reason at all. I had math teachers like this in school that seemed to pride themselves on being obtuse and fragmented. Then later someone else would explain whatever it was they were teaching in about three sentences and you'd go "Oh, yeah, of course..."

[@BoxEnjoyer]

In the US we use mx+b, although you probably already knew that if you brought it up lol.

[@Crysal]

9:28 oh god, you mentioned spanning tree, now I have to listen to the spanning tree song.

[@5hape5hift3r]

I think a variation of Euler v-e+f is to include the null face and the whole. Giving -1+v-e+f-1 = 0 in 2d polytopes this works as well -1+v-e+1 for the pentagon is -1+5-5+1 = 0 Also works with all dimensions.

[@5hape5hift3r]

Technicly iprefer the negative of this but it works anyways,

[@high2407]

In the Netherlands we used y=ax+b for a simple line :)

[@macronencer]

UK, 1970s, y = mx + c. Obviously c stands for "constant", but I honestly can't remember whether any justification was given for the use of "m", nor what it actually was. Conceptually, I think I would prefer y = a + bx because I like the idea that you start from a fixed point, and THEN add a variable thing. Others here have also pointed out that this generalises more naturally for polynomials (e.g. y = a + bx + cx^2)

[@shaunsaggers]

So good

[@Johan323232]

I’m glad my first instinct for the polytopal planar equation was correct. Also, I would like to register a technical addendum. y=mx+c cannot give you the equation of any line, x=6 for example cannot be realized this way, it only gives you all linear functions. I wouldn’t call it a correction, because the video definitely wouldn’t be improved by making the distinction, but it does explain why you have the d value in the plane equation, because d=1 and d=0 are fundamentally different cases as it turns out.

[@madsohm]

Denmark does a bit of a mix, which I expect all countries do. We do "y = ax + b" (and for quadratics: y = ax² + bx + c) in primary school, but as you enter university, most tend to move to "y = mx + c"

[@Taversham]

Huh, that's the exact opposite of how I was taught in the UK, we had y=mx+c up until Year 13 (age 17-18), then it switched to y=ax+b at university. Wasn't until I read the comments on this video that I realised people use so many other options!

[@billcook4768]

Has Matt ever covered Kepler’s theories on polyhedrons and planetary orbits? If not, they would blow his mind.

[@mananself]

Ha, I just saw myself and my son at 5:44, on the right side. How fortunate

[@PaulSteMarie]

Complete tangent, but I like the 80/20 frame for this thing. 80/20 is cool stuff to work with.

[@PennyAfNorberg]

in sweden i learned K and M

[@Oskiirrr]

Yes, specifically y=kx+m if anyone reading is confused about the order

[@GregorShapiro]

lower case: k & m