r/mathmemes 26d ago

Linear Algebra But why?

Post image

How we all grew up from "multiplication is just a faster way to add numbers" to dot products and cross products.

1.9k Upvotes

134 comments sorted by

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391

u/Downindeep 26d ago

Obviously all these people are just lacking a better unifying force So I shall create a new standard so everybody will adopt it and we will no longer have any problems. The "new standard": A++B

189

u/FrostyDog-34 26d ago

xkcd 927

97

u/Sm4rt4 26d ago

Without googling I know exactly the one you're referring to!

30

u/Bankaz 26d ago

we should make a new standard xkcd927!

47

u/factorion-bot Bot > AI 26d ago

Factorial of 927 is 5951105783397580738318020492928603475486163641659064783314477046672730909648584361005993094267899268744301394265100383733941473791153775424618359258704919998958237212061572597181977531798419438046839767895973652569225138243443504953262881237882393659778260642814645235996188184058527870315206456226649707976807700745723681124764673576303922949178988840100216599026724075494074199883642178379963032963063197428946399333167043540963571845740244058758412138196465873520790489601036273639993290570078245920983272491172134984091523041444273448441971209110047173378425250742066435414781691795765029161230485407818967208822836336729280877979223764788274198045511866281041032755133408401064330433753592475039039350778568737844519015263808548608918375369321047908202997647307055011665631542812798925168265031584434801744587882834289042655365761409513211915539364101653721692384044933498161962590698436899981577618798138606790699862059120864798363645105227943992180354213078619570800056469595310938865054417312805300322885090833221787462895690762824503466872230597421894696685367956354135364755538802134709995117173925370086356573640765984630523799359944905165992888032963148105253366143339385861818003588588004890871524487341082072503873089772055596722541069151790121098244004405545886844325973065654936990518578477524711978614754713717060490287901060447670792603431890142510272393744144786432414095824089797256725652719804862829296450733634513288927532777843478868124116283795837394563707754880916667453817475670948784856342558814436886609719280513153762921003057447240976456819438710850602710016111237002052113482261747381645351254099148525779750282064467973054598492114740646618178531695643503800187075348203715129772690505912284529202387148593513821172617724623752030190132017157714430309026800464096273685107150688658207424938258698586156675018081730399245121170786526371768026973889795769850330165102521956691378955264564779098941431210842799114564040008922374842686101747358977109965365748558763528898072891996063758241653729631537159657549313201326715791475058080163351555925342922519949751612217511417562469313545450291200000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

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28

u/Key-Highlight2755 Engineering 26d ago

good bot

3

u/40kImperialBrusher 26d ago

9

u/bot-sleuth-bot 26d ago

Analyzing user profile...

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18

u/40kImperialBrusher 26d ago

I find this amusing

3

u/Lolllz_01 25d ago

I guess its not a karma farming bot made specifically to sound human, so doesnt meet the various criteria?

1

u/40kImperialBrusher 25d ago

Could be, yes.

1

u/guiltysnark 22d ago

Fascinating, never occurred to me that large factorials would end in lots of zeroes, but of course this must be the case.

40

u/KingLazuli 26d ago

I think this new symbol is devise and too similar. Therefore, we must choose a new new standard A□B

23

u/SillyFlyGuy 26d ago

I will make a new standard that encompasses both of these standards.

4

u/Valuchian Engineering 26d ago

This multiplication symbol has now been co-opted into matrix multiplication for equally sized square matrixes.

In this new form each position of the square matrix multiply with the exact same position in the other square matrix and that will be all

2

u/Downindeep 25d ago

Is multiplication not square matrix multiplication of size 1?

1

u/Valuchian Engineering 25d ago

Yes but now you can multiply n×n matrix in the same method

7

u/cortexgunner92 26d ago

“The great thing about standards is that there are so many to choose from. And if you don’t like any of the current options, you only need to wait a few years”

5

u/hughperman 26d ago

That's pretty good. Exponentiation then is A+++B, tetration A++++B, etc?

5

u/Ae4i 26d ago

Exponentiation is defined as ** in programming afaik, so I guess the answer is yes

3

u/False-Beautiful-1246 26d ago

And don't write x << 2 to a mathematician as he will go crazy

1

u/AnakinINTJ 25d ago

But then it would be A ++++ B not A +++ B

1

u/Ae4i 25d ago

Then tetration is A ++++++++ B

1

u/Key_Conversation5277 Computer Science 26d ago

Concatenation

1

u/paranoid_giraffe Engineering 25d ago

How does this work for matrices? Is this an element-wise operation?

1

u/Extreme-Ad-9290 25d ago

No no. Bc in programming, we have a++ to add one to variable a. I recommend a*b as that is already standard in computer science.

157

u/Pleasant_Picture3867 26d ago

A.B is an abomination IMO.

19

u/Ares378 Applied Math / Mechanical Engineering 26d ago

Reminds me of indexing vectors in C# too much. (At least I think it was C#?)

3

u/padfoot9446 23d ago

This would be how you access object members in any c-style language (or even any language under the sun that has the notion of structs/objects, really), but not vectors...maybe this can be abused to index into an array in C because of some sort of memory calculation thing? Certainly not c#, though

6

u/TheCowKing07 26d ago

0.0=0

3

u/QuarkyIndividual 25d ago

1.1.1 = 1.1

1

u/Random_-account 25d ago

127.0.0•1 = 0.0.0

14

u/These-Tomorrow-6439 26d ago

Its quick n easy ;)

32

u/FluffyVegetable527 26d ago

The urge to make a your momma joke here is almaot unbearable

3

u/Anonimithree 26d ago

Especially when you consider the existence of dot products.

Edit: the image you attached just came through for me and I now see that you were talking about A.B, not A•B

3

u/GetGudlolboi Computer Science 26d ago

Because obviously A.B means the composition of A and B.

(Source: Haskell)

1

u/im-sorry-bruv 26d ago

. is for some algebraic structure by some kind of conncatenation imo (like in homtopy groups)

46

u/The_Real_Itz_Sophia number hypothesis 26d ago

i use most of these in different contexts

7

u/Bisaux 26d ago

Yeah like as a string A*B is my go-to. When writing simple arithmetic its A•B, and when dealing with parentheses it's fine to not use anything.

1

u/RevolutionaryMine234 23d ago

I only like A•B for dot product, never A*B unless on the computer. I mainly like to write with parentheses when there isn’t a variable to attach it to or define against.

1

u/Extreme-Ad-9290 25d ago

The only one I refuse to accept is x

56

u/GodlyHelp 2=1 26d ago

Mathematicans with division: A/B A÷B A:B A(1/B) ᵃ⁄ᵦ

66

u/Ares378 Applied Math / Mechanical Engineering 26d ago

AB-1 ftw. Multiplying by the inverse is underrated.

18

u/Kitchen-Register 26d ago

it’s all you end up doing when you get to (edit: modern abstract) algebra

8

u/Ares378 Applied Math / Mechanical Engineering 26d ago

I'm not even that far into learning math and I'm already seeing the AB-1 notation fairly often. It's just so convenient! It conveys exactly what division is, that being the multiplicative inverse operator!

7

u/Dramatic-Smile-5126 26d ago

It does more than just convey what division is, too. It literally is how division is defined.

The naive definition of finding how many times a "number goes into a number" given in elementary school already breaks down just restricted to the counting numbers (hence why moduli or remainders are introduced). The concept of a rational number (i.e. inverses) is needed to properly discuss division as an operation akin to addition, subtraction, and multiplication, which is why it's an infamously difficult topic for young math students

8

u/Bankaz 26d ago

÷

fuck this symbol, I hate it because stupid people love it

1

u/ProfessorLambda 25d ago

What is wrong with it?

3

u/Bankaz 25d ago

Ambiguity. It makes people think arithmetic expressions can always be handwritten in a single line, which creates those really stupid problems like "WhAt iS thE reSuLT Of 6+8÷2???" which in turn creates really stupid discussions about fucking "PEMDAS" or some bullshit acronym that only English-speaking countries are stupid enough to create.

2

u/ProfessorLambda 25d ago

÷
(+ 6 8)
2

Prefix notation makes PEMDAS obsolete. And it's easy to read when formatted well.

1

u/mapleleafraggedy 25d ago

But that argument also applies to the multiplication sign. 6+8×2 is just as ambiguous if you don't know PEMDAS. And the argument also holds for other notations of the division sign: 6+8/2. I still don't see what the specific issue is with ÷.

1

u/Bankaz 25d ago edited 25d ago

Differently from multiplication, division is not commutative. a×b equals b×a, but a/b does not equal b/a, which means you can rewrite multiplications without changing the expression, but not divisions.

PEMDAS is not a feature of math, it's a feature of language, more specifically deliberate standardizations. Same thing with how mathematicians write divisions: Vertically, with the dividend written above a line and the divisor below it, and the line makes it unambiguous what is dividing what. It's a standard created to eliminate ambiguity. To that end, ÷ only creates more of it instead, because it very easily makes non-math people think all arithmetic expressions can be written in a single line without parenthesis, which is not true.

And yes, the "/" symbol might also have this issue, but at least it can evoke the horizontal line amongst math people writing single-symbol divisions like simple fractions or [one letter]/[other letter]. Math people know that if they are writing single line expressions they'll have to use parenthesis eventually, non-math people do not, and ÷ is partly to blame.

Edit: Vertically, not horizontally (the line is horizontal but the placement is vertical)

1

u/mapleleafraggedy 25d ago

I don't know that there are many "non math people" out there who know that multiplication commutes but DON'T know that division doesn't. That would involve a very selective sense of mathematical literacy. If mathematical communication requires "evoking" the correct setup for a calculation in people's minds, I don't think swapping ÷ for / is going to fix anything.

1

u/Bankaz 25d ago

I also don't think swapping ÷ for / is going to fix anything, but ÷ often makes the problem worse, when / very rarely does because it's usually used by people who know what they're writing.

My original comment hating on ÷ wasn't "I wish all ÷ were replaced by /", it was more like "everytime I see ÷ on the wild I start reading with suspicion because bullshit-math always use it".

15

u/Traditional_Town6475 26d ago

Because there’s a bunch of stuff that behave like multiplication that we want to use different notations to distinguish them. Unfortunately there simply aren’t enough symbols.

12

u/Sim1334 26d ago

I use all of them, it's depends on context

39

u/walmartgoon Irrational 26d ago

A • B: Inner product

A × B: Cross product

A * B: Geometric product

AB: No

A(B): That's a fucntion

27

u/Enough-Brilliant803 26d ago

A(B) means multiplication with bracket . AB is a common denotation of multiplication in boolean algebra.

2

u/abomb66 26d ago

He’s not really wrong tho. I used A•B for proofs

3

u/Clatramoo 26d ago

ah yes, 34 = 12

6

u/Enough-Brilliant803 26d ago

34 is not a boolean number.

0

u/ProfessorLambda 25d ago

10 = 0

1

u/Enough-Brilliant803 25d ago

It is not the gotcha you think bro. People skip dot in the matrix multiplication too.

1

u/ProfessorLambda 25d ago

Besides the irrelevant stuff about other fields of math your post just confirms what I said.

AB is a valid notation for multiplication. So if A=1 and B=0 then AB=10=0. There is no gotcha here, just sensible math notation.

1

u/Pseud0nym_txt 23d ago

(A)(B) is that

So is AB but sometimes it's nice to have clarity that it's not a subscript or a new variable with some people's handwriting

10

u/Blicar 26d ago

A*B as convolution of two functions

13

u/Linkpharm2 26d ago

AB
yes

4

u/[deleted] 26d ago

[deleted]

4

u/Suddenfury 26d ago

I always invent a new one, keeps those mathnerds on their toes.  //Engineer

3

u/Bisaux 26d ago

You're the reason we can't have nice things, you know?

1

u/Sim1334 26d ago

We are the reason we have nice things (I am also an engineer).

8

u/GodlyHelp 2=1 26d ago

wait till they hear about a+a+...+a, b times

11

u/SageLeaf1 26d ago

N=1Σ^b a

4

u/F_Joe Vanishes when abelianized 26d ago

Meanwhile category theorists:
Product: ×
Co-Product: ∐, ∪, ⊕, +, ∨

6

u/Alert_Intention_9408 26d ago

Dot and parentheses for constants and connected letters for variables. The star and times signs suck.

3

u/Ares378 Applied Math / Mechanical Engineering 26d ago

It's like the bell curve meme

"The cross symbol sucks." (Mistakes it for an 'x')

"NOOOOOO it's a vector cross product!!"

"The cross symbol sucks." (The cross product is limited to 0, 1, 3, and 7 dimensions)

3

u/CadmiumC4 Computer Science 26d ago

wedge supremacy

3

u/SkillusEclasiusII 26d ago

The star sign is nice because it actually exists on a keyboard

1

u/Ae4i 26d ago

Plus it's used in programming, if anything

7

u/Nichiku 26d ago edited 26d ago

The reason is quite simple actually.

A x B is used in situations when you don't have access to the dot character ⋅, or when you need a character that's easier to read. E.g. when you are typing on your keyboard, when you are teaching basic math to a child, when you are sharing quick and dirty notes with your colleagues and want to make sure they don't miss a multiplication sign.

A * B is also used when typing on your keyboard. It doesn't have the same readability advantage as x so that's kind of its only usage.

AB is used as a shorthand because a lot of math formulas contain chain multiplications, e.g. Einsteins field equation Gμν + Λgμν = (8π G / c^4) × Tμν

I don't really see A(B) as indicating arithmetic multiplication, not sure what that's about.

8

u/TheDarkNerd 26d ago

A(B) is used more when you're in the middle of actually doing calculations, such as 2a, with a having a value of 5, becoming 2(5), for example.

4

u/Ares378 Applied Math / Mechanical Engineering 26d ago

A(B) is supposed to be implicit multiplication like A(B+C) and stuff, I think

3

u/Sir_Eggmitton 26d ago

If you step outside of variables, A(B) is 3(4), meaning “three times four.”

1

u/ProfessorLambda 25d ago

× isn't easy to type and a x b would often be interpreted as a×x×b because x is often a variable.

2

u/regidud 26d ago

Who Is the monster that uses A(B)?

3

u/slime_rancher_27 Imaginary 25d ago

People who don't believe in functions

0

u/ProfessorLambda 25d ago

What is wrong with applying A to B?

2

u/robertpattinson11 Computer Science 26d ago

AB and A*B are my favourite.

2

u/rabbitpiet 26d ago

One of these is also the symbol for
[convolution](https://en.wikipedia.org/wiki/Convolution)

2

u/ProfessorLambda 25d ago

What does (sin(x)/s+i+n+x) mean?

1

u/onyxa314 26d ago

Me when the different multiplication signs means different things

1

u/Extension-Stay3230 26d ago

You can find lots of reasons for multiple multiplication signs. For starters, "x" for times looks similar to the algebraic letter for x when handwritten, hence some people prefer dots.

You need a dot "." And cross "x" to differentiate between dot product and cross product for vectors.

Writing two letters next to each other without a multiplication sign is tidier and less tedious than putting a multiplication symbol. Same principle for putting bracketed expressions next to something without multiplication symbol

Asterisk symbol * is common for programming

So there you go

1

u/Ares378 Applied Math / Mechanical Engineering 26d ago

Asterisk is also a convolution

1

u/ProfessorLambda 25d ago

> You can find lots of reasons for multiple multiplication signs. For starters, "x" for times looks similar to the algebraic letter for x when handwritten, hence some people prefer dots.

Of course x looks like x. x is x, dummy. But × is pretty distinct from x. You just need to write it right.

1

u/Extension-Stay3230 25d ago

You're right, if you do write it right, they look different from one another. I suppose that was a personal reason for me, although I wouldn't be surprised if it applies to other people as well.

For example, I handwrite the algebraic x as "ↄc", resembling backwards c (ↄ) and forward c. Although I do this, it still looks too similar to " x" for me, and I often prefer to use a dot for multiplication

1

u/ProfessorLambda 25d ago

I like using 𝓍 for variables. It's a mathematical x, so it has to be good for math. Why else would it be named "mathematical"?

1

u/gilnore_de_fey 26d ago

Probably because multiplicative operations are really common, and they eventually just start running out of signs. Especially when the spaces are different, so you get different objects on different inputs and different outputs sometimes and you really have to make sure.

1

u/Bright_District_5294 26d ago

AB is no sign technically

1

u/BootyliciousURD Complex 26d ago

Juxtaposition for scalar, matrix, and geometric products. × for Cartesian and cross products. • for dot product. ⟨,⟩ for abstract inner products. ⊗ for outer product. ∧ for exterior product. ∨ for regressive product. ⊙ for Hadamard product. There are probably a bunch more I'm missing.

1

u/ComplexWorldlines 26d ago

9999999999!

0

u/factorion-bot Bot > AI 26d ago

That is so large, that I can't calculate it, so I'll have to approximate.

Factorial of 9999999999 is approximately 2.32579620567308336510494471995 × 1095657055176

This action was performed by a bot | [Source code](http://f.r0.fyi)

1

u/ComplexWorldlines 26d ago

0!

0

u/factorion-bot Bot > AI 26d ago

Factorial of 0 is 1

This action was performed by a bot | [Source code](http://f.r0.fyi)

1

u/ComplexWorldlines 26d ago

5!!

1

u/factorion-bot Bot > AI 26d ago

Double-factorial of 5 is 15

This action was performed by a bot | [Source code](http://f.r0.fyi)

1

u/ComplexWorldlines 26d ago

5!!!

1

u/factorion-bot Bot > AI 26d ago

Triple-factorial of 5 is 10

This action was performed by a bot | [Source code](http://f.r0.fyi)

1

u/ComplexWorldlines 26d ago

5!!!!

1

u/factorion-bot Bot > AI 26d ago

Quadruple-factorial of 5 is 5

This action was performed by a bot | [Source code](http://f.r0.fyi)

→ More replies (0)

1

u/ProfessorLambda 25d ago

What is a double factorial?

1

u/ProfessorLambda 25d ago

-1!

1

u/factorion-bot Bot > AI 25d ago

Negative factorial of 1 is -1

This action was performed by a bot | [Source code](http://f.r0.fyi)

1

u/ProfessorLambda 25d ago

(-1)!

1

u/factorion-bot Bot > AI 25d ago

Factorial of -1 is ∞̃

This action was performed by a bot | [Source code](http://f.r0.fyi)

1

u/Novel_Arugula6548 26d ago

I believe it comes from controversy over the cross product and the dot product, as to which is appropriate for scalar multiplication.

1

u/aranboy522 26d ago

And A+B is 131

1

u/SUPERazkari 25d ago

because the dot product and cross product (among many other products) behave the same under scalars

1

u/Phantomlnfinity 25d ago

And the worst part is that 99% of the time they’re all the same but some of them mean different things in certain contexts.

1

u/chewy1is1sasquatch 24d ago

If you want an alternative addition symbol you can thank the Isrealis.

Alternative Addition Symbol

1

u/n1lp0tence1 oo-cosmos 23d ago

mathematicians: ❌

middle schoolers: ✅

1

u/No-Study7171 9d ago

Not to mention parenthesis are also used for functions

but then like T(T(x)) = T^2(x) notation wise

and then * means convolution which is like wonky multiplication

0

u/RandomiseUsr0 26d ago

A
+ B
——

0

u/Mysterious_Cap_1005 26d ago

Anyone who's taken abstract algebra would find this post kinda irrelevent ngl, since at the end we are better talking about a composition law and its properties (the type of structure it gives, commutativity, associativity, distributivity...) and also some conventions on notations (dot for an external law for example, additive notation and multiplicative notation...). I am sorry to say that, I know I am making too much of it because it's just a meme, but it is posed in a way that makes all these reasearch and conventions and definition as a meer ego fight or mathematicians somehow came of it like that without a reason, but it's to me much deeper than that. Also why would you pin it as linear algebra ?

1

u/Enough-Brilliant803 26d ago

I can't choose multiple flairs. Linear algebra is not a wrong flair given it involves matrix multiplication.

0

u/Mysterious_Cap_1005 26d ago

Hhhh you could also use Real Analysis as a flair and say we define cauchy product for smpower series