A proof that e is irrational - Numberphile

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Professor Ed Copeland shows a proof by Joseph "Voldemort" Fourier that e is irrational.
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Ed Copeland is a physics professor at the University of Nottingham.
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And here's a previous video about e with James Grime: • e (Euler's Number) - N...
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23 янв 2021

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Комментарии : 1,4 тыс.

@OldQueer 3 года назад

Proof by contradiction always feels like ending a story with "and then they woke up and it was all a dream"

@guiorgy 3 года назад

The best kind of story For mathematicians

@fossilfighters101 3 года назад

@NoriMori1992 3 года назад

Nah, it's way more satisfying than that.

@SB-pq9dd 3 года назад

ههههههههه..... Nice!

@PrdndPhnx 3 года назад

Felt that

@duff003 3 года назад

"R has to be positive because it's just a sum of positive terms." Now that's rich coming from you lol

@Ben-kh2rh 3 года назад

And comes the Riemann zeta function of z(-1)

@TVIDS123 3 года назад

Hahahaha

@Happy_Abe 3 года назад

This made me laugh😂 Takes me back

@AlexM-xj7qd 3 года назад

Haaaaa

@shirou9790 3 года назад

well but the series converges, which cannot be said about the other example y'all are thinking about

@omgitsliamg5334 3 года назад

"e is approximately 3" Smells like ENGINEER in here

@Varksterable 3 года назад

I still prefer the classic "Pi = 3, for small values of Pi, and/or large values of 3."

@randomblueguy 3 года назад

@@Varksterable the limit as x approaches π from the left is 3.

@Das_Unterstrich 3 года назад

Pi = 3, and 3 is close to 5, so we can round up to 10

@N.I.R.A.T.I.A.S. 3 года назад

You seen that older Numberphile video with Dr. Padilla where Don Page wrote a paper like "yeah, e is approximately equal to 10"?

@johnkeefer8760 3 года назад

Engineer: π = e

@paaaaaaaaq 3 года назад

More professors and teachers should be like Ed. When you don't really know something at the moment just say "I don't really know".

@NardKoning 3 года назад

Luckily at uni I have usually found that they are like that. If you are comfortable with your knowledge, you probably are okay with it

@Headhunter_212 3 года назад

He really was stumped on the utility of e, as if he had never looked at it in that way. He had the humility to say “I don’t know”.

@stephenbeck7222 3 года назад

Daniel Sullivan e itself has tons of utility but e^x being its own derivative...well I mean there are some practical uses for it but the man was all prepared for his great presentation on irrationality, not to show some odd physics and what not.

@micha6589 3 года назад

@@stephenbeck7222 great preparation, when you are defining e as a number which does not change after diferentating xD but that number should be 0 xD

@jareknowak8712 3 года назад

...people, not teachers only.

@pleaseenteraname4824 3 года назад

"e^x is the only function that differentiated it gets back to itself" Zero function: _angry analytical noises_

@beeble2003 3 года назад

OK, OK, the only non-trivial function. ke^x: _angry analytical noises_

@godfreypigott 3 года назад

@@beeble2003 Still not general enough: y=k.e^(x+c)

@beeble2003 3 года назад

@@godfreypigott OK, OK, I'll come in again. Nooooobody expects the self-derivative! Our chief weapons are the trivial function, e^x, ke^x and ke^{x+c}.

@manuc.260 3 года назад

This is why the condition e^0 = 1 is important

@pleaseenteraname4824 3 года назад

@@beeble2003 Ah, I see you're a man of culture as well

@arasmith9076 3 года назад

"e" who shall not be named...

@seamon9732 3 года назад

Take those upvotes and get out!

@AlphaFoxDelta 3 года назад

Brilliant.

@lawrencedoliveiro9104 3 года назад

e by gum. Actually I think he’s from Manchester?

@Adhjie 3 года назад

mr. so-so?

@innertubez 3 года назад

Way underrated comment!

@ruchirkadam8510 3 года назад

This feels like an oldschool numberphile video :0

@NoriMori1992 3 года назад

Okay, so it's not just me. It's so old-school that I was thinking to myself, "Have I seen this before? Did they reupload a really old video?" 😂

@ruchirkadam8510 3 года назад

@@NoriMori1992 yeah, the thumbnail is the only thing that lets you know it isn’t one of the ancients, and even the ‘e’ in the thumbnail looks oldschool

@omri9325 3 года назад

Because the guest used to be a lot in the early videos

@francismoore3352 3 года назад

I reckon it is a bit old! We haven’t seen any videos with Ed recently which makes me think it been sitting unedited for a while. Also Brady’s camerawork is SO pre-pandemic.

@VinayKumar-vu3en 3 года назад

was looking for this comment.

@jasonpatterson9821 3 года назад

Why e is useful: When you are solving differential equations (which wind up describing an awful lot of things when you look carefully) you get lots of situations where a rate of change is related to the value of a thing. (ex: The rate of bunny births/deaths is related to how many bunnies there are.) When you find a solution, or even an approximation, for these sorts of things, e pops up all over the place. Compound interest, radioactive decay, population modeling, temperature change over time - all involve e.

@InexorableVideos 3 года назад

Fellow Eddie Woo enjoyer?

@tinyanisu1927 3 года назад

Kind of a naturally occurring number.

@luigivercotti6410 3 года назад

Basically, the e^x is the eigenfunction of the derivative operator, so it's bound to crop up in equations involving derivatives.

@Manish_Kumar_Singh 2 года назад

that's just curve fitting, donsent mean it's usefull. i say the same thing for fibonachi series, gloden number

@luigivercotti6410 2 года назад

@@Manish_Kumar_Singh Not so. Curve fitting is the fashioning of a "curve", or in other words a formula, to match a finite dataset. It is also known as interpolating, and, while generally looked down upon as an "inelegant" method, it has many a time provided essential insight in physical and mathematical matters. However, the fact that e pops up in most our differential equations is solely a consequence of the exponential function being the eigenfunction of the differential. In other words, e appears naturally around the differential simply because of what we have defined the differential to mean. If we were to look at equations based on other operators, other eigenfunctions (and, as a consequence, constants) would emerge.

@cariboubearmalachy1174 3 года назад

All mathematicians shoukd write "ta-dah!" At the end of their proofs instead of QED.

@JMUDoc 3 года назад

Tom Korner used to write "I WIN!".

@the_sophile 3 года назад

I agree

@hamudidoodi 3 года назад

I thought mathematicians draw a middle finger and address it to physicists

@coleabrahams9331 3 года назад

@@JMUDoc really?

@JMUDoc 3 года назад

@@coleabrahams9331 Sorry - it was Tom Korner, not Feynman. (And I was lectured by Korner at Cambridge, so I don't know why I confused the two!)

@lambdaprog 3 года назад

The smiliest astrophysicist in the planet is back!

@coolerdaniel9899 3 года назад

Well get him out of there already, he's got maths to do!

@felixmerz6229 3 года назад

Hollow Earthers unite!

@BeKindToBirds 3 года назад

I love him

@dommyajd9033 3 года назад

I went to a talk given by him a year ago safe to say it was super wholesome

@unclvinny 3 года назад

Ed’s the Mister Rogers of Numberphile, for sure. I’m always happy to see him!

@zacharydenboer5450 3 года назад

Love that it was pretty close to completely rigorous and had minimal hand waving

@forthrightgambitia1032 3 года назад

Including the proof of the sum!

@beeble2003 3 года назад

@@forthrightgambitia1032 Well, that actually was the biggest handwave.

@stephenbeck7222 3 года назад

beeble2003 pretty much how it’s taught in high school maths already. You need some calculus to do a real proof but the concepts are all there.

@EebstertheGreat 3 года назад

@@beeble2003 The proof for the finite case was essentially rigorous. In the infinite case, he asserts that if | _x_ | < 1, _xⁿ_ → 0 as _n_ → ∞, which technically requires a definition of a limit to prove, but which is certainly true. If you are at the point where you know enough math to even define _e_ , you probably have no trouble understanding or proving that fact. The biggest handwave might have been factoring 1/(q+1) out of the series, but that is also perfectly valid and something you would presumably know by the time you know the McLaurin series for _e_ .

@beeble2003 3 года назад

@@EebstertheGreat Yes, I'm not claiming anything was wrong, just that some details were handwaved away.

@Vodboi 3 года назад

14:28 "We know that R has to be positive, because its a sum of positive terms". The irony of this being said by the same guy who did the "1+2+3+4+... = -1/12" video.

@xxdirtytrashxx 3 года назад

Haha, thought the same, can't take him serious anymore

@txe9113 3 года назад

lol yeah

@santhosh_se5476 3 года назад

haha but this series converges unlike that one ....😛

@Ray25689 3 года назад

@@santhosh_se5476 still, saying that if you are adding positive numbers inifinite times leads to a negative number, without clarifying that you mean a different version of addition, is pretty sloppy

@hybmnzz2658 3 года назад

@@Ray25689 bruh the standard meaning of an infinite sum doesn't need clarification

@Philoreason 3 года назад

Camera man: Why is it important? Mathematician: Wrong question!

@mathanalogies9765 3 года назад

Ha! Yes! At the very least, e is theoretically important because it is "natural," in the sense that it answers that fundamental question of f'(x)=f(x). (In particular, of the general answers for this question, e^x is the one with multiplicative and additive identities as "choices" in the appropriate places.)

@tinyanisu1927 3 года назад

@@mathanalogies9765 also important to study growth/decay of things that are proportional to their instantaneous value.

@mathanalogies9765 3 года назад

@@tinyanisu1927 Yes! Used to study an object for which its instantaneous rate of growth/decay is proportional to its value, so to speak. The "natural" proportion being 1. This is why f(x)=e^x with f'(x)=1*e^x=1*f(x) is more "natural" than g(x)=e^(2x) with g'(x)=2*e^(2x)=2*g(x). And considering all h(x)=a*e^(bx+c) with h'(x)=a*b*e^(bx+c), it is most "natural" to use a=b=1 (mult. id.) and c=0 (add. id.). [I'm only adding all of the math here, now, to clarify what I meant in my first comment.] If you'd like to see a funny video about e, and why taking a=b=c=0 is actually the most "natural" choice, check out my Calc 2 (Integration Techniques and Applications) playlist on my channel - the video is called Exponential Function - How to Differentiate, How to Integrate // FUNNY/HUMOR

@Arduu123 3 года назад

There is really nothing else special about e than the fact that e^x = D(e^x). All exponential functions (including all laws of nature etc) could be written with any other number as the base, differentiating those equations is just easier when using e, thats why the convention to use e exists.

@mathanalogies9765 3 года назад

@@Arduu123 Yes, I agree (although the first sentence is a touch subjective). What you're highlighting is why the adjective "natural" is appended to the particular exponential function f(x)=e^x.

@aSpyIntheHaus 2 года назад

Prof Ed's voice is just so calming. I'm pretty sure I transcended into some dimension of e just listening to this video.

@puppypi9668 2 года назад

I see what you did there :^)

@ygalel 3 года назад

4:53 The moment you understand the choice of thumbnail

@enderwiggins8248 3 года назад

Btw if anyone’s curious about how he got that series expansion e^x = 1 + x + 1/2x^2 + 1/3! x^3 ... , a really easy way to verify that this makes sense is to use the property that e^x = d/dx e^x. If you take a derivative of each of the terms in the infinite series, they all kind of “shuffle” down. 1 -> 0 so it disappears, x -> 1, 1/2x^2 -> x, etc! (One of the reasons I think this expansion is so neat is it’s another visual way to see why e^(i pi) + 1 = 0

@forthrightgambitia1032 3 года назад

3:26 I am surprised he didn't say the obvious reason: that property lets us solve a whole bunch of differential equations that model physical and non-physical dynamics.

@SlightlyAsync 3 года назад

But how did e get into those models?

@forthrightgambitia1032 3 года назад

@@SlightlyAsync Any simple separable differential equation where there is a relationship between a function and itself will end up needing e as it represents a base case of, say, dx/dt = x which depending on the problem can then be scaled, transformed etc. This then is useful in things like radioactive decay, SIR epidemiological models, pharmacokinetics, ecological models etc etc. In fact ALL exponentials have the property that their derivative is n^xln(n) - so in fact by a scale quantity related to e (the natural log) all exponentials have this behaviour and e is in some sense the 'base case' for exponential growth that is then scaled/manipulated according to the needs. Exponential growth or decay are everywhere in nature due to the fact that many phenomena are multiplicative, more of one thing causes more of something else. And that is why e appears everywhere. Why is it that number is as fruitless a question as asking why the radius and circumference are 3.14, or why the ratios of right angle triangles follow the trigonometric functions, or the a/b = a + b/a is the Golden ratio. Nature is just that way.

@MushookieMan 3 года назад

He was blind-sided by the question. "What's the use of a newborn baby?"

@mauriciovinco6143 3 года назад

@@SlightlyAsyncit is simplification, but the sequence is roughly like this: 1. we seek the number n which satisfies this property of function f(x) = n^x such that df(x) / dx = f(x) (fixed point of differentiation ). From this we see that relative growth in function df(x)/f(x) with x is equal to increase in x, which is characteristic of exponential n^x, now n happens to be 2.71828..., we name it e because it is important, 2. we typically substitute a function which contains e^x with some additives into equations of models (they contain derivatives or integrals) and dividing by it, we get algebraic equations (i.e. in numbers, not functions) which are easier to solve. That's why e is so important

@nburakovsky 3 года назад

The number e has applications in finance, economics, growth rates, statistics, and tons of other stuff. Surprised he didn't know any applications outside of pure math

@tetsi0815 3 года назад

3:14 Brady is a brilliant interviewer. I love how he's able to ask "normal human" questions and how those are the ones that experts trip over and make them think. I bet Brady could have asked all kinds of very in depth detail questions about some obscure technicality and Prof Copeland would have had a quick answer but a simple "Why is that useful?" is not a thing that he has thought about :-D

@Dalenthas 3 года назад

That proof felt like setting up a lot of dominoes and then watching them all fall really quickly.

@tobiaswilhelmi4819 3 года назад

This would be a nice visualisation of a proof by contradiction.

@Orthosonic 3 года назад

The talk about the derivatives was a bit of a tangent...

@mysticalpie4695 3 года назад

A wild tangent 😁

@mathwithjanine 3 года назад

underrated comment

@StarGarnet03 3 года назад

GET OUT

@TheAlps36 3 года назад

I don't know, I found it pretty integral to understanding why e is important XD

@sparshjohri1109 3 года назад

@@TheAlps36 It's his area of expertise

@jellymop 3 года назад

Man I love Ed. It’s a pleasant surprise every time he shows up

@MrFireBath 3 года назад

The "tada" got me in stitches. Bravo on the presentation.

@codycast 3 года назад

It did? Really? Hm

@EebstertheGreat 3 года назад

"Tada" must be the official pronunciation of ∎.

@lotoa3383 3 года назад

"Why is that useful?" To me it is very useful when solving differential equations, a lot of the methods for solving them involve e in some way. Since differential equations describe a lot things in nature, e becomes a really important function.

@llll-lk2mm 2 года назад

plus logarithmic differentiation makes it so easy to deal a^x functions

@vgstep 3 года назад

My love for Ed has exponentially grown!

@dilemmacubing 3 года назад

yaaaaaaay another cuber that watches numberphile

@vgstep 3 года назад

@@dilemmacubing hey, sup?

@TrondReitan7000 3 года назад

After being primed by the 15 minutes of the proof, I read this comment as "My love for Ed has exponentially grown-factorial". :D

@vgstep 3 года назад

@@TrondReitan7000 haha nice

@AnnaDamm 3 года назад

Yeah but what Was the accelleration of the growth?

@Elfcheg 3 года назад

Prof Copeland's voice is ASMR in the world of math and physics. Could listen to him for hours.

@andrewlittle9063 3 года назад

The level of knowledge being laid out so deep the camera is having trouble focusing

@mysticalpie4695 3 года назад

And my brain comprehending 😅

@kadefringe 3 года назад

That moment you have to explain the function that's so important and used in basically everywhere, that you have no idea where to start with, then you simply say, "I don't really know."

@Ender240sxS13 3 года назад

I think it was more of a why is that function literally everywhere, why do physical processes behave in ways where this one number pops up everywhere. And that's what he doesn't know.

@bernardberari4250 3 года назад

His handwriting is so neat!

@Einyen 3 года назад

Yes, but why would you learn to write "x" as 2 curves? Is it due to some "don't cross the lines" philosophy?

@bernardberari4250 3 года назад

@@Einyen i think it comes from the way cursive was taught

@abdullahenaya 3 года назад

@@Einyen idk the exact reason but it helps with not confusing "x" the variable with the multipication symbol

@puremath3491 3 года назад

@@abdullahenaya that's why no one uses an x for multiplication anymore after they learn basic algebra

@HasekuraIsuna 3 года назад

_"Gauss apparently did this when he was three."_ Hahaha, misspoke! You mean third grade... right? Right?!

@HasekuraIsuna 3 года назад

@@sachinnandakumar1008 I can only handle so much Gauss-genius in one day.

@chihabmajdolin504 3 года назад

He did this when he was 3, nothing's impossible with Gauss

@oskarjung6738 3 года назад

@@chihabmajdolin504 not 3 its "e".

@StarGarnet03 3 года назад

@@oskarjung6738 Not funny. Get out.

@hexa3389 3 года назад

I wouldn't be surprised if Gauss learned calculus when he was still a fetus at this point.

@ZachGatesHere 3 года назад

"Why is that useful?" "...I'm not sure." I feel like a lot of mathematics is this. And it's part of why it's so much fun.

@WritingGeekNL 3 года назад

The number e is actually the most useful number in Applied Mathematics, so I'm not sure why he said that.

@marcellopz50 3 года назад

@@WritingGeekNL it's a hard question to answer on the spot like that

@hamudidoodi 3 года назад

You and the physicist community think that. Mathematics is mental master...

@Danonymous5000 3 года назад

@@WritingGeekNL he got caught up a bit, instead of defining e, he described an interesting property of e. Explaining why that property is important is a little harder than describing why e is important.

@ZachGatesHere 2 года назад

@@WritingGeekNL the question wasn't why e is useful, it was why that specific property is.

@laurihei 3 года назад

Well then, I was wondering what on Earth does Voldemort have to do with e's irrationality. Now I know :D

@mysticalpie4695 3 года назад

He is irrationally made as the antagonist in Harry potter I assume

@WindsorMason 2 года назад

@@mysticalpie4695 and covers the tale of his transcendence of the mortal coil.

@proxidize5738 3 года назад

Ed has such a welcoming and warm smile

@dle511 3 года назад

3:13 "why is that useful?" because it makes learning calculus just a tad easier Brady

@TheDJSyaheer 3 года назад

This definitely brings back the old-school Numberphile vibe.

@hanswurst1130 3 года назад

Why do I feel like the mathematics nerds are all just so humble people? I love it! Great to see not all of humanity is bad :)

@CodyEthanJordan 3 года назад

Feel free to see Stephen Wolfram for a counterexample lol

@vladimirjosh6575 3 года назад

@@CodyEthanJordan don't tell me he's the guy who made wolfram alpha

@KebabTM 3 года назад

@@vladimirjosh6575 Yea he's the founder of Wolfram

@jamieg2427 3 года назад

Physics people too tend to be pretty humble. This guy is a physicist, though of course he's also a math nerd 😊

@spinecho609 3 года назад

You dont get to see the bickering and backstabbing

@GoatzAreEpic 3 года назад

When he said: this is going to be just like proving sqrt(2) is irrational, i was like ok nice this will be easy. It wasn't...

@number-kv8px 3 года назад

I like how they discuss the problem the whole video instead of just solve it

@ibrahimbinkasim7419 3 года назад

- "ive been to his grave.." - "have you?" You two are real friends arent you?

@puppypi9668 2 года назад

as opposed to imaginary friends :')

@puppypi9668 2 года назад

(hey wait this gives me an idea :3 ) a fangirl fantasizes about someone and then meets them and they become friends irl > so your friendship started out imaginary but now it's also real! >> yeah, you might say our relationship is.._complex_ >> 8^)

@Jinjukei 3 года назад

Brilliant! Cheers! I like the calm, elegant and friendly way you are talking and being so keen on what your are doing.

@rewrose2838 3 года назад

e π and phi are always lurking around the corner

@CompanionCube 3 года назад

0:39 what is e? „o“

@mysticalpie4695 3 года назад

Other Vowels: I see how it is

@numberphile 3 года назад

Check out episode sponsor KiwiCo.com/Numberphile for 50% off your first month of any subscription. The crates are great! Catch more videos with Ed Copeland at: bit.ly/EdCopeland

@OG_CK2018 3 года назад

here before pin lol

@truckerob70 3 года назад

@@OG_CK2018 Still not pinned... Hmmm

@loneranger4282 3 года назад

u forgot to pin

@adityak1231 3 года назад

14:29 -1/12 enters the chat

@otakuribo 3 года назад

James Grime: we're gonna talk about e!! Ed Copeland: we're gonna *prove it*

@EMAngel2718 3 года назад

I wish more people wrote the initial 1 as 1/(0!)

@erroraftererror8329 3 года назад

How about x^0/0! ?

@llamallama4536 3 года назад

But that's 1/0, which is definitely not 1

@EMAngel2718 3 года назад

@@llamallama4536 0! is 1

@angelmendez-rivera351 3 года назад

@@llamallama4536 1/0! is not 1/0. 0! = 1.

@General12th 3 года назад

@@llamallama4536 no

@waqqiali2960 3 года назад

11:25 Gauss apparently did it when he was about, 3 Alright so...

@Duel53 3 года назад

I think he’s joking. The only thing I remember Gauss doing when he was 3 was checking his father’s books to make sure everything added up correctly.

@lagomoof 3 года назад

I think the "about three" from e itself may have had something to do with it as well.

@LandoCalrissiano 3 года назад

He did it when he was approximately e years old.

@RodelIturalde 3 года назад

The myth about Gauss have to be developed and thought to the younger generations. That it isn't precisely true is not that important. How do you think myths about other historical figures came to be?

@sumantchopde9039 3 года назад

It's the first time I'm this early to a numberphile video. Also, we came across this problem in our real analysis course a few days back. What a coincidence!

@solidazoriginal 3 года назад

Super appreciate the detail of Professor Copeland and also the graphics!!! Thank you very much

@Tehom1 3 года назад

Ed: It's the only number where if I differentiate it [meaning e^x] I get back the same number. Zero: Am I a joke to you?

@EvidLekan 3 года назад

Ce^x : I know, right?

@ShlokParab 3 года назад

Yes he forgot about zero

@jagatiello6900 3 года назад

Yeah, but the joke about integration doesn't work with zero, whereas e^x is bidirectional...

@benwincelberg9684 3 года назад

Zero: am I nothing to you?

@jagatiello6900 3 года назад

Pi talking to i on March 15... Pi: Why was e^x so lonely at my birthday party yesterday? i: Because every time he tried to integrate, he ended up with himself. Pi: Well, he would have ended up with himself and a constant, and by integrating further he could end up with any polynomial he wanted. i: Nah, he wouldn't befriend a constant. He has limits...(-inf to x)

@Euquila 3 года назад

It's such a simple proof but I would never in a million years figure it out

@skopernik Год назад

Such a pleasant voice and the manner of speech! It is a pure joy to listen.

@cameront4729 3 года назад

I remember doing this STEP question and it was one of the most beautiful yet surprisingly simple proofs I have come across!

@s8w5 3 года назад

15:05 "tah-dah!" Actually, mathematicians call that "qed".

@YaamFel 3 года назад

Mathematicians don't say "I love you", they say "$\blacksquare$", and I think that's beautiful

@cobracrystal_ 3 года назад

@@YaamFel $\hfill \square$ please

@Lefkada78 3 года назад

@@cobracrystal_ did you work for E-systems?

@puppypi9668 2 года назад

Q.E.D. is pronounced "tah-dah"

@dkranda 3 года назад

Love this video - just straight into some nice proofs!

@benoitb.3679 3 года назад

I saw Ed's face in the first frame and shouted YES! So happy to see a new video with Ed!

@BrunsterCoelho 3 года назад

This was wonderful as always Brady, thank you!

@MissHoyden 3 года назад

This was fun. Thanks.

@nexxai 3 года назад

I love the math in this one - it's so elegant, but man, the autofocus continually hunting was killing my eyes.

@colonelburak2906 3 года назад

Great to see a proof once in a while! Especially with professor Copeland. Although there is a typo in the graphics at 11:09. In the top equation, the second term in the bracket says 1/(q+1) but it should read 1/(q+2), and similarly the third term should read 1/((q+2)(q+3)). Thanks for great content!

@derrikconrad4235 3 года назад

There is a mistake at 11:18 in the equation presented. The top already factored out the q+1, so it is supposed to be q+2 and q+3. Sorry. I loved the video! Ed is great!

@IllidanS4 3 года назад

One definition of e that I like is that it is the only base for exponentiation where the slope around 0 is one. This follows the derivative definition ((e^x)'(0) = e^0 = 1), but has a nice consequence - e^x around 0 behaves like x + 1 which is useful for establishing logarithmic units: using e as the base means that multiplying by something close to 1 (imagine adding or subtracting small percentages) can be seen through the logarithm as adding that multiplier minus 1. x + y % = x × (1 + y / 100) ≈ x × e ^ (y / 100) The logarithmic unit that is based on e is called the neper (Np). Units of percentages are analogous to centinepers (cNp) but behave in a more consistent fashion.

@TheLunarNights123 3 года назад

Me: Sees Voldemort on thumbnail *So after the deathly hallows he retired and became a mathematician*

@imveryangryitsnotbutter 3 года назад

Friendly reminder that the author who created Voldemort is transphobic.

@superoriginalhandle 3 года назад

@@imveryangryitsnotbutter Passive aggressiveness I see. I still like Rowling, I just ignore their twitter and the transphobic stuff

@52flyingbicycles 3 года назад

Voldemort is not the first villain I associate with E

@PriyankitaPant 3 года назад

@@imveryangryitsnotbutter 👏

@PerMortensen 3 года назад

Mathemagician.

@malignusvonbottershnike563 3 года назад

Yo, I tried working through this exact proof yesterday; it was a Cambridge entry exam question where they guided you through it, and I could not figure out the last bit. So thanks for this video, perfect timing!

@papafreddy2123 3 года назад

One of the few proofs on Numberphile that I've actually listened all the way through and had no problems understanding it. A clear and detailed explanation that only requires knowledge of a few results that can be easily proven or learnt, hope to see more of these! Thanks Professor Ed!

@vikraal6974 3 года назад

15:05 Professor stole the spell, Lord Voldemort does not look happy.

@EleanorDrapeaux 3 года назад

"Why is that useful?" ". . . . . . . I don't know?"

@MatthewOBrien314 3 года назад

it's useful because it is a solution to the differential equation d/dx f(x) = f(x) i.e y' = y solution f(x) = e^x this differential equation shows up all the time in physics

@zakpodo 3 года назад

@@MatthewOBrien314 yes, thats the definition but why is it useful and ubiquitous. What i love about mathematics is that theres always another level, the more you know the more you can appreciate what a small amount what you know really is.

@werdwerdus 3 года назад

ed has the most soothing presentation style and voice

@nonomnismoriar9601 3 года назад

Great to see Professor Copeland again, more please!!!

@JoelRiggs 3 года назад

Professor Copeland!!! We’ve missed you.

@arrheniusleibniz 9 месяцев назад

I really enjoyed the video and also appreciate the clear and comprehensive explanation by Prof. Ed Copeland. Much thanks, Numberphile.

@deifiedtitan 3 года назад

Always a pleasure to see Prof. Copeland on the show.

@Tom_Het 3 года назад

3:13 I guess the reason why it's useful in my line of work is just that with imaginary powers, it has 1:1 mapping of radians. That makes it invaluable for frequency analysis. It's not obvious on looking at the plot, but I believe that property is inextricably linked to the other one that e^x is its own derivative. To see why, you'd have to look at its Taylor series and compare it to those of sine and cosine. It also helps you derive trig identities using only algebra.

@jangoofy 3 года назад

Never ask a mathematician why that is useful, enjoying it for its beauty is ok.

@JamesSpeiser 3 года назад

never ask why something mathematical is useful lol

@patxmcq 3 года назад

@@JamesSpeiser It's okay to ask, just don't ask a mathematician 😂

@orionmartoridouriet6834 3 года назад

Tbh it is a hard question, mainly because there's no easy explanation of what it does. For example, pi is "the circle number", and because everyone knows about circles, everyone can understand how it can be important. But e is "the differentiation number"? That to a layman doesn't sound cool, or useful. But anyone that has ever done Calculus I or greater knows how practical and everpresent e is

@jangoofy 3 года назад

I thank you all for the compound interest on this :-)

@jinjunliu2401 3 года назад

@@orionmartoridouriet6834 Then call it the "infinite interest number" and people will love it

@fackamato 3 года назад

I tried to subscribe, it turns out I'm already subscribed. Love this channel!

@numberphile 3 года назад

Bang that notification 🔔

@IDK_OR_DO_I 3 года назад

3:18 I like that humble answer!

@bertil0424 3 года назад

I've never been this early! You're Great Mr Numberphile 😁

@ShevkoMore 3 года назад

Asking based on ben3847's comment: Are you e?

@PaulPaulPaulson 3 года назад

0:36 "What actually is e?" "O"

@Master2594212 3 года назад

Good ole' Numberphile! This is the type of material that I subscribed for!

@rajibalam9748 3 года назад

Professor Copeland has a wonderful way of talking and teaching. He also seems like a very nice man. I wish I had him as one of my math teachers in uni.

@anon6514 3 года назад

3:13 "Why is that useful?" You have an analytic method for computing derivatives of similar functions with a base other than e that is simple to calculate: e.g. f(x) = 2^x 2^x = e ^ (x ln2) then using substitution (chain rule) : p = e ^ x --> f(p) = p ^ (ln2) df/dx = (dp/dx) . (df/dp) = [e ^ x] . [(ln2) p ^ (ln2 - 1)] = ln2 . [e ^ x] . [e ^ x(ln2 - 1)] = ln2 . e^ (x ln 2) = ln2 . 2 ^ x e can be thought of as a 'natural base' in the same way 'ln' can be thought of as the 'natural log' It's also why radians are the 'natural' unit for angles. Arclength / Radius = Angle if and only if you are using radians. Does this make tau more "natural" than pi? You decide.

@welovfree 3 года назад

i: [Talking to e] Be rational. e: Be real.

@zanop15 3 года назад

I literally got a hoodie with π and I having this conversation hahahaha

@josh11735 3 года назад

Pi comes along and says: I can solve both your problems if we work together.

@crumble2000 3 года назад

@@josh11735 when they get together: 1: be positive.

@smbvms 3 года назад

That is one of the clearest, most detailed explanations I have never understood in my life

@MaximeJean94 3 года назад

For the use in physics : the fact that the d/dx (e^x) = e^x can rbing us easy solutions for differentials equations (equations with functions and their diffenretials). It's important because differenciate a fonction tells us about how this functions evolve in time, and if you have a relation between the state of the system you study and the way it will evolve, you have a differential equation, and we can solve some of these with the exponential fonction.

@TheSmegPod 2 года назад

So basically for e to be a rational number there would have to be an integer that exists between 0 and 1? Am I understanding that right

@the_original_Bilb_Ono 3 года назад

*Smooth Brains:* doesn't know what _e_ is. *Big brain:* proves _e_ is an irrational number. *Galaxy Brains:* Proves that _e_ is in fact a letter.

@MrGodofcar 3 года назад

lol

@BPEREZRobertJamesL 3 года назад

*Supercluster Brains*: Proves that e is in fact, a symbol.

@gandelfy 3 года назад

Multiverse Brain: *e* is the trancendental signified

@MrGodofcar 3 года назад

@@gandelfy What is a "signified"?

@pimcoenders-with-a-c1725 3 года назад

Lovely! Beautiful proof by Fourier, and beautifully demonstrated!

@xyz.ijk. 3 года назад

So glad to be back to favorite numbers and their analyses.

@XEinstein 3 года назад

I though that nowadays everyone is trying to keep R < 1 🤔

@Smittel 3 года назад

take a shot every time he says "integer"

@darkchibi07 3 года назад

This brings me back. I remember our teacher in a Real Analysis class going over that proof.

@mikedoe1737 3 года назад

This all went totally over my head, but this guy is so amiable and pleasant, I just want to hug him.

@duffman18 3 года назад

He reminds me of the beginning of Art Attack when at 8:26 he goes: "This is an integer" "THIS is an integer" "This is.... INTEGER" 🎶🎶 da da, da dup dup, BWAH da da, da dup dup, BWAH

@sumdumbmick 3 года назад

'why is that useful?' best question ever.

@AlphaSierra5 3 года назад

It's useful because this is how interest compounds.

@LakeNipissing 3 года назад

Professor Ed has about the most pleasant voice to listen to ever !!

@PC_Simo 8 месяцев назад

”Why is this useful?” Mathematicians: *_”🤬TIGGERED!🤬”_*

@olivianunez7021 3 года назад

If you assume q>1 in 14:50, wouldn’t you have a loose end with q = 1, and then e being an integer, and therefore rational? On the other hand, even if q = 1, you would still get R < 1, so R can’t be a positive integer anyway, so why assume q > 1?

@user-he4ef9br7z 3 года назад

Yup .. that voldemort face on the thumbnail is exactly what I do when I am desperately trying to prove something but just can't.

@rebase 3 года назад

3:19 “why is that useful?” Because this way e^(cx) is an eigenfunction of the differential operator, which makes solving (certain) differential equations easy. For example, a linear dynamical system’s response to a (complex) exponential is always another (complex) exponential with a (complex) scale factor. This is one of the reasons why Fourier analysis is so useful for analysing linear dynamical systems.