Name: The mystery of 0.577 - Numberphile
Uploaded: 2020-03-27T18:07:03.000Z
Duration: 10 min 3 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

I wanna start in a nice, familiar, comfortable place, Brady.

Alright, all the way up to infinity. You remember what the answer to that was?

it's a, uh, divergent sum. Yeah, it's -1/12, isn't it?

that's that's not weird at all. so I'm not actually going to talk about that series

but I want to talk about something thats related to that series so I want to

consider what happens when we take the reciprocal of all these numbers and take

the sum of those so we take 1 + a half + a third + a quarter + a fifth.

what would we get if we added all these numbers together you can look at this

and you might think even though someone goes alright there's no way that's -1/12th

but it'd surely be infinity. you look at this and you think what is this?

i mean the numbers are getting... it's clearly getting bigger but the numbers

that you're adding again smaller and smaller and smaller so you might think

this goes to a finite answer, but actually it doesn't. this is infinite.

this is called the harmonic series, and it and it is infinite. it was first shown to

be infinite. and back in.. by a medieval, french mathematician Nicole Oresme. he

showed it in quite an elegant way, which we can go through now, he came up with

another series in which each element of the other series was less than the ones

you've got here and showed that guy was divergent. the other series that he

looked at, was the one where you asked what is the largest power of a half

that's less than the number you're interested in. so let me show you that's

probably easiest way. so 1, well it's less than or equal to that we're interested in. 1 is is

already a power of a half so we'll leave that as one. a half is of course a

power of a half so we'll leave that there. a third, well the largest power of

a half which is less than or equal to a third, is of course a quarter.

quarter, well we can leave that as a quarter. a fifth what's the largest power of a half,

that's uh, that is less than a fifth, well it's an eighth, which is a half cubed right.

for a sixth that would also be an eighth. for a seventh, that will also be an eighth. for an eighth.. it's an eighth

The next one would actually be a sixteenth, and you would go on like that, right. so this is

his alternative series, and now you look at this series and you just see that,

It actually has some nice structure so you got one plus a half, but then this combination

here gives you another half. these four here give you another half. there's

going to be eight of these 16ths, they'll give you another half, and what you're

doing is just adding a half loads of times, so it's pretty clear that this this

indeed diverges. and so by comparison, because each element here is larger than

this guy must also diverge as well. So that's the proof that that this diverges.

now.. but hang on, but each element of what you're doing up here.. *inaudible* I knew you were going to say that!

there's this is sort of nice illustrated way to think about this, this sum and

how it diverges. We've got a lousy band here right. That's what we call it in (Liverpool?).

in order to describe this, it can tell us something about this this sequence right. so what I

want you to imagine is, that, imagine this this lousy band is, is like a meter in

circumference. I know it's not, but imagine it is. And imagine we put an ant on him and

this ant, can travel one centimeter a second. okay now, after each

second we're going to stretch the lousy band, so that it's...

we add and another meter to it's circumference.

okay, so after the first second, the ant has moved one centimeter.

We stretch the elastic band by a meter, ok after the next second, the ants moved

another centimeter. we stretch the lousy band by another meter and so on

ok, question is, does the ant ever get all the way around the elastic band?

(Brady) I can't see how it could.  it seems like you couldn't,  it couldn't, right. but it can right and

it's because of this sequence, right. so let's just see why it can right. so think

about what happens in in the first second. think about, think about the

percentage of the elastic band that it manages to to traverse, okay. well it's

traveled.. the elastic band is 1 meter around it travels one centimeter that's 1%, right.

in the 1st second travels 1%. What happens in the 2nd second?

well in the 2nd second the last band is now 2 meters long

it travels another centimeter, so that's 0.5%. in the 3rd second

it travels 1 centimeter, that's 1/3th of a percent. their total length of the

ok, the same sequence, but this goes to infinity. It only has to get to a 100%, never mind infinity right?

clearly he's going to get all the way around the lousy band, and actually you

can work out how long it takes him, right. it takes him about e^100 seconds,

which is about 10^50 seconds which is quite a long time. it's so amusing

to think what would happen if you start the experiment now

and by the time that the ant finished it. well before we got to the end, you'd have..

the Sun would have become a red giant and it's probably swallowed up the earth

(Brady) you're adding one meter every second

yeah (Brady) and it's only moving one centimeter every second

(Brady) so every second the finish line is getting further away, every second.

but what it's already.. what was behind it also gets further away

so it's this is not just the distance in front but also the

distance behind and you can see what's really important is the percentage that

that it accumulates. and you can see that it just grows and grows and

grows and grows and grows. eventually it hits a 100. but you know after e^100 seconds

roughly, more or less. it gets it gets there. but yeah I mean you know, to

be honest, you know, we will even...  by the time you get to the end of this uhm...

this game, the universe has entered a black hole area where all the earth,

matter and stars and  including our experiment has ended up inside black

holes. so this is a gargantuan time, and what that illustrates is is really how

slowly this series grows. it grows very very slowly.. it grows, it grows indefinitely but

it does so very slowly and you can see that by how long it takes to go all the

way around. it actually grows logarithmically slowly. and what you can

show is, you can take, you know, this sequence all the way up to maybe the n'th

form. and you take off log(n) I'm taking the natural logarithm here,right.

what you get if you look at this guy, so each stage you take off that... the log.

well you know 1 minus.. you take the first step, 1-log(1) .. well that's just 1

1+1/2-log(2) is about... 0.8. 1+1/2+1/3-log(3)

that's about 0.7.  if you carried on ad infinitum, you actually

approach, a finite number. it's called gamma. it's the Euler-Mascheroni

constant. this is about 0.577, which is a.. if you take its square it's about 1/3rd

ok but not very much is known about this number. we know that it exists.

we know it exists, we can prove it exists.

basically the proof, it relies on the fact that this sequence is decreasing.

you can show also, that it's bounded from below.