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/sci/ - Science & Math


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5858468 No.5858468[DELETED]  [Reply] [Original]

1-1+1-1+1-1+1-1+1-1+1-1 = 1/2

>> No.5858469

>>5858468

False.

>> No.5858476

>>5858469
1-1+1-1+1-1+1-1+1-1+1-1 ... = 1/2

>> No.5858481

>>5858468

You on let's make a deal. Monty Hall has a woman show you three boxes. One has two goats in it, one has two cars in it, one has a goat and a car it. You pick one of the boxes. What are the odds that the woman has a sister?

>> No.5858480

>>5858468
= zero

>> No.5858487

>>5858476

Depending on your definition for sequence convergence, this can be true.

The problem when people write things as you have is that you are treating "1-1+1-1+1-1+1..." as if without further clarification it is an innately meaningful, physical thing. It is not.

>> No.5858495

By judicious use of parenthesis, you can make this converge to anything you like. Consider that 1=a+1-a and regroup to have convergence to a.

>> No.5858499

>>5858495

This is false; you get this result if you use intuitive, inconsistent, ill-defined ideas of what that sum means.

>> No.5858588

>>5858499
>This is false
>Repeat what I said

>> No.5858594

>infinite series
>inconsistent

we may have a completeness problem here

>> No.5858597

>>5858588
>is obviously not a mathematician

Define what you mean by the sum "1-1+1-1+1-1+1..."

Then we can talk.

Spoiler: "The number you get if you keep adding like that forever" is not a well-posed definition.

>> No.5858613

>>5858597
>is obviously not a mathematician
>repeat what I said. Again.

>> No.5858619

>>5858613
>can't even pose a problem properly

>> No.5858620

>>5858597
<span class="math">\sum_{k=0}^{+ \infty} (-1)^k[/spoiler]

>> No.5858623

>>5858619
Christ, read my original comment. I'm obviously giving a counter example. You're just stating over and over that my counter example actually IS a counter example.

>> No.5858626

Can't believe it's taken this long for anyone to say this but terms in a series have to approach zero for the thing to have a possibility to converge ...

>> No.5858629

>>5858623

You haven't provided a counter-example, because your original comment /does not formally mean anything/.

If you don't define what you mean by convergence of an infinite sum beforehand, saying "you can make it converge to anything" is an empty statement. All you have done is fool your own intuition.

Putting parenthesis in an infinite sum, by the way, is not necessarily a well-posed operation.

>> No.5858635

>>5858626

For the standard definition of sequence convergence, this is true. There are expanded notions of convergence for which it is not.

>> No.5858637

>>5858626
learn2 abel/cesaro/lindelöf convergence

>> No.5858639

>>5858629
>You haven't provided a counter-example
>Point out what my counter example is a counter example to.

>> No.5858643

>>5858639

Unless you were being facetious in your original comment, I don't see what you're getting at.

>> No.5858652

>>5858468
I knew this would surface here as soon as I saw it.
Hoping on seeing some clever ideas.

>> No.5858658

>>5858643
I was manipulting the series formally (i.e. supposing that it converged, say, absolutely) and arrived at a contradiction. The argument I gave with a=1/2 is the one that you typically see when people argue that the alternating series above converges to 1/2.

>> No.5858660

>>5858658

You were crap at communicating what you were trying to do. All you really pointed out was that notating the series as "1 + 1 - 1 +1 - 1 ..." makes it easy to do things which look intuitively correct but are formally meaningless.

>> No.5858690

>>5858660
That was my point exactly. Seems I succeeded.

>> No.5858697

>>5858660
But really, my counter-example was deeper than that. I've met people who thought that judicious use of parenthesis could only produce the result 1/2, thereby somehow singling out a probable result. My example shows that judicious use of parenthesis can produce any result, and therefore doesn't make sense.

>> No.5858705

>>5858703

confusion*

Christ, I can't type today.

>> No.5858703

>>5858697

Ah, I understand you now.

Communication through text sucks.

At any rate, confusing over that series pretty much boils down to confusion over what series convergence formally means. The only people who puzzle over it are those who haven't had any real math education.

>> No.5858719

>>5858703
Agreed.

>> No.5858735

>>5858495
could you be more specific ? i don't see how you do that.

>> No.5858756

>>5858703
what does the word "formally" means to you ? i'm asking that because the way i see it a " formal " manipulation is a manipulation of the form, ie a symbolic manipulation ( as in symbolic computation ).

Anyway, those confusions of people without real math education have lead to interesting theory http://en.wikipedia.org/wiki/Divergent_series..

>> No.5858762

>>5858756

I've always taken "formally" to mean "following strictly from a set of definitions."

>> No.5859097

>>5858735
1-1+1-1+1-1...
=(a+1-a)-(a+1-a)+(a+1-a)-(a+1-a)+(a+1-a)-(a+1-a)...
=a+(1-a-a+1+a+a)+(1-a-a+1+a+a)+(1-a-a+1+a...
=a

>> No.5859099

>>5859097
Bah
1-1+1-1+1-1...
=(a+1-a)-(a+1-a)+(a+1-a)-(a+1-a)+(a+1-a)-(a+1-a)...
=a+(1-a-a-1+a+a)+(1-a-a-1+a+a)+(1-a-a-1+a...
=a

>> No.5859100

Cesaro summation.

>> No.5859137

>>5858481
50/50. She eithers' got's one er she don't.

>> No.5859159

A = 1 - 1 + 1 - 1 + 1 - 1 + ...
1 - A = 1 - (1 - 1 + 1 - 1 + 1 - 1 + ...)
1 - A = A
2A = 1
A = 0.5

>> No.5859172

>>5859159
You can get any rational number (possibly any real but that is hard to prove).
ie if you want 3:
take the first 3 +1s and group the 1st -1 and the 4th +1, the 2nd -1 and 5th +1, each group equals 0 so you get 3.

If you want a fraction,
A = 1 - 1 + 1 - 1 + 1 - 1 + ...
1 - A = 1 - (1 - 1 + 1 - 1 + 1 - 1 + ...)
1 - A - A= 1 - (1 - 1 + 1 - 1 + 1 - 1 + ...) - (1 - 1 + 1 - 1 + 1 - 1 + ...)
3A=1.. .etc

>> No.5859605

1-1+1-1+1-1+1-1+1-1+1-1 = 0

FTFY

>> No.5859622

>>5859159
Holy shit, this is wrong on so many levels I think you just made me feel American.

>> No.5859657

>>5859622
Explain then!

>> No.5859668

https://en.wikipedia.org/wiki/Grandi%27s_series

>> No.5859699

>>5859172
see
>>5859099

>> No.5859777

>>5858487
ding ding ding ding ding... winnner.

>> No.5860241

>>5858468
[spoiler]1-1 ok that's 0. +1 ok 1.-1 ok 0 +1 ok 1 -1 ok 0.... I'm a noob at math, and i want to come to this board in order to be better at it. Why would it ever get to 1/2?

>> No.5860272

>>5860241
I think someone already prooved it, but i guess you didn't see it, so here it is:

S=1-1+1-1+1-1+1...
1-S=1-(1-1+1-1+1-1+1...)
1-S=1-1+1-1+1-1+1-1...
so: 1-S=S
1=2S
then S=1/2

>> No.5860280

>>5860272

That is not a valid proof for why the series of (-1)^n converges to 1/2 under certain definitions of sequence convergence.

The reasons for the invalidity of that "proof" have been discussed at length in the thread. Namely, the manipulation assumes that the sum converges.

If you are confused about that sum, it is because you do not actually understand what is meant by an infinite sum. Take an analysis course, and it will become clear.

Under the most common definition of sequence convergence, that sum is undefined.

>> No.5860296

>>5860280
Yeah I know that it doesn't converge if you look at it as a regular infinite sum (if we can say it like that?), I've seen that in calculus while we were studying series and infinite sums for integrating certain functions. But if we assume that the serie has a sum, then it is the answer to the sum. It just depend on how you see the problem.

>> No.5860330

>>5860296

It is true that if the sum converges, then it must converge to 1/2. Your proof was almost correct there, but your use of parenthesis is abuse of notation and the proper way to notate it is to compare the sequence with a sequence offset by one (which must, by definition, converge to the same value if the series converges).

At any rate, if you want it to converge you first must define what you mean by series convergence in a way in which it does, indeed, converge (Cesaro summation or otherwise).

>> No.5860772

>>5858469
Why?

>> No.5860794

Is there any meaning to considering the limsup and liminf of the partial sums and averaging them? This yields 1/2 in this case as well.

>> No.5862012

Alternating series test.

/thread

>> No.5862917

That's the infinite series, not the finite sum.

>> No.5862999

Divergent

Partial sums are: 1,0,1,0,1,0,1,0,...

Limit of partial sums does not exist.

>> No.5863283

>>5862999
ay, cptn.

>> No.5863315

>>5858468
Integers are closed under multiplication?

>> No.5863316

>>5863315
umm... addition, as well.
But, addition seems more relevant.

>> No.5863318

>>5859657
A = 1 - 1 + 1 - 1 + 1 - 1 + ...
1 - A = 1 - (1 - 1 + 1 - 1 + 1 - 1 + ...)
>1 - A = A dafuq
it's 1-A=1-A

>> No.5863320

>>5858637
>>5859100

>> No.5863321

S = 1-1+1-1+1-1+1-1+1-1+1-1 .....
S = 1 -(1-1+1-1+1-1+1-1+1-1+1-1.....)
S = 1 - S
2S = 1
S = 1/2

Sum of values in the sequence = 1/2

S = 1-1+1-1+1-1+1-1+1-1+1-1 .....
S = (1-1) + (1-1) + (1-1) ....
S = 0 + 0 + 0 ....
S= 0

Sum of values in the sequence = 0

S = 1-1+1-1+1-1+1-1+1-1+1-1 .....
S = 1 + (-1+1) + (-1+1) + (-1+1) .....
S = 1 + 0 + 0 +0....
S = 1

Sum of values in the sequence = 1

J = 1-1+1-1+1-1+1-1+1-1+1-1
J = 9 (1/9 - 1/9 + 1/9 ....)
9 + 99 (1/9) = 9 + 11
9 + 11 = 911
J = 911

Jews did 9/11

>> No.5863324

>>5859172

>take the first 3 +1s and group the 1st -1 and the 4th +1, the 2nd -1 and 5th +1, each group equals 0 so you get 3.

Just fyi: you cant do this.

>> No.5863326
File: 431 KB, 474x599, utilitarism.png [View same] [iqdb] [saucenao] [google]
5863326

>>5863321
my sides

>> No.5863327

>>5859137

Actually, it was a trick question. It is a goat.

>> No.5863329

>>5863318
1 - A = 1 - (1 - 1 + 1 - 1 + 1 - 1 + ...) = 1 - 1 + 1 - 1 + 1 - 1 + ... = A

>> No.5863334

>>5863329
>1 - A = 1 - (1 - 1 + 1 - 1 + 1 - 1 + ...) =
= 1-1+1-1+1-1+1 - ...
and if i repeat this sequence i get:
1 - 1 - 1 + 1 ....

>> No.5863341

>>5863334

The brackets arent there for nothing you know.

Expanding - (1 - 1 + 1 - 1 + 1 - 1 + ...)

Gets you - 1 + 1 -1 + 1 - 1 + 1 - ....

So 1 - (1 - 1 + 1 - 1 + 1 - 1 + ...)

Gets you 1 - 1 + 1 -1 + 1 - 1 + 1 - .... = A

>> No.5864141

>>5863341
Now all you need to do is solve for A.

>> No.5864808

>>5864141
But I only know how to solve for x.

>> No.5865935

>>5864808
Take a class on advanced calculus.

>> No.5866834

>>5865935
What is advanced calculus and why do we need it?

>> No.5866896
File: 49 KB, 624x420, glorious winged faggot.jpg [View same] [iqdb] [saucenao] [google]
5866896

>>5858468
No, it's 0.

>> No.5867893

>>5866834
Advanced calculus is the first higher math class. It'll teach you equivariant cohomologies and K-theory.

>> No.5868517

>>5866896
No, it's indeterminate.

>> No.5869054

>>5858468
The answer's 1. Because infinity-infinity=0, so 1-0=1. Just group the numbers up into infinites. Monty's fatal mistake was that he added a false rule- implying there is a finite limit. If that were the case then the stated limit would determine the result. The only way he's right is if it were a finite pattern that was unknown.

BTW- 0 divided by 0=infinity, for a division is but a ratio comparison and 0 has no value to require rationing with, deeming the division infinite irregardless the value of the divisor. That means any number divides infinitely into 0 and 0 divides 0 into any value that isn't 0 itself.

You fucking dumbasses.

>> No.5869125

>>5867893
Thank you.

>> No.5869189

>>5869054
>infinity-infinity=0
>0 divided by 0=infinity

How to spot a american.

>> No.5869201

>>5858468
well... on the fact that % is just - - - ... you could just as easily say that it = 1x2

>> No.5869229

>>5858499
>>5858735
http://en.wikipedia.org/wiki/Conditional_convergence

>> No.5870654

>>5869229
Doesn't apply here.

>> No.5871385

>>5869189
She is right. Are you implying Americans are knowledgeable in mathematics?

>> No.5873168

>>5871385
Didn't an american win the maths nobel prize?

>> No.5873494

>>5871385
what? how do you know anon is a gril? wth is up with people on here? uh nvm it's not important just strange

>> No.5873527
File: 88 KB, 250x250, 1369732810018.png [View same] [iqdb] [saucenao] [google]
5873527

>>5873168
>maths nobel prize

>> No.5873566

>>5873168

Are you talking about Ed Witten?

>> No.5873573

What the fuck? Why are you guys complicating it so much?
It's just a simple addition and subtraction question 10 year old kids can do.

http://www.wolframalpha.com/input/?i=1-1%2B1-1%2B1-1%2B1-1%2B1-1%2B1-1

You guys are retarded

>> No.5873598

it's cesaro convergent to 1/2, but diverges using "standard" techniques.

>> No.5873796

>>5873573
>10 year old
Addition and subtraction are taught at a much younger age.

>> No.5875364

>>5873796
not in america

>> No.5876183

>>5875364
When is it taught there?

>> No.5876196

>>5876183

it's taught when you're 7 or 8

>> No.5876213

Series does not converge?

Only took up to calc 2, what am i doing here?

>> No.5877598

>>5876196
I don't think such advanced math is taught in elementary school.

>> No.5877612

>>5877598
>amurica
shit like this is taught at ages 5-7 everywhere else

>> No.5877639

More interesting:

1-2+3-4+5-6+....=1/4

>> No.5878696

>>5860772

the series ended in OP, it was nt continuing to infinity

>> No.5878851

>>5877639
Is this some "p-adic number" trick?

>> No.5880011

>>5877612
I think you mistook it for calculus.

>> No.5880044

>>5878851

Haha, no. You can prove that (1-1+1-1+1-1...)^2=1-2+3-4+5-6..=(1/2)^2.

Of course, the series is divergent, but if it were convergent, it would be 1/4.

>> No.5880683

>>5873527
You have not heard of it?

>> No.5882092

>>5880683
No, I haven't. I don't know of a maths nobel prize.

>> No.5882716

>>5880011
What is calculus and how is it different from advanced calculus?

>> No.5882718

>>5858468
divergent series gtfo

>> No.5884192

>>5882716
Advanced calculus is more advanced than calculus. You wouldn't learn about para-quaternionic Kahler manifolds in calculus. But in advanced calculus you do.

>> No.5884196
File: 110 KB, 500x688, linklaugh2.jpg [View same] [iqdb] [saucenao] [google]
5884196

>>5884192
>para-quaternionic

>mfw people still use associative division algebras over global fields

>> No.5884237

>>5882718
Yes the series is divergent, and yes, its sum can be defined in a very clean and unique way, and its value is 1/2. More surprisingly, 1 + 2 + 3 + 4 + · · · = −1/12. See https://en.wikipedia.org/wiki/1_%2B_2_%2B_3_%2B_4_%2B_·_·_·

Remember that at some point a math teacher told you you couldn't subtract a number to a smaller number. Later you learned about relative integers. Same for non-integer powers, then for non-integer factorials and so on. Maybe your latest math teacher told you you couldn't define the infinite sum of (-1)^k, but in fact with more advanced maths you can.

>> No.5884247

>>5884237
My maths teacher also told me you cannot square root negative numbers. Are you gonna tell me that's wrong too? How much did that bastard lie to me?

>> No.5884269

>>5884247
You can using complex numbers, but most of the time it's preferable to stick to the classic square root, defined for positive reals. Otherwise you cannot use the natural and useful property sqrt(x*y) = sqrt(x)*sqrt(y).

Indeed sqrt(-1) * sqrt(-1) = sqrt(-1)2 = -1
sqrt((-1)*(-1)) = sqrt(1) = 1

>> No.5884314
File: 89 KB, 1000x1625, 1266374431326.png [View same] [iqdb] [saucenao] [google]
5884314

>>5858487
>The problem when people write things as you have is that you are treating "1-1+1-1+1-1+1..." as if without further clarification it is an innately meaningful, physical thing. It is not.

No, but there is a unique* simplest interpretation, and that's good enough.

>>5877639
>>5878851
>>5880044
>pic related

* (up to isomorphism, natch)

>> No.5884359

>>5884237
>Yes the series is divergent, and yes, its sum can be defined in a very clean and unique way, and its value is 1/2
Do you even know what divergent means ?

>> No.5884799

>>5884247
Your mathematics teacher is right. It is best to stay away from the masturbatory vendetta that is modern mathematics.

>> No.5884846

>>5858468
Ah hah, I love Numberphile. But I believe that that would converge to -1/2, because it's wobbling between 0 and -1.

>> No.5884869

Okay guys, OP here and self-saging.

seriously, I posted this shit a week ago hoping to see if anyone agreed whether or not the series genuinely converged.

It devolved into typical /sci/ bullshit about a day after, and yet it hasn't died yet.

Seriously, stop posting.

>> No.5885083
File: 86 KB, 2100x1211, 5858468.png [View same] [iqdb] [saucenao] [google]
5885083

>>5884869
The thread is being bumped by a spammer who has been fucking with /sci/ in this manner on and off for about a year now.

>>5862012
>>5862917
>>5864141
>>5864808
>>5865935
>>5866834
>>5867893
>>5868517
>>5870654
>>5871385
>>5873168
>>5873796
>>5875364
>>5876183
>>5877598
>>5878851
>>5880011
>>5880683
>>5882092
>>5882716
>>5884192
>>5884799

Compare
>>5884369
>>5882390
>>5882654
>>5882360

If you want the thread gone before the bump limit, you will have to delete it yourself. Please talk to moot (moot@4chan.org) and the mods (irc.rizon.net, #4chan) about this so that they do something to end the spam.

>> No.5885087

>>5885083
Holy shit, how many threads are you gonna spam with that paranoid delusional crap? Just because you don't like the threads, that doesn't mean there is an evil spammer. People are having science and math discussion on a science and math board. Take your meds and get treatment, you deranged psycho.

>> No.5885093

>>5884359
Yes, I do. Have you even read the wikipedia link?

>> No.5885095

>>5885087
Fuck off, spammer.

>> No.5885105 [DELETED] 

>>5885095
Jackson 5 GET!