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/sci/ - Science & Math


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2861092 No.2861092 [Reply] [Original]

Lulz. I bet half the retards on here won't get this.

>> No.2861109

i dont get it. please enlighten me.

>> No.2861124

>>2861109
The girl on the left is wrong because the type of fraction isn't specified.

>> No.2861126

This comic is wrong.

The definiton of a rational number is that two _whole_ numbers can express the number - pi is not a whole number, and thus pi/1 does not demonstrate that pi is a rational number.

>> No.2861131

>>2861124
bingo

>>2861109
retard

so i was right in OP

>> No.2861135

The comic isn't wrong, one of the characters in the comic is wrong. The comic is right actually.

>> No.2861139

But it NEVER ask that

The art of learn mathematics is the art of learning to troll with formalism!

>> No.2861140

>>2861126

retard, see >>2861124

>> No.2861138 [DELETED] 

>>2861126
>This comic is wrong

You mean the girl on the right is wrong. Thatsthejoke.gif.zip.jpeg

>> No.2861155

Idiots. It says FRACTION, and PI/1 is clearly a fraction.

Losers.

>> No.2861162

>>2861092
This is correct because female mathematicians wear glasses, have messy hair and never dress in pink.

>> No.2861197

1pi/1 = pi

>> No.2861207

ITT: OP is proven right.

>> No.2861212

>>2861162
>where*** glasses

>> No.2861227

What does π mean?

>> No.2861232

>>2861162

one of our (female) lecturer's favourite tops this term was a pink stripy thing.

she did also wear glasses and have messy hair.

>> No.2861240

>>2861227
>look up acii character
>requires knowing the name of the symbol in the first place
:|

>> No.2861243

>>2861240

I laughed until I realized:

http://en.wikipedia.org/wiki/Greek_alphabet

>> No.2861440

Denominator is 1, so it's not a fraction. If she'd said tau/2 that'd be a different story.

>> No.2861449

>>2861440

If Pi/1 is not a fraction, so is tau/2.

>> No.2861455

>>2861449
OK, that's not a grammatical English sentence.

>> No.2861456

>>2861092
Relationship between 2 INTEGERS.

Fixed that A+ math student

>> No.2861464

>>2861455

OK, that´s not related to this thread.

>> No.2861470

>>2861456
Relationship between 2 RATIONAL NUMBERS.

Fixed that /sci/bro.

>> No.2861476

>>2861464
OK, it's hard to explain in reply to something that doesn't make sense.

>> No.2861480

>>2861207

u got that right.

>> No.2861489

Can "i" be expressed as a fraction?

>> No.2861504

>>2861476
It's pretty easy to see where he was going with that, unless you're a retard.

>> No.2861522

>>2861489

i/1

>> No.2861523
File: 8 KB, 283x283, wat (2).png [View same] [iqdb] [saucenao] [google]
2861523

24/7

>> No.2861536

http://www.wolframalpha.com/input/?i=ln+-1+%2F+i

YOU JELLY?

FIELDS MEDAL PLEASE.

>> No.2861543
File: 82 KB, 376x500, wizard.jpg [View same] [iqdb] [saucenao] [google]
2861543

>>2861536

>> No.2861564

>>2861536
>logarithm of a negative number
>divide by imaginary unit

Shit just got imaginary, son.

>> No.2861581

>>2861536

> implying logs of negative numbers exist

I thought wolfram alpha was smart.

>> No.2861582

>>2861536

Why the fuck is that pi?!

Why the fuck does there exist such a relationship?

Fucking mathematics.

>> No.2861593

>>2861536

>log(-1)
>i*pi

This is just mathematicians arbitrarily assigning shit, right?

>> No.2861595

>>2861536
Sure is Euler's Identity in here

>> No.2861601

>>2861536

wait a second. when did you learn this, from HIGH SCHOOL ALGEBRA!?


LOLOLOLOLOLOL

EVERYONE KNOWS THIS LULZ

>> No.2861602

>>2861582
>>2861593

>doesn't know about Euler's identity

>> No.2861607

>>2861582
because ln(-x) = i pi + ln(x)

>> No.2861611

>>2861607

AHAHAHAHA

ln(-x) is undefined for real x.

>> No.2861616

> implying Euler's identity has anything to do with this.

>> No.2861630

>>2861611
No, it's what I said, moron.
http://www.wolframalpha.com/input/?i=ln+x

>> No.2861640

>>2861616
Do you even know what Euler's Identity is?

>> No.2861645

>>2861630
Wolfram alpha is wrong there.

Ln(x) is a function. As such, it must have unique solutions everywhere. According to your formula, ln(e^(i3Pi)) is also Pi.
Also, the limit does not converge for negative real values (which basically means the same).

mathematicians would be laughing hard at this.

>> No.2861648
File: 21 KB, 241x230, haah.png [View same] [iqdb] [saucenao] [google]
2861648

>>2861109
Is the retard who got it wrong.

>>2861124
Is the retard who got it right.

>> No.2861651

>>2861640

Do you?

>> No.2861663

>>2861651
<span class="math">e^{i \pi} + 1 = 0

<span class="math">e^{i \pi} = -1

i \pi = ln(-1)

\pi = \frac{ln(-1)}{i}[/spoiler][/spoiler]

>> No.2861665

>>2861648

Actually, he didn´t get it wrong, he didn´t get it at all.

>> No.2861674

>>2861663
Fucking tags

<div class="math">e^{i \pi} + 1 = 0</div>

<div class="math">e^{i \pi} = -1</div>

<div class="math">i \pi = ln(-1)</div>

<div class="math">\pi = \frac{ln(-1)}{i}</div>

>> No.2861676

>>2861611
No matter how you slice it, you're wrong.
1) For negative real x ln(-x) is defined trivially, as <span class="math">\int_0^{-x}\frac{1}{x}dx[/spoiler]
2) Otherwise, we can extend the definition of log to all complex numbers pretty simply. Considering finding the inverse expression to e^z = c where c is an arbitrary complex number. Write c in polar form and z in rectangular. e^x*e^(iy) = r*e^(i theta). So clearly, x = log(r), r is a positive real so the real definition of log applied. Also, e^(iy) = e^(i theta). This is of course a family of solutions, that is y = theta + 2pi*k where k is an integer, (you know e^(ix) is 2pi periodic right?). So there you go, log(c) is a set valued function such that
log(z) = log|z| + i(Arg(z) + 2pi*k) k = 0,+/-1,+/-2...

>> No.2861680

>>2861663

Oh boy. Where do i start?

First two are correct, third line is false, therefore also the fourth, as you CAN´T use ln on negative real numbers, as explained before.

You can however specify the numbers that when exponentiated will return -1. Those will be inPi, the n being essential.

he correct equation therefore would be inPi/i which is nPi and therefore useless.

>> No.2861684

>>2861665
You get the post anyways.

>> No.2861685

>>2861616
e^(i pi) = -1
e^(i pi + x) = e^(i pi) * e^x = -e^x
ln(-e^x) = x + i pi

>> No.2861697

>>2861676

> No matter how you slice it, you're wrong.

I disagree. So does mathematics.

> 1)

wut?

> (you know e^(ix) is 2pi periodic right?)

Yes, exactly, and that´s why Ln(x) is not a function for these values, as it is not unique/the limit does not converge.

> log(c) is a set valued function

Misnomer, since it´s not a function. It´s a relation.

>> No.2861703

>>2861685

see

>>2861680

>> No.2861710

>>2861645
You're a full-blown idiot. Not all functions are 1-to-1. For example y=x^2 is not 1-to-1, as x=1, and x=-1, both give y=1.

ln(e^(i3Pi)) = pi i
ln(-1) = pi i
this does not imply that ln(x) is not a function.

>> No.2861715
File: 10 KB, 333x331, sadPikachu-Crying.jpg [View same] [iqdb] [saucenao] [google]
2861715

>>2861676

> use ln as a function
> then say it's not a function
> mfw almost nobody on /sci/ knows how to math

>> No.2861726

>>2861680
You're wrong. There is no magical law saying you can't take logarithms of negative numbers. The logarithms of negative numbers are complex, which is why there's an i involved.

Protip: When wolfram alpha disagrees with you, just assume you're wrong.

>> No.2861728

>>2861710

You get that wrong, sir.

Look at the graph of y = x². There are no two points "on top of" each other.
Now look at Ln(-x) interpreted as a "set-valued function". Infinitely many points on top of each other -> not a function.

>> No.2861744

>>2861726

For a value to exist, the limit must converge. It does so, if it converges for coming from every possible "direction" in any case. This is not true for the logarithm, as well as for roots by the way.
Do i seriously have to write down the proof for you now? Can´t you just look at how the logarithm is defined?

>> No.2861747

PI is defined as the ratio between the Circumference and Diameter. Why is it irrational?

>> No.2861750

>>2861645
>ln(e^(i3Pi)) is also Pi.
holy fuck you're an idiot. e^(i 3 pi) = -1. So you're just restating the fact that ln(-1) = pi i

>> No.2861757

>>2861747
Because there is no circle for which the circumference and diameter are both integers.

>> No.2861767

>>2861744
ln(x) is a continuous function. It's value at every point exists as a limit from both directions. Give it the fuck up.

>> No.2861770

>>2861750

Oh my god are you retarded or wut? I meant ln(e^(i3Pi)) is also -1 of course, obvious as fuck.

And no, i´m saying that you are wrong, because ln(-x) is not a function for real x, as explained before.

>> No.2861774

>>2861728
What are you smoking? There are no points "on top of each other" in ln(x) or ln(-x). Each point in the domain maps to a single complex value.

>> No.2861792

>>2861767

Sigh, guess i have to. I´ll use the example of -1. I´ll use polar coordinates (z = r*e^(i\phi)) and set r = 1 for convenience.

from the left of \phi = Pi:

c_1 = lim ln(e^(i(Pi-d))) as d -> 0 = lim iPi + d = iPi

from the right:

c_2 = lim ln(e^(-i(Pi+d))) as d -> 0 = lim -iPi + d = -iPi

c_1 != c_2

> It's value at every point exists as a limit from both directions

HURR DURR I´M SMURT

>> No.2861799

>>2861774

No you fucking idiot. The solution is i*n*Pi, which obviously all lie on the y-axis of the complex plane.

>> No.2861803

>>2861792

That should be a "lim iPi - d" in the line where c_1 is calculated.

>> No.2861831

ITT: People unable to do math.

The ln is only defined on the SLICED complex plane for a reason, dumbfucks.

>> No.2861842

>>2861792
y=ln(x) is a function that maps from x to y, where x is real and y is complex. You're changing it to a function from a complex x to a complex y. You're a complete idiot.

>> No.2861854

>>2861842

Now you just went full retard.

Either you´re talking abour ln(x):R->R or Ln(z):C->C. Your magical "function" is non-existant, as no limit at all can be computed.

>> No.2861860

>>2861842
logs are defined for complex numbers as well

>> No.2861868

>>2861715
You're right, there's ambiguity. But we can fix that. Some definitions:
Define ln(x) as above taking only positive reals as imput.
Define log(z) as a set-valued function http://en.wikipedia.org/wiki/Multivalued_function
taking a complex number z as an input and returning a related infinite set of complex numbers as output. You don't seem to like this idea, but there is a fix! We just need to make an arbitrary choice. So let's go with the convential choice, define Arg(z) as the angle the complex number z makes with the x axis, such that -pi<x<=pi. This choice is of course entirly arbirary but it means that our branch cut of log (let's call it Log(z)) will be discontinuous (note: not undefined) on the negative reals.
If you dont like this definition of Arg, I can make it more explicit, let's say <span class="math">Arg(x+iy)=2 arctan\frac{y}{\sqrt{x^2+y^2}+x}[/spoiler] except on the negative reals where it = pi. This gives the expected valued. So finally we have an honest to goodness function to invert e^(z)=c.

Log(z) = ln|z|+i(Arg(z))

Tada!

>> No.2861874

>>2861799
Right, that's why e^(i*2*pi)=-1. Oh, wait, it doesn't. Fucking moron.

>> No.2861877

>>2861842

http://en.wikipedia.org/wiki/Logarithm#Definition

"For the logarithm to be defined, the base b must be a positive real number not equal to 1 and y must be a positive number."

Also:

http://en.wikipedia.org/wiki/Complex_logarithm

>> No.2861898

>>2861868

> Define log(z) as a set-valued function

As said, it´s not a function then, it´s a relation. You can´t use it like a function in that case, what you did before.

I agree that undefined was the wrong word. It is still not the inverse function of exp on the negative real axis.

Chosing the principal branch doesn´t really help. I could have chosen any other branch and would have gotten a different result when evaluating "Ln(e^(-Pi))".

It does not solve our problem.

>> No.2861908

>>2861874

Goddammit you know what i mean. i (2n+1) Pi.

>> No.2861920

>>2861898
The principle branch is the branch for which the logarithm is defined. just like sqrt(4) only gives 2, ln(-1) only gives pi*i, not 3*pi*i etc.

wolfram is correct, ln(-1)/i=pi

just like sqrt(4)/2=1, not -1

>> No.2861925

>>2861920

Dude, did you even read what i had written?

I can chose any other branch and get a false result. Therefore, your deduction was wrong.

i(2n+1)Pi/i != Pi

I can chose a branch in which n = 3 for example.

>> No.2861928

>>2861877
>believes wikipedia over wolframalpha on math questions

>> No.2861936

>>2861928

I believe what i can deduce myself. I have provided the proof for your deduction being wrong. If you don´t trust mathematics, that´s not my problem.

>> No.2861941

Hey guys, what is the third root of -8, without looking it up?

>> No.2861943

>>2861925
but ln(-1) doesn't equal i(2n+1)pi you fuck

it equals i*pi

you don't pick whatever branch you like, you have to pick the priciple branch because thats how the logarithm is defined

>> No.2861953

>>2861943

No, that´s how YOU defined it. I am free to chose whatever branch i like to.

-1 = e^(i(2n+1)Pi), NOT e^(iPi)

Seriously.

>> No.2861963

>>2861941
-2 is one of them

>> No.2861971

>>2861963

lolno. You confuse this with the solution to x³ = -8. Completely different.

>> No.2861976

>>2861953
so arcsin(0) = 10*pi ?
does sqrt(4) = -2 ?

no they fucking don't
you don't get to define the logarithm, it's already defined

>> No.2861977

>>2861971
well then what's the difference between a cubed root and a third root?

>> No.2861981

>>2861953

Me again, just to ylear this up:

I am only arguing that this deduction:

>>2861685
>>2861674
>>2861663

is false, as explained.

What can be done though is to chose the principal branch of the logarithm to be its value at the negative real axis. In that case, one is free to write Log(-1)/i = Pi.

The deduction using Euler´s formula is still wrong.

>> No.2861986

>>2861941
3 3rd roots of -8, given by
2e^(i * theta)
where
theta = -pi/3, pi/3, pi

>> No.2861991

>>2861977

The solution to x³ = -8 is {-2,2\pm isqrt(3)}, whereas the third root of -8 is 1+isqrt(3).

>> No.2861993

ITT: OP wins the bet.

>> No.2861996

whenever anyone mentions complex numbers /sci/ becomes full retard

>> No.2861998

>>2861996

It´s weekend. That´s the main reason i guess.

>> No.2862028
File: 93 KB, 360x270, Math-That-looks-right-to-me.jpg [View same] [iqdb] [saucenao] [google]
2862028

OP here. Retards sure didn't disappoint. I love the Internets.

>> No.2862039

Can we all agree to use x as a real variable and z as a complex one, so we don't get problems like >>2861991

>> No.2862049

>>2862039

I don´t see a problem there, but ok.

>> No.2862058

>>2862049
Just makes it absolutely clear whether or not we're looking for real answers or all answers.

As far as the ln(x) stuff from before, it's clear that ln(x) is undefined for negative x and ln(z) is defined for negative z.

>> No.2862064

>>2862058

> ln(z) is defined for negative z.

You didn´t read, right? ln(z) is not a function if you let z be a negative real value.

>> No.2862073

Cool story bros
I still don't get why the math student thinks differently than the mathematician

>> No.2862077

>>2862064
see: >>2862039

>> No.2862084

>>2862058
>it's clear that ln(x) is undefined for negative x
That's just fucking wrong. We can argue about it not being a function because it has multiple answers, but it is sure as fuck defined.

>> No.2862085

>>2862077

I don´t think you get it. z = -a+ib with b = 0 and a>0 is a complex number (on the negative real axis), for which the ln(z) is not a function.

>> No.2862088

>>2862073
The math student doesn't think. The mathematician answers the question. Also the math student is using a wrong definition of irrational.

>> No.2862091

>>2862088

And just repeating definitions he learned by heart and most likely not yet understood well.

>> No.2862094

>>2862085
Bah, fuck. I can't think on the weekends it seems. You're right...I think. Probably.

>> No.2862096

>>2862084

He obviously means the real ln in that case.

>> No.2862099

>>2862096
In that case, I should have been clearer.

Disregard most of what I said, I suck cocks.

>> No.2862108
File: 121 KB, 468x349, wtf.jpg [View same] [iqdb] [saucenao] [google]
2862108

How can you guys derive all those equations from that question?

>> No.2862123

>>2862108
What question?

>> No.2862125

>>2862123
>can pi be expressed as a fraction?

>> No.2862128

>>2862108

Guy says ln(-1)/i was a possibility. Some people didn´t get it. Someone then tried to deduce this from Euler´s equation, which was proven to be impossible.
Then shitstorming.
Now we´re here.

>> No.2862133

>>2862125
You mean all the logarithm stuff? Complex analysis. Just a bunch of things that equal π.

>> No.2862134

>>2862085
>>2862064
>>2862084
ln(x) where x is <0 is defined as a fuction though
by definition you take the primary branch

I though we were over this
ln(z) is defined as a function for all complex z except 0

>> No.2862140

>>2862125
I don't know why that's even a hard question to answer. How about fuckin' C/d... you know, the first thing you learnt about pi.

>> No.2862143

>>2862128
tldr; it wasn't impossible. 2 people did it.

>> No.2862148

>>2862134

> ln(x) where x is <0 is defined as a fuction though
by definition you take the primary branch

> implying there exists an a so that e^a is negative

>> No.2862164

usually, irrationality proofs begin "suppose # can be expressed as a fraction a/b where a and b are in LOWEST TERMS..."

pi/1 is not in lowest terms because 1 divides pi.

>> No.2862168

>>2862164

exactly.

>> No.2862170

>>2862164
>implying any of that matters, π/1 is still a fraction