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7:00 PM
 
@Hippalectryon hehe, nice! Pi chocolate!
 
Ah @Chris'ssis is back
 
@Alizter :D
 
@Chris'ssis Merge those two, turn into a girl
 
@Hippalectryon llooolllllllllllllll, nice! Where do you find all these pictures?
 
7:02 PM
Hi @alizter, I missed you too lol.
 
@JasperLoy I have been studying
and avoiding studying
 
Back in 20 min.
 
Can someone answer my stupid question?
 
@Chris'ssis Google Images ? :D
@UserX You should ask first :-)
@Alizter Try to avoid avoiding studying
 
I already did. if $f(x_1)\neq f(x_2)\implies x_1\neq x_2$ then $f$ is one to one. Right or wrong?
I know that if the implication was the other side around it would be true
An iff would be true
 
7:05 PM
no
$f(x)=5$ is a non-bijective function
or ignore me
I am tired
Back to read about homology
 
Wait what
 
@UserX What is $f$
A usual function ?
@UserX If so, that's false
 
A function...
 
@UserX Urm well it depends of the domains you are considering
 
The whole domain of f
Whatever that is
Real functions though
 
7:10 PM
@UserX Oh so you restrict the domain to $Im(f)$ ?
Im for Image, not Imaginary
 
Yea
 
@UserX continuous ?
 
Yea
And $x_1,x_2 \in D_f$
 
Yeah I guessed that one xD
@UserX Why doesn't what Alizter say work ?
f(x)=constant
It does verify $f(x_1)\neq f(x_2)\implies x_1\neq x_2$
But isn't one to one
@Alizter ^
 
$f(x_1)=f(x_2)\forall x_1,x_2\in D_f$ though
 
7:13 PM
And so ?
It still verifies $f(x_1)\neq f(x_2)\implies x_1\neq x_2$
 
We can't have$ f(x_1)\neq f(x_2) $ though
 
That's not a problem
 
If the hypothesis is false, our results won't be any usefull(the implies)
 
$1=0\Rightarrow anything$
False => True
@UserX Let me show you a similar example
@UserX What is the set of natural number that, if they are odd, are divisible by 3 ?
 
I like how the site awards fast answers over good answers.
 
7:17 PM
A_3 if it's the set of odd natural numbers divisible by 3
 
Same with simple questions over hard ones.
 
@N3buchadnezzar I don't.
 
@UserX Not exactly
@UserX I didn't say the set of odd numbers divisible by 3, I said the set of odd numbers that, if they are odd, are divisible by 3
 
Yea I know :P
 
@Alizter you are a drop in the wast ocean ;)
 
7:19 PM
I don't think he does either, @Alizter...
 
@Hippalectryon dunno how to represent it, but I get the idea
 
@UserX It's {3,9,15,...}+the even numbers right ?
 
Yea
 
Well it's the same here. An even number does not satisfy 'is odd', and therefore is in the set. However, the result is useful
When you look at the set of points $x1,x2$ such that $f(x1)\neq f(x2)$, because it's empty doesn't mean it's useless
 
You still haven't convinced me but I'm not sure I get your point.
 
7:23 PM
It means that $f$ satisfies any statement $f(x1)\neq f(x2)\Rightarrow\dots$
 
hi
 
@felix Hi
 
anyone got any ideas about math.stackexchange.com/questions/1038624/… ?
it's a bit sad when I can't even understand the answers, let alone answer the questions :)
 
@felix upvoted
 
@Hippalectryon thanks,, do you understand what is going on?
 
7:25 PM
@UserX Likewise, a function from the empty set to the empty set is bijective
 
@Hippalectryon I am worried that it's obvious to brainy people :)
 
@felix Unfortunately not, otherwise I would have answered :-)
 
@Hippalectryon oh you read fast :)
 
@Hippa @UserX @Mike \o
 
@Hippalectryon okay so any implication from there woyld be true so that implication is too
I think studenmath will clear things up for me
@Studentmath hi there :D
 
7:27 PM
@felix No, I remember you came here some days ago (am I right) and asked that
 
@Hippalectryon that's quite possible :)
 
I need a more obvious counter example :P
@Studentmath if $f(x_1)\neq f(x_2)\implies x_1\neq x_2$ then $f$ is one to one. Can you give a nice counter example?
 
Hmm, let me think
What about a constant function?
 
Lol
 
7:31 PM
What's up?
 
I hope $3$ times will do it hehe
 
A counter example that's not a constant function? :D
 
@Studentmath You're the third to give that answer hehe
 
@UserX A function with constant parts, increasing
 
7:32 PM
@Hippalectryon that's the same thing :D
 
@UserX :D
 
Well obviously @Hippa, but I wonder if there is an example without constant parts at all
 
Hey!!! @MikeMiller @DanielFischer If $I$ and $J$ are ideals and if $f \in I$, $g \in J$, why does it stand that $f \cdot g \in I \cap J$?
 
Would this make a good MSE question? Prove that there is no non-constant function as a counter example to that?
 
So what, consider $f(x)=|x|$
 
7:33 PM
@UserX Hey wait
 
Wait, I think I am being stupid
 
@Studentmath Do isotopes differ in chemical properties?
 
@Studentmath You're right I think
@Studentmath Even sin(x)
 
@Hippa yeah
 
@UserX LOL
 
7:34 PM
@UserX basically any function that is continious holds the fact $f(x_1)\neq f(x_2) \to x_1\neq x_2$, or is that taking it too far..
 
@UserX HEHEHEHE that's so stupid
 
No, it's true.
 
@Studentmath Wait
@Studentmath What's the contrapose of $f(x_1)\neq f(x_2) \to x_1\neq x_2$
 
Think about the definitions, @evinda.
 
It's $x1=x2\Rightarrow f(x1=f(x2)$ @Studentmath @UserX
Which is always true
 
7:35 PM
@Hippa true for every function
 
:D
@UserX Basically what you said is implied by the definition of a functio
 
@Alizter iirc no, only in physical properties
 
Wait what the fuck yes
 
@Hippa I like how we got from "wait it's true for $f(x)=|x|$" to "oh it's always true"
 
@Hippalectryon here you might find more info on chocolate that you don't know yet. There are serious studies behind them. perfecthealthdiet.com/2012/11/…
 
7:37 PM
@Studentmath :DD
@Chris'ssis Thanks
 
@MikeMiller Do you mean this definition?

$$I(x)=\{ f(x_1,x_2, \dots, x_n) \in K[x_1,x_2, \dots, x_n] \text{ such that } f(\overline{a})=0, \forall a \in X\}$$
 
@Chris'ssis the sources of that article are unsourced articles
 
$\int\mathrm{C}(x)\mathrm{d}x$ where $C$ is the $@Chris'ssis-Chocolate$ function
 
I'm wondering, I've shown that if $G$ is simple, of order 60, than it is isomorphic to $A_5$. As hinted, I have shown it by considering the sylow-2 subgroups, and 'easily' proven they must be either 5 or 15. One way or another, I've shown a subgroup of index 5, homomorphism to $S_5$ and completed. It made me wonder how many sylow-2 subgroup actually are.
 
No, I mean the definition of an ideal. I'm not going to commwnt further. You can solve this.
 
7:38 PM
@Hippalectryon :D
 
So I checked in $A_5$, it's easy to see there are 5.
I wonder why in proving the isomorphism, I could've went along with 15 sylow-2 subgroups too - why does it work that the isomorphism exists even in a case which, apperently, doesn't exist?
 
@Studentmath is that an exercise in a book?
 
Not only doesn't exist, isn't true..
@UserX showing the isomorphism itself, yeah
 
@UserX For instance, look at that - ncbi.nlm.nih.gov/pubmed/21875885?dopt=AbstractPlus "The highest levels of chocolate consumption were associated with a 37% reduction in cardiovascular disease (relative risk 0.63 (95% confidence interval 0.44 to 0.90)) and a 29% reduction in stroke compared with the lowest levels."
 
@UserX I am moving back to toplogy for a while now.. guess will be back at abstract algebra in a couple of days
 
7:40 PM
@Chris'ssis now THAT is a nice source
@Studentmath I hope you'll remember AA as I'll definitely have more questions soon
 
@MikeMiller Do you mean the following?

$$f \in I \text{ and since I is an ideal} \Rightarrow f \cdot g \in I$$

$$g \in j \text{ and since j is an ideal} \Rightarrow f \cdot g \in J$$

$$f \cdot g \in I \text{ and } f \cdot g \in J \Rightarrow f \cdot g \in I \cap J$$
 
@Chris'ssis I randomly clicked one of the sources(of the first article) and the article citated the original article back lol
 
:-)
 
self reference?
a=a
 
I bet that's why people hate science. "Wrong! Muh sources". @skullpatrol nah. It was another article. But that other article had the original one as a source. So a=b=a
 
7:45 PM
so symmetric reference
 
Can all the references construct a permutation group?
16 sources. Say $n$ of them with $n\leq 16$ do "symmetric reference". Then the group $G$ of the sources is isomorphic to $D_{n}$ which is isomorphic to $S_{n}$
That must be the most meaningless usage of group theory ever.
 
@UserX $D_n$ is isomorphic to $S_n$?
 
I think I proved it somewhere I might be wrong. Wait.
 
So you proved $2n=n!$? :P
 
8:00 PM
Nope, so I didn't :D
 
You probably did with $S_3$ and $D_3$
 
@Studentmath yup I did that in class today I think.
 
Wait, aren't you 16 or so?
 
I think I confused n with 3
I'm 17
I did that along with some other isomorphisms in a paper during history class or something that I don't pay attention to
 
So in class you mean in your free bored time?
 
8:05 PM
Well yea
 
Now it's clear
 
I also realised that it's so hard creating a cayley table yourself.
 
They provide good insight but you really should not use them for $n>8$. $n=8$ is even pushing it a bit.
 
@MikeMiller n=7 would take me a year to do by hand
 
@MikeMiller
I need your help.
 
8:10 PM
No, $n=7$ should be pretty quick. :)
 
I need a Möbius transformation that fixes $|z+1|=1$, fixes $0$ and sends $-1$ to $i$.
Can you help, @MikeMiller?
 
No, sounds awful.
 
You said you knew how to handle this things.
 
I told you to use cross ratios or whatever.
 
The cayley table of S_7? How?
 
8:13 PM
@UserX Err... I meant $n$ as the order of the group.
 
@MikeMiller I don't know how to use cross ratios.
 
Dear Christ don't try to write out the catley table of $S_7$.
@Pedro Iunno. I'm going to take a nap.
 
@MikeMiller that's what I meant :P
 
That Cayley table has over 25 million entries... so yeah, no
 
I don't even know where you'd draw a 5040 x 5040 table
I mean, okay, Microsoft Excel, but that's cheating.
 
8:24 PM
Okay who can do it on Excel :P
I wanna see that
 
@UserX: I wrote an answer to that DE question (more or less what another guy posted after he removed his wrong answer). ... Oh, and I heard back from Mike Spivak. Did you tell me that your version of his book was an e-book or hardback?
@Mike: Did you stun your students with a great recitation? :P
oh, @Pedro ... You need to learn cross ratio and some projective geometry. I sent you that stuff ages ago :P
 
@TedShifrin Where?
 
No, @Ted. There were three there. I talked about the midterm and when they didn't want to talk more we all went home (and cried?)
 
@TedShifrin Look.
 
That's the last chapter in my algebra book, @Pedro. I think I sent you a .pdf over a year ago.
 
8:29 PM
My geometry is as poor as Haiti.
 
Only three, @Mike? Sigh ... I guess most people leave early for Thanksgiving. At MIT no one dared do that ...
 
Nothing was covered in class since the midterm. I think people generally skip post-midterm recitation because all we do is cover the midterm...
 
Hello @robjohn, I won't say that same thing again, lol.
 
Ah, fair enough, @Mike. And of course they all knew how to do that perfectly :D
 
Right... that's why the average was below 70.
 
8:32 PM
@TedShifrin you have an algebra book?
 
@Ted is probably weeping over my geometry skills.
 
@UserX Yes. He wrote a linear algebra one and an abstract algebra one.
 
@TedShifrin hardback
 
@Jasper is my advertising agency, even though we totally disagree on learning/teaching styles :P
That's what I thought, @UserX. I'll confirm with him.
 
@TedShifrin Where in your chapter are cross products?
 
8:33 PM
@Pedro: Your exercise is confuzling me.
 
I shouldn't talk about books in front of him, lest I set him off.
 
@TedShifrin I help many authors advertise books, since I know many books, though I don't know much math, lol.
 
@TedShifrin Why?
 
Cross ratios are in section 2, @Pedro, mostly, although they come up in later stuff too.
 
@TedShifrin You sent chapter 8.
 
8:34 PM
Right, @Pedro. Section 2 thereof.
 
I don't know how Ted feels that I'm trying to pirate his book...
 
...Maybe you should have the tact not to say that to his face...
 
Pirate Spivak's book or mine?
 
@UserX Maybe if you just asked for it nicely he'd give the part you need.
 
This question has three close votes, all for different reasons. We should get someone to close as a duplicate and someone to close as primarily opinion based.
 
8:35 PM
Ted is used to lack of tact. René would accuse Ted of far worse ... oh wait, he did.
 
Which would you make you less mad?:P hint; I already have Spivak's book.
 
Yeah, somehow I don't think they have permission to do what they did with Spivak, but I'll just let him know.
@UserX: You told me it said 5th edition, right?
 
I download the books to see if I want to buy them, then I delete the files after reviewing.
 
@TedShifrin The cross ratio is always real or infinity?
I didn't know that.
 
@Pedro: That's all working in $\Bbb P^1(\Bbb R)$.
 
8:37 PM
@TedShifrin Oh.
 
I discuss the complex case in the last section.
Exercise (which appears in the last section): Real cross-ratio of complex numbers iff they all lie on a line or all lie on a circle.
 
@TedShifrin yea
 
LOL, @Pedro. Did you vote to close as opinion-based?
 
@MikeMiller Of course.
I did it for the Lulz.
 
I'm going to find a dupe.
 
8:39 PM
@TedShifrin I have no idea how to prove that.
It's in my problem sheet.
 
@PedroTamaroff Use yer braain
 
My brain has no geometry.
 
actually agrees with @Mike that @Pedro whines too much sometimes :D
You do need a bit of high school geometry for that one.
 
@TedShifrin How?
 
@MikeMiller look the obvious dupe I found...
 
8:41 PM
Start by assuming the points are on a line or on a circle. Worry about the converse later.
Why are two of the biggest mathematical guns on this site wasting their time answering that stupid quadratic formula question? Utterly ridicu-louse.
 
@TedShifrin how is your book structured? Can I pick it up from next to nothing knowledge on AA?
 
I don't agree that that one's an obvious dupe... it doesn't make any mention of coefficients. A standard reading would be "why does the quadratic equation work for all polys with real coeffs?"
 
@TedShifrin That can probably be done by bruteforce calculation, so I'm actually worried about the converse.
 
Yes, @UserX, but it does rings before groups. And I don't want to argue with Jasper or anyone else about why.
No, @Pedro, don't do it by brute force.
 
@TedShifrin how is that even possible?
 
8:43 PM
@TedShifrin I never said anything about rings before or after groups, lol.
 
@TedShifrin Especially ironic, given Martin's comment earlier today.
 
I'm sure you complained once, @Jasper :D
 
@MikeMiller at least it was closed with 5 different reasons. But that question DOES answer the othe rquestion...
 
@TedShifrin I didn't find anything on complex stuff.
 
Indeed, @Mike. I just don't get it.
 
8:43 PM
@TedShifrin I'm sure I did not, lol. But nvm, lol.
 
In the last section, I talk about hyperbolic geometry in terms of the upper half plane and $\Bbb P^1(\Bbb C)$, @Pedro.
 
Did that one guy who wanted to integrate $w_1$ ever get back to you?
 
Yes, @Mike, he gave references. But I think he's still deluding himself.
 
@TedShifrin Oh, OK.
Do you know how to handle my question?
 
Ah, he's pulling it out of physics papers, @Ted. So it's probably nonsense. :)
 
8:46 PM
@MikeMiller What are you talking about?
 
@MikeMiller lol
 
@Pedro: Here's a warm-up exercise for you (regarding your circle one). Find a conformal mapping fixing the standard unit circle (as a set, of course), fixing $1$, and sending the origin to infinity. Now can we do it sending the origin to $2i$? to $3i/2$?
 
@TedShifrin OK.
 
I don't think those papers ever said they could do Stiefel-Whitney. I think they were talking about Chern mod 2, but maybe I didn't look carefully enough, @Mike.
 
@TedShifrin The first is inversion.
 
8:48 PM
Right. So are the others :P
 
What do you mean?
 
They're the composition of an automorphism of the disk with an inversion. :)
 
@TedShifrin The second one was talking chern mod 2 (and denoted it steifel whitney). The first one... just writes down stuff like $\int_X w_4$.
 
Right, @Mike, but doesn't the latter denote an evaluation on the mod $2$ fundamental class?
@Pedro: I like it when you say "oh." :D
 
8:50 PM
That almost makes sense... except for the 1/2 in front of each integral.
If that's the actual notation for evaluation against the fundamental class, though, ugh.
 
Hmm, I'll go back and look when I'm done writing my recommendations. On a cooking break right now.
 
That's often the custom, @Mike.
 
Physicists...
 
It makes me think they're doing something spin, @Mike. I should ask my former student who's now an NSF postdoc doing math physics at Cambridge.
 
8:52 PM
:18778553
I am not sure how to change the transformation to get $2i$.
 
Oh, so maybe I was wrong, @Pedro. Compose the other side. :)
Oh wait. But then the circle won't be preserved any more.
 
@TedShifrin I already said that the automorphism $$\frac{z-\alpha}{1-\bar \alpha z}$$ preserves $|z|=1$ and fixes the two roots of $\alpha/\bar \alpha$.
Here $|\alpha|\neq 1$.
 
SE put out an iPad app, @Ted
 
@MikeMiller the android app sucks
I bet apple app sucks too
 
I have been using it for ages, @Mike, but (a) it doesn't do chat, (b) I was a beta-tester and could no longer get their latest versions after I updated my OS (and Safari is still a total disaster).
 
8:56 PM
I don't use the iPhone app, but the iPad one is actually made for the system (a system where it's easy to type), so it's probably nice.
 
No, I like the app pretty well, @UserX
 
The iPhone one is not very good. I just use my browser.
 
@TedShifrin I hate the app... did you find a way to get chat on mobile?
 
Yes... use your web browser.
 
The app has nothing to do with chat. Because Apple messed up Safari, I can only chat on Chrome, on which I cannot load MathJax.
 
8:57 PM
The only browser that works is the built-in browser which is lame.
 
I use Chrome on my desktop just fine, but on the iPad I can't load robjohn's MathJax link.
 
@TedShifrin those sentences weren't meant to be connected
 
When I'm in chat on my phone I don't see TeX... I just parse it myself.
 
As a future mathematician, @UserX, you need to be careful about connectivities.
 
@TedShifrin I don't want to be a mathematician.
 
8:58 PM
That's what I used to do, @Mike, but then I got spoiled. Robjohn's thing will load on Safari. I had to be sneaky to do it (and I've forgotten how).
 
The world's better off with fewer of us, so it's all for the best.
 
It's just a hobby I find fun into.
 
Then why are you here all the time, @UserX?
 
@TedShifrin I use Safari but I never got it to work.
 
@TedShifrin because I like math.
 
8:59 PM
I had to load a dummy page with the link on it somehow, @Mike. If you want, I can try to recapture how. I told robjohn in here over a year ago how I did it. Perhaps he saved what I said.
 

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