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4:00 AM
but I bet that is what most have tried. So there is probably a wierd example out there somewhere
@DemCodeLines did you take the derivative?
 
Yeah, my TA who put the problem up as a joke during a class said he believes the answer is negative too.
 
it seems like a simple question we should be able to answer
what if you replace $\mathbb C$ with $\mathbb R$ ?
 
@ComTruise No, I am just going through the "Help me solve this" guides to see how they do it and hoping that I am fortunate enough to do well on the test.
 
$A$ must have infinite krull dimension
@DemCodeLines ah, math lab?
 
Yes...
what a freaking waste of a course
 
4:06 AM
oh?
 
@seaturtles How obvious is that?
 
And this is calculus 1. With this understanding, there is no way I will make it out of Calculus 2 and 3. All thanks to this great teacher
 
In a scale of 1 to Lang.
 
@PedroTamaroff if R->S is a surjective ring homo then dim R > dim S (exercise)
 
maybe your teacher just doesnt know how to communicate with you. I bet if you go to office hours or meet after class they will be able to help you more.
 
4:07 AM
@seaturtles Yes, take preimages.
 
use lattice correspondence and prime ideal iff quotient is integral domain
 
Well, $\dim R\geqslant \dim S$.
One can always take the identity. =P
 
@ComTruise The teacher is pretty bad.
 
I used Hungerford's Algebra. Hated it for a while but eventually came to like it ok.
 
When you ask questions, he gets irritated sometimes
 
4:08 AM
well thats not good
 
If you keep asking the same thing because you don't get it, he just says "No" without the explanation and moves on.
 
teachers should love questions
 
It's not good at all. This is a summer course. Meaning a 3 month course into 1.5 months.
3 tests covering 2-3 chapters each. It's bringing my GPA down, really down.
and ticking me off.
 
You're not a math major I guess?
 
use the ebook, homework, quizzes and infinite practice tests to get familiar with the questions that will be on the test. as long as you don't do this too late, you will be able to ask questions about what you don't understand along the way, either in class or in office hours or even through email
 
4:11 AM
Computer Science, but some how you need to know calculus 3 to do programming. #SchoolLogic
 
@DemCodeLines I know some CS people -- some friends, others friends of friends -- and they are very knowledgeable in math. It is a great asset.
 
math should help you think
 
@PedroTamaroff Knowing Math is great, but destroying someone's future computer science career just because he didn't do well in a course that had nothing to do with his career is just wrong.
But anyways, that's not the discussion. It's that I have a test and the teacher will put questions that will be mind-boggling.
 
I think linear algebra or discrete math would be more appropriate for CS
 
@seaturtles Suppose we're given $n$ integers $a_1,\ldots,a_n$. Then there exists $1\leqslant k\leqslant \ell\leqslant n$ such that $a_k+a_{k+1}+\ldots+a_\ell$ is divisible by $n$.
I should use the pigeonhole principle.
sigh
 
4:15 AM
while one can use anything from calculus 3 to group theory in computer science applications, depending on what you want to be and do all you should need is some practical linear algebra and discrete math knowledge/intuition
 
@DemCodeLines Just practice a lot. You'll get used to it. You can also pick a good book like Apostol's or Spivak's calculus.
 
hmm, firefox wants me to change math to maths. must be british.
 
@PedroTamaroff My teacher said that if I can do derivatives then I should be good for the test.
 
@DemCodeLines Seems like it.
 
Except I don't trust him, cause I know he will put some really difficult problems on there.
 
4:19 AM
just dont make it more difficult that it is
tell yourself "it's easy", and just do work!
that always worked for me
 
If y = sin^3 x, then dy/dx = (dy/du)(du/dx), where (a) u = x^3, (b) u = 3sin^2 x or (c) u = sin x
 
u-substitution?
 
4:49 AM
@ComTruise Are you, by any chance, on reddit?
 
Is there a simpler restatement of math.stackexchange.com/questions/838031/… ? I think the wall of text will turn off people from trying to answer it
 
@user939259 probably not. is each parent's height-genome picked uniformly from the possible genomes with height 7?
err, sorry didn't finish reading
 
5:07 AM
@seaturtles what is your specialty?
 
err, I like numbers and representations I guess
 
I like infinitary combinatorics
 
@seaturtles Did you read my problem above?
 
@seaturtles Yes uniformly picked
 
@PedroTamaroff the open problem I responded to?
 
5:12 AM
Hi @Pedro
 
The question was from a job interview I had today, and the interviewer asserted it could be done without long calculations, but it's not at all clear to me how to do that
 
@seaturtles No, the other one. It's a few lines above.
@AntonioVargas Hey.
 
@PedroTamaroff I'll think about; finishing answer to user##@#$%#$%
 
<3 new handle
 
I get an expected height of 8.
Oh wait I read it wrong.
 
5:19 AM
if an interviewer seriously asks me a math question I will leave
I do math in seclusion
thats the only way
 
@user939259 moi?
 
Really? I get probability brainteasers all time in interviews
@seaturtles Thanks for the answer, that makes sense and I think it's the answer the interviewer gave as well
 
I dont do brainteasers
 
<shrug> everyone from Silicon Valley to Wall Street asks them, so I figure refusing to do them drastically reduces my job options
 
so they decide to hire you based on these questions?
 
5:24 AM
In part, yes
 
ugh
really inappropriate if you ask me
 
I don't disagree, but why do you think so?
 
they won't find out how you work that way, so it could be misleading.
I mean its good to be able to think on the fly, but most of the time math research is a deliberate, long term process
 
I agree with you. I think they would say that it's an indicator of general intelligence, flexibility of thinking, and how you handle yourself when confronted with new problems
 
is your area related to probability?
 
5:28 AM
I think I read somewhere that Google did a study and found that performance on brainteasers had essentially 0 correlation with performance on the job (at least at google)
Yeah, my area is finance stuff, so it definitely involves an understanding of prob and stats. Although I've gotten brainteasers at pure software firms as well
 
maybe I just dont like interviews in general
 
Yeah, it's definitely stressful. But there doesn't seem like a good alternative?
 
I might have to take graduate Probability
but my foundations are pretty weak in this area
think it is called Modern Stochastic Processes
tell me all I need to know :)
 
What field are you in?
 
very abstract topology
 
5:39 AM
Academia?
 
I am a student
but required to take a variety of courses
 
Unless you go interview for tech jobs, I feel like your likelihood of encountering brainteasers like the one I posted is pretty close to 0
 
is that going to be hard for me? 2 semester graduate probability?
I usually like more abstract math
 
5:55 AM
Does there exist a function $f: \mathbb{R} \to \mathbb{R}$ that takes each value in $\mathbb{R}$ three times?
Find one if it does :D
 
sure you don't want more conditions than that?
otherwise partition R into three uncountable parts, say (-inf,-1), (-1,1) and (1,inf), and biject each with R
if you impose continuity, the answer is intuitively no, and it isn't too difficult to prove
 
@seaturtles Oops, I meant with continuity. Can you prove? :P
 
6:10 AM
hint: let a be the first zero and b the third zero. (I assume you want each value taken precisely three times, not just "at least three" times)
 
Precisely, yes.
But then? :O
 
it's a hint. think about it. what can you do/say from there, using continuity? let the force guide you, luke.
 
:/
 
hey, I'm not your homework-doer
2
 
@blue Oh, you think this is a homework :D
 
6:16 AM
the image of [a,b] must be some interval [c,d]. how many times can the value c - epsilon be taken?
 
This is continuous and takes all values three times :O
 
ah, my reasoning is erroneous
skulks away inconspicuously
 
:P
lol
 
@IlanAizelmanWS to solve quadratics, you can use completing the square or the quadratic formula
also, you got responses very quickly, there was no need to come here too
 
7:16 AM
@blue You're also anon, right?
 
many masks
 
@blue Have you ever looked at Hilbert's 13th problem?
 
heard of it, but not really
 
My bad.
Counting stuffs above 10 is too hard for me.
I meant 13 instead of 17.
 
ah, hadn't heard of that one
 
7:21 AM
@blue yep. definitely interesting, don't you think?
 
yeah
 
AFAIK, I only know that a sextic cannot be solved by elliptic functions - I have in general no idea about holomorphic functions of single variable. I just don't know enough alg. topology to appreciate Abhyankar's proof yet.
The elliptic function case is respectively easier : You have to prove that $A_6$ is not isomorphic to $\rm{PSL}_2(\Bbb F_p)$ for any prime $p$.
It's a near miss, though : $A_6 \cong \rm{PSL}_2(\Bbb F_9)$
 
@BalarkaSen And sea turtles of course.
 
@blue are base fields $k$ s.t. $\bar{k}/k$ is prosolvable always easy to make?
for example, how about $\Bbb F_p$ adjoined with some transcendental $t$?
are all the galois extensions solvable?
 
7:38 AM
dunno
 
well, $\rm{Frac} \, \Bbb F_p[[t]]$ seems a much doable candidate.
every galois group on $\Bbb Q(t)$ get considerably much smaller in $\Bbb Q((t))$
plus, $\Bbb F_p$ pushes it to something more small.
 
 
2 hours later…
9:19 AM
?
 
 
1 hour later…
10:45 AM
i have a puzzle
looking for answers
+__+__+__+ = 30. 1,3,5,7,9,11,13,15. you can repeat numbers
sum of 5 numbers
 
@GTR I faved that Cambridge page but i have so many thing to finish before :/
 
@Hippalectryon That's too mathy!
 
@Sawarnik yeah lol i know, but otherwise i always forget about those awesome pages i haven't visited yet :)
@Sawarnik I also have yet to read all the posts on MSE i've favorited
 
@Hippalectryon And what are those blackened out thingys? :O
 
@Sawarnik Things you're not supposed to see xD
 
10:57 AM
@Hippalectryon Oh!
 
@Sawarnik No one shall see :D
 
r9m
@robjohn are you awake ? :) (ping)
 
@Hippalectryon Why so secret?
@r9m The one with the blue mean square is awake :D
 
@r9m yes
 
r9m
@robjohn is it your birthday today ? or was it a few day/weeks ago ? :D :P
 
11:00 AM
@Sawarnik Ok ok -____-
 
@r9m it was a while ago. why do you ask?
 
r9m
@robjohn sorry belated Hb'day then :P .. I just noticed 54->55 in your M.Se profile :P lol
 
^right.
 
@robjohn He wants to send you money as a present. You just need to give him your bank account number and pass :3
 
@Hippalectryon Uh, that was not what I thought! Nothing so secret type!
 
11:03 AM
@Sawarnik What were you thinking (_(
 
@Hippalectryon Maybe some NSA documents!
 
@Sawarnik You'd really think i'd show THOSE ? :D
 
r9m
@Sawarnik I woke up just now .. :P lol the holiday is spoiling me :P lol
 
@r9m When did you sleep?
 
r9m
@Sawarnik 4am :P .. 12 hours straight :P
 
11:05 AM
@r9m Good, that's pure owl-like behaviour.
I slept at 2 and woke at 11.
 
r9m
@Sawarnik (y)
 
r9m
@BalarkaSen do you wear specs ?
 
@r9m thanks. it was something like 6 weeks ago
 
@BalarkaSen Jus 5 hrs?
 
11:08 AM
@r9m not yet, no. i will have to though, i think. i can't see stuffs in distance.
 
@BalarkaSen Wi?
 
r9m
@robjohn oh .. LOL !! welcome ;)
 
:D
 
@Sawarnik "Optical method"
 
Do you wear specs @r9m ?
 
r9m
11:09 AM
@Sawarnik you have seen me on fb :P lol .. don't you know ? :P
 
@r9m Yes I know :) Power?
 
r9m
@Sawarnik -1.5 on both balls(eye) :P
 
Thats too good :(
 
Really, @Sawarnik, how thick could you be to not to get the joke?
 
@BalarkaSen ?
 
11:12 AM
3 mins ago, by Balarka Sen
@Sawarnik "Optical method"
 
Yes so?
 
Gah, forget it.
 
I know what you mean. But its a very poor joke.
 
snif
Do you have any more of those functional equations, @Sawarnik? You saw my last answer, I presume, BTW?
 
@BalarkaSen Yes.
 
11:15 AM
Yes and yes or yes and no or no and yes or no and no?
Ambiguous answer.
 
Yes and yes or yes and yes.
 
@BalarkaSen I didn't give you my functional inequation did I ?
 
Nah.
 
@Hippalectryon No. What's it about?
 
@BalarkaSen find all $f:\mathbb{R}\rightarrow\mathbb{R}$ such as $\forall (x,y)\in\mathbb{R}^2,\ f(x+y^2)-f(x)\geq y$
 
11:17 AM
Uh-oh.
 
@BalarkaSen Didn't I give you this once? :D
 
@Sawarnik Which one?
 
@BalarkaSen Don't worry the answer fits on one line :D
 
Hippa's.
 
@Sawarnik I don't recall.
 
11:18 AM
$f:[0,1]\rightarrow \mathbb{R}$
$f(x) + f(1-x) + f\left(\sqrt{x^2+(1-x)}\right) = 0 $$ f\left(\tfrac12\right)=0
$$
 
Let me think, @Hippa.
@Hippalectryon Are you sure you don't have continuity imposed? otherwise, $f(x + \delta) - f(x) \geq \sqrt{\delta}$ violating continuity.
 
@BalarkaSen There is no restriction at all >8)
 
Hell. You're evil, @Hippa.
 
shower before or after getting a haircut?
 
Never said it had to be continuous muhahahahahaha
@N3buchadnezzar after
 
11:21 AM
Mmm
 
@N3buchadnezzar That will clean the remaining cut hair
It's awful to have some stuck on you :3
 
@Hippa I give up. I am not familiar with noncontinuous jobs much, in case of functional equations.
 
@BalarkaSen xD Fastest surrender ever
 
Yes.
 
I don't know anything special about it either lol
@Sawarnik Don't you surrender :c
 
11:26 AM
Hey all ;)
 
hey @IlanAizelmanWS
 
Actually, @Hippa, I don't think I am going to give up.
 
@BalarkaSen :D
 
$f(x + \delta_1 + \delta_2) \geq f(x + \delta_1) + \sqrt{\delta_2} \geq f(x) + \sqrt{\delta_1} + \sqrt{\delta_2}$
This looks fun.
 
And so
 
11:28 AM
$f(x + \sum \delta_i) \geq f(x) + \sum \sqrt{\delta_i}$
 
Doesn't seem false
 
How about taking $\delta_k = 1/k^4$? $f(\pi^4/90) \geq f(0) + \pi^2/6$
Very suspicious.
 
How is that suspicious ?
The function might be discontinuous on every point
 
@Hippalectryon STAHP
 
Hey i'm here to disturb you while you're thinking :3
 
11:34 AM
@Hippa I think I got it.
 
@BalarkaSen :D
 
Uuf, my internet!
Q1. $f:[0,1]\rightarrow \mathbb{R}$
- $f(x) + f(1-x) + f\left(\sqrt{x^2+(1-x)}\right) = 0$
- $ f\left(\tfrac12\right)=0$
 
Take $\delta_k = 1/k^2$. Then $f(x + \sum^n 1/k^2) = f(x + \pi^2/6) \geq f(x) + \sum^n 1/k$ as $n \to \infty$
Now set $x = 0$
$f(\pi^2/6) \geq \lim_{n \to \infty} \mathcal{H}_n$
 
@Sawarnik STAAAAP
 
This is impossible, left side diverges.
 
11:36 AM
@Sawarnik remove that
@Sawarnik That's the answer
@Sawarnik :c
 
:16140665 Wuzzat?
 
I had given him this question and he doesn't remember!
 
@Sawarnik lol
 
@Hippa Did I do alright above?
 
@BalarkaSen I think there's an error somewhere
 
11:38 AM
@Hippalectryon OK. Point me out.
 
@Hippalectryon I have one for you. Want it ? :)
 
@Sawarnik sure
 
First point me out, @Hippa
@Sawarnik STAHP
 
@BalarkaSen You didn't use the fact that $f$ was growing did you ?
 
@Hippalectryon Nope.
 
11:39 AM
That's what bugs me
 
- $f:[0,1]\rightarrow \mathbb{R}$
- $f(x) + f(1-x) + f\left(\sqrt{x^2+(1-x)}\right) = 0$
- $ f\left(\tfrac12\right)=0$
Prove that infintely many solutions exists.
 
@Sawarnik Ok i saved it on my comp
 
@Sawarnik This is an old, poor, sick-looking lame problem.
 
@Hippalectryon No try it.
@BalarkaSen It wasn't for you and its not lame. You couldn't do it either.
 
@Sawarnik I gtg in like 5 minutes so i'll focus on finding @BalarkaSen's error :c
 
11:41 AM
Ok.
 
@Sawarnik I can't do it and I won;t try it. Thank you.
 
@BalarkaSen Who is telling you to try?
 
@Sawarnik well you just spelled "to try".
 
?
 
You're thick. You don't even get logic jokes.
 
11:44 AM
Nobody gets your poor silly jokes.
May 19 at 20:46, by Sawarnik
@BalarkaSen Is there any function such that $f(x+y^2)-f(x)\ge y$? It won't be differentiable, and is increasing seems obvious, though.
 
OK, I didn't recall it.
 
@BalarkaSen I can't find any error -_-
 
in Room for General Mathematical Conversation, May 20 at 10:07, by Sawarnik
$f:\mathbb{R}\rightarrow\mathbb{R}$
$\forall (x,y)\in\mathbb{R}^2,\ f(x+y^2)-f(x)\geq y$
 
@Hippalectryon So the winner is?
 
@BalarkaSen Idk it still bugs me
:D
I gtg anyway
 
11:46 AM
@BalarkaSen Post as an answer and see the downvotes coming (and the winner) :P
 
OK. I'll ask @robjohn.
 
Lalalalala
* away *
 
Lalalala
 
100
Q: How to evaluate $ \int_{0}^{\frac{\pi}{2}}\frac{1}{(1+x^2)(1+\tan x)}\,\mathrm dx$

juantheron$$\int_{0}^{\frac{\pi}{2}}\frac{1}{(1+x^2)(1+\tan x)}\,\mathrm dx$$ My Try: Let $$\tag1 I = \int_{0}^{\frac{\pi}{2}}\frac{1}{(1+x^2)(1+\tan x)}\,\mathrm dx.$$ Put $x=\frac{\pi}{2}-x$. Using $\int_{0}^{a}f(x)\,\mathrm dx = \int_{0}^{a}f(a-x)\,\mathrm dx$. So $$\tag2 I = \int_{0}^{\frac{\pi}{2}...

 
Let the public decide.
 
11:47 AM
Anyone up for Trying that question?
 
@Sawarnik I rarely post answers to medium hanging fruits.
 
@BalarkaSen I have an integral type thing for you.
 
@robjohn you here?
 
@BalarkaSen -_-
 
@Sawarnik not in the least interested.
 
11:47 AM
@BalarkaSen yes
 
@robjohn You saw @Hippa's question?
 
First it wrong, and its not easy, and you don't do anything great by not answering anything.
@Bala
 
@BalarkaSen which one?
 
@Sawarnik ?
 
$f:\mathbb{R}\rightarrow\mathbb{R}$
$\forall (x,y)\in\mathbb{R}^2,\ f(x+y^2)-f(x)\geq y$
 
11:48 AM
@robjohn I have an answer. Can you check it for me, please?
 
$$\int_1^{\frac{\pi}2} x \cos(\frac1{x}) dx>\frac12$$
@BalarkaSen Prove it :P
 
@Sawarnik No wish to do that.
 
Yes, I knew.
And then you will say later its a lame, poor, blah question.
 
It's ugly though.
But otherwise looks fine. Not poor at all.
 
@BalarkaSen: Dude! Which problem you guys talking about?
 
11:52 AM
@Sawarnik wasn't that shown using substitution then the power series for $\cos$?
 
$f(x + \delta) - f(x) \geq \sqrt{\delta}$. Thus $f(x + \sum \delta_i) \geq f(x) + \sum \sqrt{\delta_i}$. Take $\delta_i = 1/i^2$ then $f(x + \sum^n 1/i^2) \geq f(x) + \mathcal{H}_n$. Taking $x = 0$ and limit $n \to \infty$ we have $f$ not defined at $\pi^2/6$ @robjohn
 
It was for other guys :)
Besides, I have modified the question a bit :)
 
@Sawarnik Have you completed matrices and vectors?
 
@BalarkaSen Found it boring beyond my limit so left it.
 
Yes, so did you get to Abstract Alg.?
 
11:54 AM
@BalarkaSen for what $\delta$ is $f(x + \delta) - f(x) \geq \sqrt{\delta}$ true?
 
@robjohn Any real.
That's what is given.
6 mins ago, by Sawarnik
$f:\mathbb{R}\rightarrow\mathbb{R}$
$\forall (x,y)\in\mathbb{R}^2,\ f(x+y^2)-f(x)\geq y$
 
@BalarkaSen have you checked to see that such an $f$ can exist?
 
@robjohn I believe it doesn't.
4 mins ago, by Balarka Sen
Yes, so did you get to Abstract Alg.?
 
@BalarkaSen Nah.
 
@BalarkaSen by that condition $f$ is monotone increasing, therefore differentiable almost everywhere. However, by the condition given it is nowhere differentiable.
 

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