The h Bar - 2016-10-05 (page 4 of 5)

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4:14 PM

Everyone is a phd to you, though

4:32 PM

@EmilioPisanty Could have been better, as in, what happened was not the best of all possible ways to handle the situation? Sure, I agree with that.

In general, I think the best way to handle bad answers is to explain to the author why the answer is bad and how to improve it.

Nobody really did that.

@EmilioPisanty I like your table of Fourier transforms.

For those who want to nerd out -- I thought it would be cool to take a look at how elections progress over time. The only metric I have that I know of is to look at when the constituent badge is awarded in a year an election occurs. The data for the current election isn't dumped yet, but the query set up for the 2014 election on SO looks like:

data.stackexchange.com/stackoverflow/query/551108/…

I've drawn that before. It was an "aha" moment when I figured it out.

Hahaha, Pontryagin duality :D :D :D

@Danu ?

Just funny to see such high powered math :)

4:43 PM

I never know how to vote on questions like this

0

Q: Gauss's law for cylinder with infinite height with a spherical cavity

Imagine there is a cylinder with a charge density of +Q per unit volume and of infinite length. Now place a spherical cavity inside it with a diameter equal to the cross-section diameter of the cylinder. Is there an electric field inside the sphere? If so, is it possible to calculate the E-field ...

electrostatics electric-fields gauss-law

@DanielSank How about down? :P

Why would you vote that down?

^ That

Because I think it's not* interesting content for this site.

@Danu I meant in the review queue.

4:45 PM

Ah

i was always under the assumption that if it were interesting & a good question you vote it up. If it is uninteresting but follows question-asking guidelines then you leave it alone

If you don't like it, feel free to vote it down.

I think about it this way: Am I interested in seeing questions like this one on the site more often?

Clear nope in this case, so I'm voting it down.

@0celo7 did you ever take a logic class? I'm having a test in an hour on connectives. I don't get how $p \to q$ works

@Danu Review queue...

@DanielSank I was talking to Obliv

In review, I'd probably skip. It's kind of a close call.

4:48 PM

@Danu I know.

@danu "Voting up a question or answer signals to the rest of the community that a post is interesting, well-researched, and useful, while voting down a post signals the opposite: that the post contains wrong information, is poorly researched, or fails to communicate information."

@Obliv Wrong.

I think it's pretty much a personal question

It's on the site help center though

I know, but I disagree

For me, it's a personal indicator of "interest aroused"

¯_(ツ)_/¯

4:51 PM

:)

@DanielSank thanks, I'm pretty proud of it myself

Doesn't really get enough playtime, I think

Particularly because it's so simple

It's just putting together a bunch of really tough material that undergrads already know

Gives it lots of form and structure

For essentially no work at all

@EmilioPisanty So do you know anything about Pontryagin duality? Or were you just throwing that name around

Because I'd love to hear about it

@Obliv I am famous for disagreeing with most of classical logic.

No, I have not taken a course on it.

user228700

@JohnRennie: Are u there? (If u are, are u too tired to have a small discussion with me?)

Hi, yes I'm here

5:02 PM

too tired?

@0celo7 I get it. It's like really stupid though.

it's only 6 or 7 over there

he's not that old

@Obliv $p\to q$ means it is impossible for $p$ to be true and $q$ not. Or something like that.

$(p \to q)$ is like : if p is false, the statement is true regardless. Though, if p is true, then q must also be true.

user228700

@0celo7 But dude, he keeps himself busy! It's not like he's just sitting around, amirite @JohnRennie? :-) Are u too tired tho?

it just takes it for granted that if it's a false antecedent, that the statement will be true.

But when the antecedent is true then the consequent must be true.

5:04 PM

No, I'm fine, carry on :-)
Though as it happens I have been lazing around all day because I've got a cold.

how did you catch it

user228700

@JohnRennie Oh :/ I hope u feel better soon.

Cold? More like ebola

user228700

Learning about vehicles moving along a horizontal curve and the centripetal force that causes this motion.

@0celo7 dude I was tripping out ~1.5 years ago when i was still taking adhd medication. had a panic attack cuz i thought I had ebola

5:07 PM

@Obliv From my niece (age 14), who presumably caught it from her diseased schoolfriends :-)

@Kaumudi OK

i didn't know what a panic attack was and I initially thought the side effects of the medication were symptoms of ebola then I started panicking and thought that was also part of the symptoms. @0celo7 ah.. good times

user228700

@JohnRennie So static friction makes it move along the curve, which is a part of the bigger circle, yes?

to be fair though it was like a week after i heard people were spreading ebola a couple towns away.

Static friction? Maybe you need to take a step back and explain what the system you're considering is...

user228700

@JohnRennie Oh, sorry. I'm considering a car moving on a circular track.

5:11 PM

@Kaumudi Ah, OK, yes static friction keeps the car from sliding outwards.

@yuggib Ok the proof wasn't hard in the end

user228700

@JohnRennie Yes, I don't understand this.

user228700

You know what, sir? I'll come back t'row and we'll discuss it then. My brain is sort of fried, so I'm going to go to bed. Is that OK?

OK :-) I'll be here around 05:00 UTC tomorrow.

user228700

As usual :-) Bye!

5:14 PM

If $f:E\to F$ is differentiable at $x\in E$, then $\dd f(x_0)(v)=(vf)(x)$, for any $v\in E$.

This is clearly true for $v=0$, so we restrict attention to $E-\{0\}$. Let $t\in\Bbb R$ and $\epsilon>0$. Since $f$ is differentiable at $x$, there is $\delta>0$ such that
\[\frac{||f(x+tv)-f(x)-\dd f(x)(tv)||}{||tv||}<\frac{\epsilon}{||v||}\]
whenever $||tv||<\delta$. Assuming $t\ne 0$ and dividing by it gives
\[\norm{\frac{f(x+tv)-f(x)}{t}-\dd f(x)(v)}<\frac{\epsilon}{||v||}\frac{|t|}{t}||v||\le \epsilon \]

\dd is a \mathrm{d}

@EmilioPisanty Agreed. I've actually been doing a series of lessons on signal processing and noise for my group. So far it's been very well received.

My boss particularly liked the table of the four transforms.

(We both came up with exactly the same idea, by the way. Great minds...)

My writeups are on github.

I've got a very nice one on aliasing.

@DanielSank I have some interesting pathological things you might be interested in

@0celo7 Uh oh.

@DanielSank First: there is a function $\Bbb R^2\to\Bbb R$ for which both partials exist at the origin, but it's discontinuous there

@0celo7 Give function.

5:20 PM

Nothing is as pathological as a continuous surjection $\Bbb R\to \Bbb R^2$.

@DanielSank $f(x,y)=\frac{xy}{x^2+y^2}$ for $(x,y)\ne (0,0)$, and $f(0,0)=0$.

(you cannot make it continuous by picking a better value at $(0,0)$)

@0celo7 k

so that's cool, but wait, there's more

a function $\Bbb R^2\to\Bbb R$ for which all directional derivatives exist, but it's not a linear function of the vector

and then the worst

@0celo7 Is the gradient of this function particularly interesting?

a function $\Bbb R^2\to\Bbb R$ for which all directional derivatives exist but the function is not continuous

5:26 PM

Usually, the cute pathological functions with a single bad point are related to a closed by not exact form.

hmm

I'll examine it later.

-1

Q: Math and engineering questions: where do we draw the line?

I was reading through some old meta discussions recently, and I was interested in some of the points made, but they never quite seemed to be implemented or discussed further. So, I am asking the current community what they think of the following points; I will post my own personal opinion as an a...

discussion scope questions

user116211

6:16 PM

28

Q: Topology and the 2016 Nobel Prize in Physics

I was very happy to learn that the work which led to the award of the 2016 Nobel Prize in Physics (shared between David J. Thouless, F. Duncan M. Haldane and J. Michael Kosterlitz) uses Topology. In particular, the prize was awarded "for theoretical discoveries of topological phase transitions an...

reference-request at.algebraic-topology dg.differential-geometry mp.mathematical-physics

user116211

Same sort of question has been asked at Physics yesterday; but that was really in a bad shape; the MO is quite well-structured and has a legit reasonable query.

@MAFIA36790 any progress on sections 1-9 of Bredon

6:34 PM

@0celo7 this class is retarded

the notation is so unintuitive

What class?

logic. I just took the exam

Why are you taking a logic class?

credits

i didn't take 11 APs like you

Take topology

6:39 PM

It looked like this : $(\text{~T} \to (\text{P} \to \text{C}))$ on the first line

P

Is ACM dead?

$\text{~C}$

@Obliv what?

then triple dots in a triangular formation followed by T

user116211

@0celo7 He is preparing his victory speech.

6:40 PM

Is the argument valid, if not find a counter example

How the hell would I know? I'm fairly sure logic is all wrong

It doesn't make sense

I had no idea what the hell the argument even was. That if P and not C, then T (following that function above)? eh i give up

user116211

@Obliv Never seen such thing.

Are you failing that course?

user116211

@Obliv They are statements, if you are studying propositional logic.

6:41 PM

no i had like >90's on all quizzes

What higher level math course has quizzes?

the connectives , $\cdot , \vee , \to , \equiv$ were easy and thats what the quizzes were
on

Literally no clue what any of that means.

user116211

@0celo7 I've not gotten the book yet. Our University is closed for nearly one month now.

WTH?!

6:43 PM

it's not a high level course and it's not math either. it's labeled under philosophy

user116211

@Obliv logic, dude.

Logic is PhD level mathematics...

i had to take a philosophy course for humanities requirement. I didn't feel like taking actual philosophy so there you go

user116211

I'm studying Propositional and First Order Logic in my off times. They are interesting.

@mafia if you give me values for logical statements I can process the validity of the argument. But, the way they were presented made no sense to me.

6:46 PM

@Obliv let's learn about Banach spaces

user116211

Logic is not necessary for learning mathematics though.

user116211

Bourbaki never cared for logic.

I have to take a calc exam in 3 and a half hours .-. it'll probably be easier than that logic b.s.

user116211

@Obliv Be ready for some infinite series and analysis.

Calc 3 is PhD level too depending on how bad the integrals are.

6:48 PM

how do you be ready for something

@MAFIA36790 calculus has no analysis

user116211

@0celo7 We have analysis included in Calc 1 course.

user116211

@Obliv Proving every theorems and doing exercises...

i'll do the theorem proofs but exercises are so boring

I have a question

6:50 PM

i don't have an answer

user116211

PhD level ._.

Does India actually produce a significantly higher quality work force to justify all the shit they put you through?

user116211

@0celo7 No idea.

does india have nuclear bombs yet

I think so.

user116211

6:51 PM

@Obliv Get idea by googling.

@0celo7 lets make bombs for india if not

user116211

@Obliv Putin will be helping India to make one of the largest nuclear plants in Asia.

Putin is a great man.

i wonder what's worse: if you have 1 huge nuclear plant for a country that when something happens to it: it kills the country, or if you have a bunch of small nuclear plants across the country that inevitably increases risk of meltdowns

user116211

Trump loves Putin.

user116211

6:54 PM

I love Putin memes.

There have been two meltdowns in history. Both were completely avoidable.

[citation needed]

wait are you talking about the japan one? How could that possibly be avoidable @0celo7

Read the Wiki pages.

yeah it could have been avoided, by not building it

user116211

> Russia is currently collaborating with India in building one of Asia’s largest nuclear plant with eight reactors (each 1000 MW) in the southern Indian state of Tamil Nadu. Russia will be building atleast 16 reactors in various sites in India by 2030 with a cumulative investment of more than US$ 45 billion.

6:55 PM

@Obliv The tsunami walls were not high enough despite them knowing such high tsunamis were possible.

Do you even know what happened?

Water got into the system and disabled the cooling system, right?

The tsunami took the cooling system offline. They didn't have a good backup and the initial thing was avoidable.

i don't think they had 'tsunami walls' in the first place.

user218912

what effect did it have on the environment?

killed things and probably ruined water supply

6:57 PM

[citation needed]

i think they also let the water in at some point to cool down the reactor manually. Then the radiation spread through the running water

okay that part i'm not sure about lol

user116211

@Obliv, Notice, Russia will be building not 1 but sixteen nuclear plants in India.

user218912

@ACuriousMind hey!!!

@0celo7 Why is your profile picture a deformed cat?

@IceLord heyhey

user218912

@ACuriousMind it could have been a blue deformed cat.

user218912

7:00 PM

20 hours ago, by IceLord

user image

thats cute

@ACuriousMind I thought you had died :(

@ACuriousMind because it's good

user218912

@0celo7 why do you always assume the worst case?

Because I'm usually pleasantly surprised.

it's an unwritten rule of the internet that when someone is away for more than 2 days, they are assumed dead @iceL

user218912

7:02 PM

@Obliv [citation needed]

user116211

ACM is not away for two days.

Unwritten

user218912

@0celo7 yes but it's written that it's an unwritten rule somewhere.

user116211

Then why it is unwritten...

LOL

7:03 PM

@0celo7 I was merely away for 24 hours...

Right, so you understand.

I basically couldn't do math @ACuriousMind

Without you to hold my hand I cannot do it

user218912

same with me for qft.

You better not be leaving him alone like that if you want his vote @acuriousmind

user218912

I finally bought a tablet.

user218912

I am now broke.

7:08 PM

Steal a purse

^don't do that

@iceL which tablet did you end up getting

user116211

rob a bank

user116211

Find Nazi hidden treasures.

user218912

@Obliv ipad pro 9.7 with the stylus

user116211

Be like Indiana Jones.

user116211

7:10 PM

@IceLord Nice!

I didn't know nazi's hid treasures. I don't think you'd want whatever they're hiding @mafia

user218912

it feels like writing on real paper.

@Obliv Contrary to what it might seem like sometimes, I do have a real life I occasionally have to deal with :P

user116211

@IceLord Good ;)

Whoa I want an iPad Pro

Can you get a stylus for an iPad Air 2?

user116211

7:11 PM

@0celo7 Buy Surface Pro.

user218912

@0celo7 no.

user218912

not the apple one.

Hey guys. Was Skull Patrol banned? Or is he actually dead?

Skull petrol?

user116211

No idea.

7:11 PM

Yeh

user116211

Didn't see him for quite a long time.

@ACuriousMind what does that mean in AI terms?

Oil change?

Lubrication is important.

oh wow he hasn't been seen since Aug. 1

user218912

I thought skull patrol would show up when I posted this

user218912

7:12 PM

20 hours ago, by IceLord

user image

user218912

but he didn't.

@ACuriousMind Ah, I see ;)

@ACuriousMind you can be forgiven for that

oh no

this is not going in the right direction

user116211

> It should be remarked that the definition of ordering is not very restrictive. For example, $X\times X$ is an ordering of $X,$ but a rather uninteresting one. Relative to this ordering, each member of $X$ is an upper bound, and in fact a supremum, of every subset. The more interesting satisfy the further conditions: if $x$ is less than or equal to $y$ and $y$ is less than or equal to $x,$ than $y=x\,.$ In this case there is at most one supremum for a set, and at most infimum.

What

user116211

7:15 PM

What does Kelley mean here by uninteresting?

He literally says it right underneath

user116211

Also, he said the definition of ordering is not very restrictive; what does that mean?

user116211

Oh, by ordering; he means partial ordering.

@0celo7 what is that transformation on the cat called

or what transformations does it include

@iceL do you know

user218912

7:36 PM

I think affine transformation.

@IceLord no

user218912

I don't remember.

user218912

I just remember arnold talked about affine transformations.

it's volume-preserving

isochoric (?)

user218912

okay.

7:39 PM

although it clearly is not volume-preserving.

@IceLord I need to resort my books

Should I put the geometry books on the shelf, then physics, topology, and analysis books on the desk?

I can probably fit topology and geometry on the shelf

user218912

no only the physics books should be on the desk obviously.

the physics books are on the desk

user218912

I meant only.

(the shelf is more prestigious btw)

user218912

:(

7:41 PM

@IceLord not possible I have too many books

what is your skype?

it says you are offline

user218912

I lost my password to my old skype.

user218912

when I reformatted my computer for university.

IceLord is offline

user218912

oh right I added you.

user218912

k i'm online now.

7:55 PM

I'm having tons of trouble right now figuring something out and any help would be appreciated.

I have a basic understanding of RSA encryption (though I got somewhat confused with some of the math; the Khan Academy videos aren't great)

and I'm trying to understand Shor's algorithm. I read the Wikipedia article, and I didn't get it at all.

Could anyone explain?

8:12 PM

@ValterMoretti hi welcome back to chat, youve been nominated as a guest chat speaker for the site, our next slot is open. it is supported by mod DZ & co-organizer DS. any interest? more details/ prior sessions here meta.physics.stackexchange.com/questions/7783/…

@heather shors algorithm is an advanced topic that is not even covered in a lot of undergrad QM classes. try this arxiv.org/abs/quant-ph/0109004

@vzn who are DZ and DS?

@0celo7 you can figure out advanced topology but initials of chat room regular(s) and a site mod elude you?

@vzn, thank you for the link!

Rethinking some of what I've learned about Shor's algorithm:

@vzn what happened to that other guest DS was talking about

I forgot his name but he did a phd on quantum decoherence i think

I've read that the fastest algorithm on the classical system for prime factorization of numbers runs in O(exp((64/9)n^1/3(log n)^2/3) and Shor's algorithm runs in O((log n)^2 * log log n) and then O(log n) steps of post processing on a classical computer.

8:19 PM

@Obliv click on the link. CJ Riedel date is still "TBD"

What is O here? I know n is the number of bits used to represent the number. Also, in the expression for Shor's algorithm, is n expressed in classical or quantum bits?

@heather sounds about right (not up on the exact details). big-O is landau notation. try reading about P-time in complexity theory. en.wikipedia.org/wiki/Big_O_notation ... are you in the US?

@vzn, yes, I am in the US

@heather what state?

@vzn Iowa

8:22 PM

& why do you want to learn about shors algorithm?

so O o omega etc just show the level of complexity? and O is the most complex?

Sorry, because

I'm trying to understand quantum computing

and I'm trying to learn that because

a. I got interested in it

@heather big O notation is a way of mathematically characterizing/ bounding the growth of other fns. its used a lot in CS complexity theory and sometimes in math.

well, basically that

bounding the growth of other functions...hmm, I'll have to read more. =)

so do you want to get into university physics someday?

I'd love too, honestly

But, you know, right now I'm in middle school

And I honestly kind of hate science class

(earth science right now)

8:25 PM

@heather yes see your profile. why dislike science class?

Well, because it moves sooo slowly, doesn't really go in depth, and passes by all the interesting stuff

there are probably a bunch of youtube videos on shors algorithm, try it out

I did

I watched a few, and I get some kind of idea

but not a very good one.

So I'm reading various sites and papers and Q&A on stack exchange, including the computer science one

@vzn, your mention of big O notation really helped: I found this computer science question, and I think I get it enough now to continue reading. Thanks!

@heather yw, its just a start. youre attacking some big ideas that require a lot of fundamentals, do recommend some complexity theory, there are some nearly-intro references out there

@vzn, okay, thanks...googling =)

8:32 PM

@heather you might start out by learning about basic classical factoring algorithms & understanding why they run in exp time. do you have any interest in coding?

@vzn, yes, I am teaching myself Python currently, though I've also messed with languages like JavaScript, HTML/CSS, Wolfram Mathematica, and others

exp time standing for exponential time, I assume?

@heather ok cool. good intro exercise, try writing/ debugging a basic factoring algorithm.

@heather yes

@vzn, okay, I'll try that! Should be fun.

@heather :)

@vzn, thanks for all your recommendations - sounds like I've got a lot of stuff to explore. =)

8:35 PM

sure. fyi QM involves a lot of linear/ vector algebra, suggest getting into that too. many ppl in the chat rooms can help with that.

@vzn, yeah, I found shankar's textbook as a pdf online for free, I heard that uses mainly linear algebra, which is good, because I don't know calculus, but I know some linear algebra because I learned some so I understood what was going on in the quantum computing videos and papers I've been looking at.

@heather just keep in mind "most" QM is basically never taught in HS and not even all that much in college except typically to physics majors in later years.

@heather Shankar is a very nice first introduction :)

@vzn, yeah, I know. But it'll be good to learn the more basic stuff too.

@Sanya, well that's good! =)

Okay, so I've been reading about what polynomial and exponential time is, but why is it such an advantage for it to be in polynomial and not exponential time?

@heather P time is considered/ almost defined as "efficient"

8:40 PM

@heather Because polynomials grow much slower than exponentials.

@heather a nice conceptual area for beginners to study this concept is travelling salesman problem, check it out... started with that myself yrs ago around your age

@ACuriousMind, oh...

@vzn, oh, yeah, I've read about that! I'll go back to it, because I didn't really look into the math behind it.

@heather also recommend after you write your factoring algorithm try it out on larger numbers & do timing experiments

@vzn, there will be an exponential curve when plotted as number on x-axis and time on y-axis, right?

@heather right! roughly though. getting the details right is not trivial. if youre interested in science its an excellent beginner exercise/ experiment

8:44 PM

@vzn, okay, sounds cool

Wow, I think I just saw the light with Shor's algorithm!

If you look at RSA encryption, Alice can send a message to Bob by having two prime numbers, p1 and p2, and multiplying them together to get a number N. If you then say that there is a sequence x mod N x^2 mod N x^3 mod N etc etc, if x isn't divisible by p1 or p2 the period of the sequence is (p-1)(q-1) which I think is Euler's phi function. Then you create a superposition over all the numbers in the sequence and figure out the period...okay, so I need to figure the last part out.

@heather RSA security is based on whats known as the "discrete log" problem which is nearly the same/ conjectured to be equivalent in hardness to factoring.

@vzn, oh, huh, I watched some videos that said it was based on factoring...I guess there are multiple ways to do it?

ACM

HALP

QUIK

@heather coincidentally the illustrious @Bernard was asking about shors algorithm & dredged up some links for him somewhat recently, maybe you two can collaborate some

@vzn Sup man

What's going on?

8:52 PM

@BernardMeurer, hello

Howdy

@ACuriousMind I need your German body

I hear you are also interested in shors algorithm?

@BernardMeurer you know, once in a blue moon, its a female geek :P

Or, not your body but your brain

@vzn once in a blue moon a geek from middle school **

8:53 PM

@Obliv, yup

@BernardMeurer What ails you?

LOL bernard

Middle school?

@BernardMeurer, yeah...

Oh, okay, I'll hold my language. Yes, I quite like Shor's work

8:54 PM

@BernardMeurer dont be afraid of younger ladies :P

lol

But anyway, I was wondering if you by any chance understood what in the great wide universe a quantum fourier transform was?

@ACuriousMind My Chemistry course is going insane and I cried today, skype halp

@vzn I'm afraid of the FBI

@BernardMeurer lol (arent we all...) maybe she'll be "legal" when you graduate college :P

@heather Not really, I did some reading on it but it's above my mathematical & physics skill right now. Soon enough though, I have time :)

@vzn Doesn't make it less wrong in any way

8:55 PM

@BernardMeurer, well, thanks =)

@heather fourier transforms, another rather advanced topic, try studying the math version 1st, and biasically QM has a version specific to it connected to QM physics...

@ACuriousMind The granpa started doing Schrodinger on 3dim spherical coordinates

@ACuriousMind I am so lost

@BernardMeurer :( I have to type something up for tomorrow though and am kinda busy

@vzn, yeah, I read the first sentence of the wikipedia article and I got lost but somehow got that it was the quantum equivalent of the discrete fourier transform, whatever that is. I'll work on it

@ACuriousMind It's okay, it doesn't have to be today, but I will need some saving b/c right now I feel like it's Portuguese class all over again and I can't conjugate my verbs

8:58 PM

@BernardMeurer you could ask me

@BernardMeurer what exactly troubles you about it?

Middle school was hardcore

@0celo7 Skype?

no

ask what you were doing to ask ACM

@heather wondering about whats in your bkg to attempt such challenging topic(s)...? do you have STEM parents? inspiring teacher? scifi moives? etc?

I don't see why it has to be ACM

8:59 PM

@Sanya The fact that I'm a freshman and the fucker wants me to do infinite integrals of the wave equation somehow

and don't you need help with analysis?

@0celo7 because he was active on the chat when I came for help?

@0celo7 You do, yeah, I appreciate that a lot

@BernardMeurer I'm always around +/- a few hours

@vzn, my parents are STEM-y, my dad's an engineer and he's pretty interested in physics

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