Mathematics - 2013-02-05 (page 3 of 3)

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5:02 PM

What a stupid thing...

Orange Harvester

@Charlie Well, it works. One of my friends went to germany for an internship, and was looking for an apartment to lease. He was told to transfer the funds into an account 'over the phone'. He fell for it. He was only saved by the bank cashier who refused to take his payment since it was based in Nigeria.

Its more a cause of carefulness than stupidness.

:-|

Hava naguila hava nagila hava naguila

Venismecha

5:23 PM

I don't like this edit math.stackexchange.com/posts/295572/revisions

why did he do this? what should I do?

Orange Harvester

@user58512 Do you mean the link to wordpress thing?

yes

Orange Harvester

Add the link back, and drop a comment pinging the editor to ask why he did so.

I don't like $\pi$ either but that doesn't matter

Orange Harvester

@user58512 I think $\pi$ is better.

5:37 PM

what are the minimal generating sets for symmetric groups

I think S_4 has (1 2 3 4), (1 2 3)

gap> MinimalGeneratingSet(SymmetricGroup(5));
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 2nd choice method found for `IndependentGeneratorsOfAbelianGroup' o\

that sucks...

gap> SmallGeneratingSet(SymmetricGroup(5));
[ (1,2), (1,2,3,4,5) ]
gap> SmallGeneratingSet(SymmetricGroup(6));
[ (1,2), (1,2,3,4,5,6) ]
gap> SmallGeneratingSet(SymmetricGroup(7));
[ (1,2), (1,2,3,4,5,6,7) ]

there is a pattern :D

@Sanchez I am still waiting for a reply from my supervisor.

5:58 PM

@orange Jayesh Badwaik

this group theory is like a rubiks cube

and I can't solve the cube ......

Orange Harvester

@Charlie Yo.

@OrangeHarvester you know it...

Orange Harvester

@Charlie Yes I know it.

6:22 PM

@Charlie btw, thanks for the translation earler :)

6:35 PM

Hello, I have a question that I think is math related. It's sort of a game theory problem, but it's also about sending reliable messages across an unreliable channel. Here is the problem set up. Is there a name for this kind of problem? Is there any way for Alice and Bob to collaborate without putting themselves in danger?

youtube.com/watch?feature=player_embedded&v=-reFBJ4R9iA suffer with me.

@Kevin, cryptography ??

I'm guessing you didn't write this?

I did write it. I'm paraphrasing an article I read long ago about the subject, but I forget how it was resolved.

I don't know if cryptography would be useful. The opposing army can't intercept any messages, because they are sleeping. The only risk is the pigeon not delivering a message at all.

ok

just because they were called alice and bob

Yeah, I picked those names because it seemed to be the style for problems of communication in general

6:45 PM

@Kevin, after it goes back and forth a few times they could just say "If you get this send me one back but dont expect a reply"

Hi everyone. If I have an answer for a very old problem should I answer it?
The user has been inactive since 1.5 years...
http://math.stackexchange.com/questions/47795/number-of-edges-required-to-have-a-perfect-matching

@YongHaoNg, I think it's good to - but maybe it wont be accepted

Orange Harvester

@YongHaoNg If the question has been already answered, then it might not be good for you to do so. If the question has NOT been answered however, then you SHOULD post an answer!

Just curious, if I post an answer, will it be then flagged as active such that the rest will see it?

Orange Harvester

Not sure about the technicalities of software.

6:50 PM

Thanks for the responses. =)

On stackoverflow's meta, there's a very old question that gets frequent new answers. It shows up on the "new" tab all the time.

I will just post it then.

@YongHaoNg A new answer will move an old question to the top of the stack, and others will be able to see it.

Orange Harvester

@Kevin For danger, we have public key encryption. I encrypt the message with my key, and transmit to you. You encrypt that message with your key and transmit back to me. I decrypt the message with my key and then, send it to you, where you now finally decrypt with your key and read the message. This of course assumes that the message does get delivered.

@YongHaoNg However, you might not get a response from the original poster, given that they have moved on

Orange Harvester

6:51 PM

@EdGorcenski Others might find it useful.

Indeed, so it is certainly worth posting.

Orange Harvester

@Kevin For the pegion delivery thing, in modern world, there are timeouts. We estimate the time taken by the pegion to reach you, and then we wait for approximately the double time to get the message back. Once that has been done. If the reply does not come back in that time, we retransmit the message using another pigeon.

@EdGorcenski Noted. Perhaps someone searching on Google may find it useful.

All problems come from somewhere :)

7:09 PM

OMG! mersenne.org newscientist.com/article/…

does anybody here have an access to Springer articles?

Orange Harvester

yes. just saw it on reddit I think.

I'm not at the university at the moment

@Jonas ?

Sir.

@Ilya You can login with NetID.

@JonasTeuwen how?

7:14 PM

Just go to login on Springerlink.

Then select Delft.

@Jonas: thanks, man

8-)).

going to Prague tomorrow and saving some readings to iPad @Jonas

@Ilya Alrighty.

If you drop me an e-mail of things you need when there I'll send them in a quify.

Orange Harvester

@JonasTeuwen Yo man. Wassup?

7:19 PM

Hi. Sir.

Orange Harvester

Being English today eh?

Pretty okay math today.

I'm trying to learn to speak like Richard Dawkins.

Orange Harvester

Good. :-)

Hahaha.

Yes.

@JonasTeuwen thanks, already done with that

trying to see how co-algebras are related to what am I doing

Orange Harvester

7:23 PM

@JonasTeuwen I am trundling through Hoffman Kunze Linear Algebra. What should I move onto next?

How about something else?

@Ilya I know some co-algebra guys!

Shall I provide you with their contacts (PhD Cand.)

@Jonas: not so fast, please :)

Orange Harvester

@JonasTeuwen I mean what comes after Hoffman Kunze linear algebra? More advanced linear algebra?

Not so slow 8-(.

@OrangeHarvester Functional analysis.

Orange Harvester

@JonasTeuwen Ohh, must complete theory of measures before doing that I suppose.

7:27 PM

You have not done so yet?

Orange Harvester

Currently in process.

@OrangeHarvester that's easy to learn by yourself

@Ilya What should we do with this guy??

@Ilya What is not easy to learn by yourself?

@JonasTeuwen encourage :)

@JonasTeuwen nothing!

Try Aliprantis!

7:28 PM

(encouraging)

@Ilya where did you encounter co-algebras?

Orange Harvester

What is Aliprantis?

@Tobias in computer science they use co-algebras to talk about dynamical systems

@OrangeHarvester *who

A splendid dead mathematician.

Orange Harvester

Ohh?

7:29 PM

@Ilya ahh, cool, I didn't know they were used there

@Tobias neither me

@OrangeHarvester Riesz spaces in economy? Huh...

@Ilya I have good math.

@Ilya There is a conference in Leiden in July.

AND I INVITED YOU NOW OKAY?

(There will be cookies for your eating pleasure)

@JonasTeuwen tell me the details, please

I'll come

websites.math.leidenuniv.nl/positivity2013

@JonasTeuwen thx, in my evernote. Does it require registration/some fees?

7:39 PM

Registration I think so, but fees will be waived I am sure for you.

@JonasTeuwen what do you mean?

That you will not have to pay I think (I can ask the organizer, but I am pretty sure 8-)).

ok

I'm gathering some stuff now, so let's talk a bit later - and thanks a lot for the invaitation, @Jonas

Okay, thanks.

Good luck.

thanks!

7:47 PM

im confused is <a,b|a^i =1, b^j = 1> infinite?

the group

freely generated group

yes

how come relations don't decrease the dimension of a group?

as you can take an arbitrary long product alternating between a and b

groups don't have a dimension

I just find that weird!!

some relations will be sufficient to make the group finite, some not

8:02 PM

<a,b|a^i =1, b^j = 1, (ab)^k = 1> always finite?

I mean what about (abb)(abb)(abb)... maybe that goes on infinite..

yeah, unless the numbers i,j and k are "small" it will not be finite

that's weird!!

why?

for which (i,j,k) is it finite? I know $$ I: \langle s,t \mid s^2, t^3, (st)^5 \rangle\ $$ is finite because it's here en.wikipedia.org/wiki/Icosahedral_group

hmm, good question

so maybe if we try to see why $st^2$ has finite order in that, we can see what is going on

8:09 PM

(s t^2)(s t^2)(s t^2)(s t^2)(s t^2)(ts)(ts)(ts)(ts)(ts) = 1

I found on wikipedia $$D_{2n} = \langle r, f| r^n, f^2, (rf)^2 \rangle$$ en.wikipedia.org/wiki/Presentation_of_a_group

in that case $rrf$ has inverse $rf r^{n-1}$ and neither of obviously have finite order

in this case, the equation $(rf)^2 = 1$ together with $f^2 = 1$ tell you that you can switch the order of $f$ and $r$ as long as you invert $r$

I see!

that shows it's always a finite group

I asked it on the site

@Tobias, I think what you said may be the completely general proof for all cases

because they all have an element of order 2

then again that would need shown

In mathematics, a triangle group is a group that can be realized geometrically by sequences of reflections across the sides of a triangle. The triangle can be an ordinary Euclidean triangle, a triangle on the sphere, or a hyperbolic triangle. Each triangle group is the symmetry group of a tiling of the Euclidean plane, the sphere, or the Hyperbolic plane by congruent triangles, a fundamental domain for the action, called a Möbius triangle. Definition Let l, m, n be integers greater than or equal to 2. A triangle group Δ(l,m,n) is a group of motions of the Euclidean plane, the two-dimensio...

I wonder which book I can learn about triangle groups in

I just realized there is a BPP algorithm for zeta(5).....

8:58 PM

hi

9:16 PM

@anorton, what robots did you make?

9:41 PM

Are a direct product and a cartesian product of vector spaces the same? Thanks.

10:16 PM

@user58512 I built robots for FIRST LEGO League competitions (elementary through early high school), and then progressed to building robots for the VEX robotics competition. For a sense of scale: https://picasaweb.google.com/117193422174132582321/VEXMidAtlanticChampionship2012#5719912416331487170

I've also had experience with the BOE-bot platform, and am now starting to dabble in Arduino. I'm much more interested in the programming/control system for these robots than with the construction thereof.

@anorton do you wwrite an AI for it?

user19161

@anorton Wow, interesting.

@user58512 eh. I haven't done so yet--FIRST LEGO League was more geared towards pre-programmed behaviors, and VEX was more for remote control (so my programs were more to help the controller and the robot become "one")

I'd love to learn AI, though. I'm taking a mobile robots course on Coursera, but it doesn't look like it will cover AI. Eventually, I'll get into that--I'm certainly going to try to take an AI course when I get to a 4-year school.

I tried to write an AI once but it didnt work

They're pretty tough... :)

10:20 PM

do you like Coursera ?

I mean is it good to learn from

I really like it. Now, i will say that it requires a bit more self-drive than a normal class, because there's no penalty for doing poorly. That said--I took an Algorithms class and learned quite a bit. I'm going to take a lot more from there now.

wow they even have one on equine nutrition

haha

I never noticed that one... I tend to only look at the Sci/Tech/Math classes...

yeah

last time I looked there were only I think 6 courses

user19161

Hey @anorton so have you decided where to apply to? I forgot if you told me...

10:30 PM

@JasonBourne I don't think I have mentioned it... Anyway--I'd like to stay in Virginia for school, so I know I'm going to apply to UVa and Virginia Tech. Periodically, I change preference. :)

Outside of Virginia, I haven't thought much about that yet. I'm going to finish up another year at the community college after graduating highschool (to gather more transfer credit), so I have some time to think it over.

user19161

@anorton Oh OK. Have you thought about your career plans? Math or computers or what?

Funny you should mention that. Yeah.

I'm mainly thinking about EE or CS right now. CS would be fun (I love programming), but I think I'd learn more in EE. I like to learn, so that has its appeal.

Math would be fun, but I'd have to do a bit more research before going that route.

With respect to careers: I have an interest in number theory/cryptography. However, I'm not sure I want to make a career out of that, or if it should just be a hobby. I also like programming robots, so that could easily be what I do, too. Being a professor has its draw, but I'd have to go and "shadow" a rese…

There is never a shortage of things to do in Controls.

@anorton Didn't realize you were in Virginia. I happen to know quite a bit about UVa's programs.

@EdGorcenski Really? Cool!

I've visited their CS dept (I participated in a programming contest there last year), but haven't had time to get up there again to see their EE dept.

Their EE department is very strong. My company has many, many close ties with them.

10:46 PM

That's good to hear. I like UVa (right now, that's my preferred school), but was wondering about their reputation as an engineering school.

It has a good reputation. It all depends. Tech schools get a lot of distance out of having "tech" in their name, but it is far from the only option out there.

VT is also a great school, obviously.

True. I have several friends at VT or who recently graduated from there, and they encourage me to go there...

If you don't mind sharing, what company are you with? (Just wondering...)

In my immediate group of co-workers, there are 2 PhDs in EE from UVa, one MS in EE from UVA, and one MS in EE from VT. Company-wide, I think 9 or 10 PhDs are affiliated with UVa either as former students or faculty.

That's really cool. Does your company do work in controls? (just guessing from the number of EEs you've just listed...)

Orange Harvester

@EdGorcenski Controls is an expansive field though. :P I guess you have to be proficient in almost everything from mechanical design to sensor behaviour to actual electrical goods.

10:54 PM

@OrangeHarvester Not necessarily.

There are sub-specializations.

Actuator guys do actuators. Algorithms guys do algorithms

Sensor guys do sensors

Orange Harvester

Okay. What kind of controls does your company does?

Historically, we've done navigation and control; we've done work in adaptive control for air, sea, and land vehicles, intelligent control systems, control in the presence of damage, fault detection and isolation, that sort of thing.

Orange Harvester

Ahh. Cool.

Lots of model-based analysis, this is actually my specialty.

Holy nickers, Wordpress is the crappers on LaTeX.

11:02 PM

Where you have a mathematical model based on first principles, and you have data, and you extract patterns from data using that model. Also, you can develop algorithms around models that have certain nice properties, e.g. guaranteed convergence, etc.

Hey @OrangeHarvester

Orange Harvester

@math101 Hello. :-)

@JonasTeuwen Good you know that when you are starting out. :P Try Blogspot. It lets you include your custom MathJAX code.

Mmm. I'll do it this way! I just wrote a small converter.

Orange Harvester

@JonasTeuwen You wrote a converter?

It supports LaTeX, but not completely.

Orange Harvester

11:04 PM

@JonasTeuwen What does it convert to?

Wordpress HTML

Orange Harvester

Okies. Its basically replacing delimiters with LaTeX in front I guess?

11:24 PM

Yes.

user19161

@jonas Are you going to change your photo soon?

@JasonBourne Into what?

user19161

@JonasTeuwen Into another photo? I just love seeing new photos of yours bro.

The most recent one is on my blog...

user19161

@JonasTeuwen Don't worry bro, I am not into you.

11:31 PM

I am not worrying.

And that would be more like a compliment than a worry to me 8-).

user19161

@OrangeHarvester But not out of the box right?

Orange Harvester

@JasonBourne Out of the box is impossible I think, since everyone will have his/her own personal preferences.

And its not even difficult. Basically, you copy paste the following code:

Is Orange our new Fuhrer?

Orange Harvester

@JonasTeuwen YEAH.

I am hungry! I need to write a letter of recommendation! I don't know what to do now!

Orange Harvester

11:37 PM

People ask you to write a letter or reco?

@OrangeHarvester Where'd the code go?

(I was just about to copy it... :P)

Orange Harvester

pastebin.com/hzhMhLhE

@OrangeHarvester Yes, Sir.

Thx.

Orange Harvester

@JonasTeuwen Cool.

11:39 PM

I was surprised as well.

Orange Harvester

@anorton I have not done anything myself, I have just copy-pasted from the source page of the math.stackexchange.com site.

Ah. ok.

Orange Harvester

and made it more pretty. (I am a coding style freak)

A carefully weighted opinion. That will be the content of the letter.

Orange Harvester

@JonasTeuwen I hope you are not suspended from recommendation writing for that. :P

11:52 PM

:P.

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