Wolfram Mathematica

rcollyer

01:05

@OleksandrR You may have the same issue my wife had. The expected idioms on Windows are just enough different than the idioms on OSX that it takes a good bit of time to do things "correctly" on either platform when you have been working on the other. It took me a while to adapt to OSX from Windows, but it has taken my wife much longer than it took me.

Hi @belisarius

belisarius

@rcollyer Hi !!

rcollyer

What's up.

belisarius

just drinking my last beer :)

rcollyer

That's too bad. I wish I had beer.

belisarius

mmm

I may lend you some

rcollyer

01:07

It would take a few days to get here.

Although, there's the uubp protocol.

belisarius

I think there should be BTP (beer transport protocol) over IP somewhere in my machine

ha!

great minds ... etc

rcollyer

Do you know anything about Group Theory?

belisarius

I used to. But not anymore

rcollyer

That's to bad. I have a question up on Mathematics that I'd like answered.

belisarius

In fact I worked hard in solid state for a few years ... decades ago

rcollyer

01:12

It has been pissing me off for several weeks now. Once I discovered how wrong my current method was. :P

@belisarius Ultimately, I'd be using it in Tight Binding model, so it applies.

belisarius

I think it could take me a few days just to understand what you are asking

rcollyer

That's my problem. I've spent days on it.

belisarius

Ask a fellow mathematician

rcollyer

The first part is easy. The second part is where the difficulty lies.

belisarius

are you at uni?

rcollyer

01:14

@belisarius That's why I posted it on Mathematics.

@belisarius In a sense. I'm working remotely.

belisarius

oooo ... thats bad

rcollyer

But, I do hang out at my Alma Mater, so I could go ask someone from there ...

belisarius

you should drink your morning coffee with mathematicians. All difficult questions are due to be answered by 10:30

I did that for yars

never fails

rcollyer

Ah, true. But, I only know 1 prof there any more. I should go bug him; he was my differential equations prof, and he may just know, or who to ask.

belisarius

Thats the key point of being at uni. Know people. ask questions. get answers :)

a very underestimated gain

rcollyer

01:20

For this problem, the idea is I'd like to block diagonalize the Hamiltonian according to the irreducible representations of it symmetry. Of course, this has the added benefit that doing that gives me all of the eigenvalues, too. But, the real benefit is you can create a model with that symmetry using the projections onto those subspaces, i.e. a modified spectral theorem.

belisarius

What is a "block diagonal" matrix?

rcollyer

You turn a regular matrix into
$$\begin{pmatrix}\mathbf{A}&0 \\ 0&&\mathbf{B}\end{pmatrix}$$
where $\mathbf{A}$ and $\mathbf{B}$ are matrices themselves.

belisarius

mmm my chat cant read TeX

rcollyer

Damned alignment is off. :P

belisarius

What should I do with it?

rcollyer

01:27

Better yet, read this link.

belisarius

ok done

so you want something like a Jordan form?

where each eigenvalue subspace is closed under the linear transform?

rcollyer

It is like a Jordan form, yes. The idea is that the eigenspaces correspond to irreducible representations of the symmetry group, so using group theory you can, in principle, get part of the way to a full diagnolization by using that.

@belisarius bingo.

belisarius

bingo ... what?

rcollyer

Now, different instances of the same representation still interact with each other, but different representations don't.

@belisarius just saying your comment was correct.

belisarius

ahh ok

ok. now I understand the problem. Still miles away from having a useful idea about how to solve it

rcollyer

01:33

If you look at the character table for the symmetry of a triangle, you'll note that the functions $(x^2 - y^2, x y)$ and $(x z, y z)$ each correspond to the $E$ representation and form their own little supspaces.

The issue at hand is how do I automatically generate both of those representations and not their sum?

And, that's my question on Mathematics.

I can generate their sum easily. Teasing them apart requires trial and error. I'd prefer there was less of either.

belisarius

Sorry, my math is too rusty. It will take me a LOT of effort to remember all those once loved details

rcollyer

Don't worry about it. It is a lot more familiar to me, and I don't see it.

Unfortunately, the books I have seem to make it into a chicken and egg problem. :P

Hi @acl

belisarius

Did you ask a GR physicist? They tend to work a lot with irr. representantations

rcollyer

No, but none are local.

belisarius

damn

acl

01:40

@rcollyer hi

belisarius

hi acl!

rcollyer

@belisarius looking at mathoverflow, and it may have been a mistake. I don't understand 3/4's of what they're writing!

belisarius

@rcollyer ask the rubber duck

rcollyer

@belisarius I've tried. it told me to quack off!

belisarius

@rcollyer At MO they are real mathematicians. They are not people, you know.

rcollyer

01:45

@belisarius I'm aware. I was hoping, though.

belisarius

I wonder how you do science in remote. It usually takes a lot of interaction

like in this case

rcollyer

Computers are a wonderful thing.

belisarius

yes and no

a shared coffee table makes wonders

a mail to an unknown person doesnt

rcollyer

That's the truth. But, my son was allergic to where we were living, so we moved. And, I'll finish up remotely.

belisarius

a good reason indeed :)

rcollyer

01:50

it ins't easy, but it is doable. and, with any luck, I'll have this wrapped up soon.

belisarius

Is it your PhD?

rcollyer

yep.

belisarius

nice

bout what?

rcollyer

with luck, I'll be employable, also.

@belisarius condensed matter. specifically, dft (density functional theory). more specifically, dft on the B20 structure, and whatever else I can think of throwing at it.

belisarius

what uni?

Do you know him?

Leopoldo Máximo Falicov

Leopoldo Máximo Falicov (June 24, 1933 – January 24, 1995) was an Argentine theoretical physicist, specializing in the theory of condensed matter physics. Life Falicov was born in Buenos Aires with both parents of Eastern European Jewish origin. His father, Isaías Félix Falicov, was Argentinian and his mother, Dora Samoilovich, emigrated to Argentina as a child. Falicov attended the Colegio Nacional de Buenos Aires and then attended the School of Engineering and Natural Sciences at the University of Buenos Aires, where he got his undergraduate degree in chemistry in 1957. In 1958, he rec...

Not albert, of course

rcollyer

01:57

@belisarius I know the name, but it has been a while.

CHM

@rcollyer @RM I've seen your comments. I don't think of SO anymore for MMA questions.

rcollyer

@CHM but, enough people do. We need to bring them over to the true "church."

belisarius

@rcollyer I worked with him for a while

rcollyer

@belisarius cool!

belisarius

@rcollyer many years ago

rcollyer

01:58

@belisarius life moves on.

belisarius

Ok. off to bed

nights all!

rcollyer

night.

I'm going to follow his lead: Good Night or Day (as the case may be) to you all.

CHM

@rcollyer so the question will be moved?

Oh, night.

And it has already been moved.

You guys are too fast.

rcollyer

02:14

@CHM It was moved just a short time ago. Flagging can take a while, but my "helpful" flag count on SO is slowly getting to the point where it may come up in the queue a little faster.

@Verbeia the number of questions just exceeded our visits per day for the first time today!

Verbeia

@rcollyer I should patrol SO for migratable questions more, then. I already have a Marshal badge there.

rcollyer

Should help. Likely it will be most successful when we have a dupe here.

Verbeia

@rcollyer That's cool. Even more interesting to my mind: the entire time of the beta, visits has exceeded users.

We have 59 enthusiast badges now

anyway gotta go, was just dropping in. I promise to do the 90-day graphs tonight.

rcollyer

And 8 days away from a chunk of fanatic badges.

Night.

F'x

07:50

Verbeia

09:43

Updated site usage graphs on meta - now available

Heike

09:58

robjohn

11:56

@Heike cool! I remember missing that by a little.

@Heike I did have 12345 :-)

Sjoerd C. de Vries

12:13

@Fx That's 5 times The Number of the Beast

Well, almost

90 days in beta. What's next?

J. M.

Hi Sjoerd!

robjohn

@JM welcome :-)

J. M.

@robjohn (I said hello to you already, so... :P)

robjohn

@JM You said hello to some weirdo on Math :-p

J. M.

He was a most peculiar fellow, yes...

...after all he underwent binary fission during April first...

Verbeia

12:26

Hey @JM are you back now?

Szabolcs

@JM Hi JM

Verbeia

ooh bedtime - see you all in the morning

Szabolcs

Good night @Verbeia!

J. M.

@Verbeia Wish me luck; if all goes well with this week I should be a regular visitor again...

@Szabolcs Hi also!

Heike

@JM in that case, almost welcome back

robjohn

12:29

@Verbeia it is the morning. :-)

Heike

no it's not, it's the afternoon

robjohn

afternoon is certainly an odd time for the sun to be about to rise :-)

J. M.

@Heike Hi.

Heike

my visit to the apple store went smoothly. No it's fingers crossed that I doesn't take too long to get my macbook back.

robjohn

@Heike what was the problem?

Heike

12:32

@robjohn broken dvd player

robjohn

Ah, I have an old computer with a DVD player that doesn't even light its power light. I think it is too old to fix or find a replacement.

It makes it hard to reinstall software since most of the software was on CDs or DVDs.

Heike

@robjohn I have an external dvd player as well so I'm not too handicapped without it but it's covered by applecare (just) so I thought I might as well get it repaired for free.

J. M.

Always a good idea to exploit warranties... :)

robjohn

@Heike I have an external one as well, but the software to use it seems to be missing. Reinstalling it is problematic without the internal DVD player.

Heike

@robjohn apple distributes most of their software via their app store nowadays so I have hardly any software on dvds/cds

robjohn

12:40

@Heike This is a computer from the 2000-2001 era most software for it is on DVDs or CDs

Heike

@JM I have 3 days left on my applecare so it was now or never.

@robjohn I'm surprised it's still working.

robjohn

@Heike I have several Apple computers from 1997 or before that are still running.

Heike

Then again, my brother in law was still using his Power Mac G4 until last year or so

robjohn

I am planning on upgrading sometime soon, so that my server can run more current software.

Heike

@robjohn That was his main reason for upgrading as well.

I guess it's getting harder and harder to find software that still runs on a power pc.

robjohn

12:46

indeed

I remember when they switched from Motorola chips to PowerPC. I was wary then.

I was also working at Apple.

Heike

even my other macbook which is 6 years old is too old to run lion.

robjohn

11 years makes it even harder :-)

Heike

@robjohn Can't have been a bad move. They became quite big with their PPCs.

@robjohn I think I still have a Pentium II with FreeBSD installed gathering dust somewhere.

robjohn

@Heike It was the right thing to do. I was just disappointed that all of my investment into 68K programming was going for naught.

I wrote a 68K assembler and used it for quite a bit of programming.

Heike

@robjohn I never knew there were 68k processors.

robjohn

12:55

@Heike Those were the Motorola chips that were used before the PPC.

J. M.

@Heike High-end TI calculators use processors like those. Motorola's, IIRC.

robjohn

The oldest Macs used 68000, then 68020 and 68030. I think the final revision was the 68040.

Szabolcs

13:40

@JM Do you perhaps have any suggestions what's the right keyword to search for to learn about adaptive sampling You are quite familiat with numerical methods, I think. Yes, that question is not very well written ...

F'x

@SjoerdCdeVries even weirder, we have 25 questions with score 25, and 25 answers with score 25 (here)

3

Eli Lansey

@Szabolcs I would look to see what people do for finite element adaptive meshing

J. M.

@Szabolcs Unfortunately, I'm far away from my personal library, so I can't give concrete references. The usual terms are "recursive subdivision", and yes, "adaptive sampling" (though it seems that search is "noisy", as you said).

Eli Lansey

since they also want to minimize number of mesh cells, and have large mesh in "boring" regions, finer mesh in "interesting" ones

interesting usually defined by some metric changing a lot with increasing mesh

Szabolcs

@EliLansey I don't know much about FEM, but don't they determine the resolution based on the shape of the boundary?

Eli Lansey

13:44

@Szabolcs somewhat. but imagine a plain rectangular space with changing material properties

if the function varies greatly in a particular region, adaptive meshing should give a finer mesh

if it varies slowly, then it's a finer mesh

Szabolcs

yep

that's right

Eli Lansey

now, this might end up being more computationally intensive

but, if you can come up with some computable metric that you want to get below some threshold, or minimal variaiton etc

you can probably use FE adaptive meshing approaches to sample your space more effeciently

this is sort of useless, but look at slide 33

ansoft.com/workshops/hfssinfoday/…

this may be more helpful

cs.rpi.edu/~flaherje/pdf/fea8.pdf

Szabolcs

Alright, I'll try to search about FE

Yes, the second one has some interesting things

Eli Lansey

14:03

@Szabolcs this one actually deals with discontinuities

R.M

16:21

Eli Lansey

@RM Congrats!

J. M.

Shouldn't that be dancing?

rcollyer

17:01

@RM Congrats!

That makes for some scary statistics on the site, though. 4 of 5 10k users are non-mods. The next highest rep user is a mod with 4300 points less! Below that, there seems to be an exponential tail, maybe a bit fatter than exponential, but definitely a quick fall-off.

Heike

18:12

@RM Congrats, @RM

@rcollyer Is that a good or a bad sign?

R.M

Thanks, @Eli @rco @hei :)

robjohn

18:29

@RM Congratulations!

rcollyer

@Heike I don't know. It can be interpreted as a select group of individuals outpacing everyone else, and whether or not this is considered "unhealthy" is open for debate. At this point, it is just an observation. Of course, the 12k folks have slowed down their contributions considerably ... so, likely we won't be seeing much from @RM very soon now.

@robjohn Hey do you know anything about group representations? I've been beating my head for a while now about this, and would like to see if it can be answered.

Deven Ware

18:44

Hey guys I have a quick question about a differential equation in mathematica that I was hoping someone could help me with here, as its not really substantive enough to pose as a question on the main site

Essentially I'm just trying to get mathematica to solve the differential equation for a forced harmonic oscillator,

It seemed to me, like "sol1 = DSolve[{x1''[t] + 2 b x1'[t] + w0^2 x1[t] == a Sin[w t],
x1[0] == 0, x1'[0] == 0}, x1[t], t];"

should solve it, but then when I just define a function to be the solution to this and evaluate its derivative at 0 it doesn't give me 0, which was my initial condition

R.M

@DevenWare f[t_] = x1[t] /. First@sol1;

f'[0]//FullSimplify gives you 0

Sjoerd C. de Vries

19:00

@r.m Congratulations!

Deven Ware

See I knew it wouldn't be substantive enough for a question on the site ;)

R.M

@SjoerdCdeVries Thanks (and @robjohn) :)

8 more days and then I can take a break :)

robjohn

@RM why in 8 days?

Sjoerd C. de Vries

@RM Same here. Just in time for my vacation. Getting Internet abroad just to get fanatic is a bit... fanatic. Well, I did it before.

R.M

@robjohn 100 days — fanatic badges :)

robjohn

19:23

@RM Aha :-) I just passed 200 days; unfortunately, that means nothing.

When I was at 62 days or so, I worked on a problem for a couple of days. I didn't realize that simply working on a problem, which seemed online, did not constitute as being connected. I was a bit upset that I had to start over.

I am at 202 days consecutive :-)

P. Fonseca

22:43

For those interested in a Mathematica speed question: lwsffhs.wordpress.com/2012/04/17/…

Verbeia

@rcollyer I've been meaning to fit some sort of distribution to the reputation distribution I get from the API, but I haven't had time to date.

@RM Congratulations, by the way.

@rcollyer I am not too worried about having a small number of high-rep users. We already have 23 users with 3000+ rep, and looking at rep in the past month, there are many new contributors.

See also new users by reputation for people who joined only in the last 45 days.

The top four users are very active, but they also had a head-start that will soon diminish.

Rojo

And today you have one more user with 4000+ :)

Thank you, thank you

Congrats @RM

10k looks even better than 12.9k, let alone with a gold badge on the right

R.M

23:13

@Rojo I cast the vote that pushed you over :)

Rojo

@RM, thanks, hehe.

I haven't had lots of time lately so my rep was stuck

but now what you've pushed it over the 4k barrier, it's a whole different story

R.M

@Rojo time to unleash the mma beast inside you? ;)

Rojo

I'm browsing to see what I've been missing

Nice to see that most of the questions have good answers already

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