Wolfram Mathematica

Gustavo Bandeira

12:10 AM

When I click the blue little In[x]: and Out[x]: A rectangle appears in front of it, why does this happen?

Verde

12:25 AM

@GustavoBandeira If you hear a "tic-tac" sound, kill the kernel immediately.

Gustavo Bandeira

I guess it's not so urgent

@Verde Look

Verde

Gustavo Bandeira

???

Verde

the tic-tac sound I warned you about

J. M.

@GustavoBandeira Sometimes, you just have to try things out. Don't be so scared to make mistakes... it's not like Mathematica will explode if you screw something up.

@GustavoBandeira Always a good idea to look at the docs before fooling around.

@R.M Probably a professor's sick joke... :)

R.M

12:55 AM

@J.M. even if it does, it is only a Quit or in the case of FE, a reopen away from normal

Except when you have a Write without closing streams... that Fortran in mma question yesterday nuked my 16 gigs of ram and all the space on my drive before the OS shut it down

J. M.

@R.M Ah, that... I'm a bit surprised textbook code was not sufficiently debugged...

R.M

@J.M. Yeah, it wasn't sanitized. It worked if you gave it the input it liked, but since it was full of Writes in While loops, it was a recipe for disaster

J. M.

In any event: one should not have to simultaneously teach numerical methods and programming in one class. ;) People won't know if the mistakes they made are due to the programming, the numerics, or both...

Verde

@J.M. most probably both. Murphy rules

You know. The light at the end of the tunnel is the headlamp of an oncoming TGV

J. M.

Two Russian hunters meet. "I shot a gigantic bear yesterday", says Ivan. "Look at the hide!"
"How do you find such huge bears?", Sergei asks.
"Easy," says Ivan. "You stand in front of a cave and whistle. When the bear comes out, you shoot."
Weeks later the two meet again. Sergei is covered in bandages. "Didn't you follow my advice?", Ivan asks.
"Sure I did. I stood, in front of a cave and whistled," Sergei replies.
"And what came out?"
"To me, it looked like the Trans-Siberian Express."

Verde

1:22 AM

ha!

nice

J. M.

One good joke deserves another.

Verde

let me write another bear hunting one. In English it will take me a few minutes

R.M

We should do 'yo momma' jokes using mma functions...

4

Yo momma's so fat that Flatten[yomomma] crashed the kernel

Verde

1:39 AM

A CIO invites his star programmer to his cabin in the mountains, in a hunting experience.
They arrive, and after unpacking, the boss says: “Start chopping the onions, I’ll go for the first beast”. The programmer lights the fire, and starts preparing the vegetables. An hour passed.
Then, suddenly, a loud “HEEELLLP” outside made him wonder if bears could speak in English. He approached the window and saw his boss, running towards the house, and a BIG black bear viciously grinning and clapping its jaws behind him.

The boss came crying, jumping and panting. “Open the door … Open the door … HELP”.
Our poor programmer, his pants wet, opened the door and prepared to close it as soon as the old man came in. But the boss, at the last moment, showing an incredible agility jumped aside and let the bear enter the cabin. Then he shut the door, and laughing aloud cried “Start boiling it, I’ll go for the second one”. And disappeared into the beautiful snowed mountain.

R.M

lol

Verde

sorry for my English, anyway

a joke is a difficult piece of text ;_)

R.M

The English is pretty fine :) I could understand it very well

Verde

@R.M You are too kind :)

J. M.

2:00 AM

@Verde I see nothing to apologize for in what you wrote. :)

Verde

@J.M. spoiling a joke for lacking the precise wording is unforgettable

I have a pretty good command of Spanish, so I hate my English :)

J. M.

Ah, well... I can't help you there. :)

Since @R.M asked for it: "Yo momma's so fat, if she were a simple graph, Girth[yomomma] would return Infinity."

"Yo momma's so fat, FindShortestPath[] would choke on her."

Verde

"Yo momma's so fat, that Thinning[Yomomma] returned No way"

"Yo momma's so fat, that Dilation[Yomomma] overflowed HAL 9000`

Gustavo Bandeira

@Verde lol

R.M

2:17 AM

"Yo momma's so useless, that ValueQ[yomomma] returned False"

Verde

HA!

MemberQ[Yomomma] allows some nasty jokes

R.M

"Yo momma's so huge that PossibleZeroQ[1/yomomma] returned Duh!"

Verde

2:41 AM

"Yo momma arse's so huge that BottomHatTransform[yomomma] returned No more hats available"

J. M.

5:04 AM

@r.m What do you need from me?

(though I'll be leaving in a few minutes)

R.M

@J.M. This is back to an integral I was puzzling with a few weeks ago (picking it up now)... belisarius helped locate a credible source which returns mma's solution, but I'm looking for some insight to getting to the solution (involves elliptic functions)

In its simplest form, it is Integrate[Exp[-x] BesselJ[0, x]^2, {x, 0, \[Infinity]}

J. M.

@R.M Looks like a Laplace transform...

You've tried looking at a table of Laplace transforms?

R.M

Well, I have the solution, so that's not the issue... what integration strategy do you use to solve this?

J. M.

The mechanical method would be expanding the Bessel function and integrating termwise; since the series is absolutely convergent, swapping integration and summation won't be a problem.

(BTW, the result is an elliptic integral, not an elliptic function; different things, much like sine and arcsine are different functions.)

R.M

5:20 AM

Sorry, meant elliptic integral

Getting the arguments between different references sorted out is a pain in itself

well, not really a pain, but would've been nicer to not have to do it

J. M.

LaplaceTransform[BesselJ[0, s]^2, s, t] /. t -> 1 does look simple, yes. There might be a more clever way than the mechanical method, but it's not coming to me at the moment.

R.M

No worries, I'll grind the papers some more down this path for a while... thanks :)

J. M.

@R.M Just in case: have you seen this paper?

(Carlson takes a hypergeometric route before converting to elliptic integrals in that paper.)

Well, gotta go. I'll ping you if I see other pertinent papers.

R.M

@J.M. no, I hadn't and after looking through it, it looks like exactly what I need! thanks :)

Verde

5:46 AM

@R.M Just curious ... why do you need another refrence?

R.M

@Verde I didn't need an additional reference for the solution, yours was good enough. I was trying to understand how one would end up with an elliptic integral, so something with a proof was what I was looking for

Verde

@R.M Ahh Ok!

Gustavo Bandeira

I opened the link and saw:

SIAM J. Math. Anal., 11(3), 428–435. (8 pages)

R.M

It's not a porn site; sorry to disappoint you :P

Verde

8 pages for an integral

Gustavo Bandeira

5:49 AM

My eyes were focused on two screens. I was like: Anal what?!

Verde

@GustavoBandeira siam.org/journals check that

sooner or later you'll pray to them

Gustavo Bandeira

@Verde Why?

Verde

@GustavoBandeira Because they publish all important Maths results

Gustavo Bandeira

I just wish I could understand it.

Verde

Take your time

30 years

R.M

5:53 AM

Them and AMS

Verde

:D

@R.M yep

@R.M I love this title "Journal on Uncertainty Quantification". Sounds like ...OMG

R.M

"I now know how much I don't know and hence it should be published"

Verde

yep

very funny

@R.M Imagine the first sentence on a paper there "The authors believe their results are certain with a probability of 87.5% under NTP, Tuesdays and Fridays (mostly evening)"

R.M

Fridays*

(* beer has been observed to have an effect on the uncertainty results)

suddenly, yo momma's not so fat anymore

Verde

@R.M I am sure the actual Journal is not so fum to read. A pity.

gotta go

Bye!!

R.M

6:09 AM

bye

Rolf Mertig

8:21 AM

Not sure why. But in M8 there are no errors and I get {4.132650114325803*^-21, 4.132650175888174*^-21, 6.051831136439662*^-20,
6.051831071742674*^-20}

Oleksandr R.

@Verde uh... that's a different message than I got in 7.0.1. What version were you using? This looks like a bug in the properties calculations for v7.

@RolfMertig thanks. Yeah, I don't see any messages in v8 either. I assume the bug was fixed.

This is the reason I was asking (he's using 7).

@Verde thanks for the analysis! But the data were contrived, so I knew the parameters already. :)

Oleksandr R.

8:44 AM

@Verde I got Function::slotn which is hard to understand why it happens without context (i.e. could have been my fault for confusing NonlinearModelFit). With Internal`LocalizedBlock::novar, you only get it if Internal`LocalizedBlock was called incorrectly, so the situation is a lot clearer--looks like someone else's fault. :) The fit is still done correctly and without messages; it's just (some of) the properties calculations that give trouble.

Rolf Mertig

10:03 AM

Does anybody know if there is a deeper reason that Simplify[1. x, Element[x,Reals]] or Refine[1. x, Element[x, Reals]] does not simplify to x ?

Eli Lansey

1:26 PM

Not sure if this is question-worthy, but I've been banging my head against the wall for a while about this

If I have a 3d dataset (i.e. {x,y,z,val}), where val is itself a list {v1,v2,v3} (i.e. a vector field)

and i interpolate it

how can I compute vector derivatives (grad, div, curl) on the interpolated function?

Eli Lansey

1:53 PM

nm, asked the question anyway

0

Q: Derivatives (gradient, curl) of interpolated 3D data

How can I calculate vector derivatives (divergence, gradient, curl) of interpolated data? For sample data, you can use: f[x_, y_, z_] := Exp[I z] {1, 0, 0} testdata=Flatten[Table[N@{x,y,z,f[x,y,z]},{x,0,4 Pi,Pi/10},{y,0,4 Pi,Pi/10},{z,0,4 Pi,Pi/10}],2]; intf = Interpolation[testdata] I know th...

interpolation

R.M

@RolfMertig probably because the precision of x is unknown, whereas that of 1. is...

J. M.

2:22 PM

@EliLansey At least div and grad are easy; have you seen the syntax D[{f[x, y, z], g[x, y, z], h[x, y, z]}, {{x, y, z}}] before?

Eli Lansey

@J.M. no, I have not

reading help file now

not sure how i missed that before

but that still requires seperately interpolating the {x,y,z} components of the vector field

to get the f, g and h

J. M.

You can then use Tr[] for div, and Diagonal[] for grad.

Eli Lansey

@J.M. that's quite nice, actually

J. M.

@EliLansey Not really, you could use something like D[intf[x, y, z], {{x, y, z}}] where intf[] is the InterpolatingFunction[] you have.

Eli Lansey

@J.M. hmm, nice.

I had tried other things like

div = {D[#[[1]], x], D[#[[2]], y], D[#[[3]], z]} &

and then

div@intf[x, y, z]

but that always gave {1, 1, 1}

wait, D[intf[x, y, z], {{x, y, z}}] gives a 3 entry list, not a 3x3 list

oh, div there should read grad

J. M.

2:36 PM

@EliLansey Using the intf[] in your question, D[intf[x, y, z], {{x, y, z}}] gave me the Jacobian after substituting in numerical values of x,y,z...

Eli Lansey

@J.M. comparing D[f[x, y, z], {{x, y, z}}] /. Thread[{x, y, z} -> {.4, .2, .3}] and D[intf[x, y, z], {{x, y, z}}] /. Thread[{x, y, z} -> {.4, .2, .3}] shows they're transposes of each other

J. M.

Ah, right. That would be important for deriving the curl; for div and grad, since the diagonal elements are untouched by transposition...

Oleksandr R.

@J.M. we seem to have converged on a rather similar color scheme...

J. M.

@OleksandrR. Apparently; I am currently going through a purple phase with my artwork...

Oleksandr R.

@J.M. I think purple/red/blue are encouraged by the site theme. I changed mine from green because it clashed horribly with Jin's design.

J. M.

2:47 PM

@EliLansey In any event: curl would be a matter of choosing the right entries in the skew part of the Jacobian (that is, jac - Transpose[jac]).

Eli Lansey

@J.M. yah. this is far better than my crazy way of doing it.

J. M.

@OleksandrR. Green does stick out a bit with our current color scheme, no?

Eli Lansey

@J.M. i think this is a pretty good approach for an answer. are you going to write one up?

J. M.

@EliLansey You could edit your answer for these... :)

Eli Lansey

@J.M. i'd rather give credit where credit's due...

but if you'd prefer i can edit my answer

J. M.

3:00 PM

Yeah, just edit it... it's not like I gave a complete answer; just ideas. :)

Stack Exchange

3:24 PM

Blog - Stack Exchange: Join us for our opening reception of Stack Exchange: Denver!

Bethany Marzewski on September 06, 2012

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Wolfram Blog Feed

6:03 PM

Wolfram Blog: Computer-Based Math Education Summit 2012

posted on September 06, 2012 by Michael Belcher

In November 2011 we held the first Computer-Based Math Education Summit in London. Over two days we brought an unprecedented cross-section of people with a stake in STEM education to address the question “In an era of ubiquitous computing, how should we rebuild math education from the ground up, to keep pace with and drive [...]

Verde

7:06 PM

@EliLansey youu here?

Eli Lansey

@Verde hi

Verde

hi

try this pleae

Cross[{d[1], d[2], d[3]}, {2, 3, 4}] /. Times[a_, b_] -> b[[a]]

post the result :)

Eli Lansey

wait, the first solution seems to be working now, though

after a kernel restart

Verde

hehe

please try this

Cross[{d[1], d[2], d[3]}, {2, 3, 4}] /. Times[k_, b_] -> b[[k]]

Eli Lansey

wow, really nice

@Verde same error

Verde

7:10 PM

post the last line

copy it here

no images

Eli Lansey

{(-3)[[d[3]]] + 4[[d[2]]], (-4)[[d[1]]] + 2[[d[3]]], (-2)[[d[2]]] +
3[[d[1]]]}

Verde

it is different

Eli Lansey

that was second one

first:

{d[2][[4]] + d[3][[-3]], d[1][[-4]] + d[3][[2]],
d[1][[3]] + d[2][[-2]]}

Verde

and I only changed the letter a in the pattern for a k

damn

gotta go,I'll be back in 20 min

Guillochon

I'm getting confused by the Mathematica debugger...I have "break at messages" selected, but it's not breaking on a message

Nevermind, I checked the box in the middle of an evaluation

It works now

Verde

9:42 PM

There was a question about memoizing partial arguments ... Anyone?

R.M

9:53 PM

@Verde I think it was JM's Charlier polynomials question...

Here:

24

Q: How can I implement dynamic programming for a function with more than one argument?

Dynamic programming is a technique for avoiding the repeated computation of the same values in a recursive program. Each value computed is immediately stored. If the value is needed again, it is not computed but simply looked up in the table. (1) I use orthogonal polynomials a fair bit in my...

programming recursion memoization

Verde

@R.M Yep Mike Bantegui's answer.

R.M

Is this what you were thinking of?

Verde

Yes! Thanks

R.M

no problem :)

Verde

@R.M Want a riddle?

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