well, vector field on the double cover might not give you a vector field on the base but you still get chi = 0

I know

That's why I said direction field

oh. what's a direction field?

I guess it's a Grassmannian line field, in the end

Well, not exactly

there's still a "length" associated

It's basically the doublet $(-V, V)$

With $V$ some vector

ah, ah, gotcha. choice of the unit sphere bundle i guess

I guess it's the sphere bundle with antipodal identification

which I guess is what the Grassmanian is

The projective space

right

Although...

No I think I'm good

I still need to show that if M is orientable the existence of a direction field is equivalent to a vector field, I guess

Well I suppose any vector field is a direction field

there

bam

I need to write this down clearly once I'm not in class :p

Learning about bloody CSS

@Slereah I don't know what a Hadamard state is

@ACuriousMind Hadamard state is a state that has not too divergent properties

Such that $\langle A \rangle \approx f(x) r^{-1} + g(x) \ln(r) + h(x)$

So that all the divergent parts are encoded by $f$ and $g$

@nbro: GR gives us an expression for the four-acceleration, and this includes both terms due to non-inertial motion and due to spacetime curvature: $$ a^{\mu}= \frac{du^{\mu}}{d\tau}+\Gamma^{\mu}_{\alpha \beta}u^{\alpha}u^{\beta} $$

Well, more like $\langle A(x,y) \rangle \approx f(x,y) r^{-1} + g(x,y) \ln(r) + h(x,y)$

with $r$ the distance $xy$

That way the only divergences are from the short scale

@nbro: the proper acceleration is the norm of the four-acceleration of an observer in that observer's rest frame. So the proper acceleration can be due to non-inertial motion, spacetime curvature or both.

The QFT in GR people go on and on about Hadamard state, but I can't think about what a non-Hadamard state would be

@Slereah Right, it's not too hard.

Alright

So I think I can (kinda) prove Steenrod's theorem

Wew

that took a while

It's weird that nobody ever calls it "steenrod's theorem"

since it is literally only proven there

@0celouvskyopoulo7: wow, it was a bug in Mathematica!

That's really unlucky - a bug in Mathematica surfaces just as you install it for the first time.

@JohnRennie He caught my disease

that's not unlucky

of breaking every software I touch

Mathematica is full of bugs

@BernardoMeurer :-)

@Slereah to be fair there aren't may bugs that kill it dead. It's the timing of doing a fresh new installation just in time for a bug to kill your fresh new installation, and of course it's natural to think that's your fault.

@BernardoMeurer no-one is that old

@JohnRennie lol

@JohnRennie not even you? :p

JD was Einstein's best friend

What's important is to keep your mental age at the right level. I find a mental age of 13 suits me best.

Always a popular thing to say on the internet but not so relatable when you meet actual children

Asking the real questions.

Lol

Humans

I do wonder if OP actually expected to find an in-universe explanation for a inconsistency in a Pokedex entry.

@JohnRennie my question was tweeted by someone!

@0celouvskyopoulo7 ...the SE twitter bot? :P

What's that

@0celouvskyopoulo7 I have to admit it would never have occurred to me that it was a bug in Mathematica.

Each SE site has a bot that tweets popular questions. Most sites hate it. :P

You just thought my code was shit. Admit it.

If you know that the question was tweeted because it's in the edit history, then that's the bot

@0celouvskyopoulo7 well no, because I ran the code and it worked.

I would have guessed it was some weird configuration problem.

@JohnRennie I just saw a function in C that returns other functions

We usually downvote our twitter ad into oblivion

And also C templating

@ACuriousMind That link is wrong

At least it wasn't a rickroll :P

Fixed

@BernardoMeurer that's pretty standard. In C (and C++) you can have pointers to functions and pass them as arguments or return values. The typing is a bit of a nightmare though.

@JohnRennie Function pointers I knew about, but templating is new for me

(in C)

eh it's alright

@JohnRennie Now here's a shitty problem

@BernardoMeurer I didn't know you could do templating in C ...

If I have a C string like this 7/28/2011 17:40:00

@BernardoMeurer constipation?:-)

I don't think you can

How can I turn that into a "native" time variable

blog.pkh.me/p/20-templating-in-c.html

the closest equivalent is to just use "anonymous" pointers in your functions

You can, it's just hackish

@BernardoMeurer time conversion is a balls ache in all environments. I'm fairly sure there is a standard C library with time conversion functions.

Isn't it just time.h

@Slereah Yeah, but time.h has a lot of things

and I'm not sure 7/28/2011 17:40:00 is standard

having a lot of things is the opposite of a problem

@BernardoMeurer If you know the format is always going to be that just use sscanf.

An embarrasment of riches

look at this stuff, isn't it neat

wouldn't you say my collection's complete

linux.die.net/man/3/strptime

Aha!

I've got gadgets and gizmos a-plenty

I've got whozits and whatsits galore

You want thingamabobs? I've got 20!

Sounds like a Dr. Seuss story :-)

It's actually a song from the little mermaid

Just like the little mermaid, @BernardoMeurer has all he could need yet wants more

@JohnRennie welp, it broke itself again

garbage software

the joys of Mathematica

@Slereah Oh stop it you, I'm blushing

You could try using the firewall to block access to the Mathematica patch server ...

At least until they've fixed the problem.

@JohnRennie There's a solution on my quesion, but fuck it

I just won't use mathematica

You may remember that scene from the Little Mermaid as the one where Ariel doesn't know what a fork is

Despite the fact that her father literally holds a giant fork at all times

@0celouvskyopoulo7 I guess there are alternatives. It's just that for things like Christoffel symbols MM is fantastically useful. Calculating them manually is a massive pain.

Use Matlab instead

or get undergrads to calculate things for you

@JohnRennie how do I launch a standalone kernel is mathematica

as people did in the olden days

@Slereah I am an undergrad

Get weaker undergrads

@JohnRennie It says to put it in the command line but OSX doesn't have one?

Isn't OSX based on linux these days

How can it not have a command line

@BernardoMeurer halp

how do I go to /Applications/Mathematica.app/Contents/MacOS/WolframKernel in the terminal

cd /Applications/Mathematica.app/Contents/MacOS/WolframKernel

ah, cd

how do you know that?

"not a directory"

well fuck

I have rode the computer horse before

Try just cd /Applications/Mathematica.app/Contents/MacOS/ maybe

@0celouvskyopoulo7 Go doing cd one directory at a time

then type ls to display all files

and doing ls in between

to see what you got there

ok, how do I go one level down

also throw your mac in the trash and get a PC

Level down is cd ..

do I have to type the full path every time?

no

just the name of a folder in the current folder

cd folder?

that doesn't work

well that doesn't work

doesn't it

odd

well maybe it does, but I can't use tab to autocomplete

I don't know how Mac OS works but since it's a layer on top of linux these days

it should work

Ok, so WolframKernel is in that path, but it's a file, not a directory.

How do I run it?

file/prgram whatever

just type the name of the file directly

type "WolframKernel" in the console

Or ./WolframKernel, eventually

"Command not found"

is ./ working?

aha!

Ok, I'm in

Time to run this code...

@0celouvskyopoulo7 is in the matrix

I can't tell what I just did

it works!

hurray!

Ok, I want to make this curve smoothing prettier now

it doesn't work...

I hate this

ah, forgot braces

so did u get it to work

oooooohhhh, boy

Questions by number of deleted answers

is this ever a goldmine of terrible terrible posts

(with apologies to <10k'ers)

> By detecting a mathematical inconsistency in our world, for example that 2+22+2 can be equal both to 44 as well as 55; such an observation would make the existing

> alternatives of string theory conceivable alternatives because all of them are mathematically inconsistent as theories of gravity; clearly, nothing of the sort will occur; also, one could find out a previously unknown mathematical inconsistency of string theory - even this seems extremely unlikely after the neverending successful tests

(found from one of the questions in that link)

and I don't know what 2+2=5 physically will look like

I mean, beat that for sheer off-topic-ness

What's with all that nuclear power stuff in those posts, I amstarting to wonder whether they are the same person

@Secret those specific answers, or the query in general?

Those are answers to this one, so a nuclear theme is understandable

Ah I see

@Secret you piqued my curiosity about that 'offensive' post, while at that

and wow is it brain-hurtingly bad

(well actually, my rep is not enough to see it, I just kinda assume all 4 of them judging form that screenshot had something to do with pushing a nuclear agenda of sort, thus my prejudgement is likely wrong)

@Secret no, it's just that past some threshold of visibility, posts tend to accumulate a fair amount of cruft

particularly over the years

> Is there any reason why nuclear power plants cannot be constructed five miles off the shoreline. A plant floating five miles out would be safe from both tsunamis and earthquakes. I can't think of any technological problem for this.

I gotta ask you

seriously?

uh, that will be even worse if a leak occurs, as the mobile nature means it spreads out more, and you wil be directly f*** up the ocean ecosystem

not to mention you need thick cables to relay that energy back to the shore

"There exists a refinement of the notion of wave front set, based on the concept of Sobolev spaces."

Noooooooooooooooo

It's the microlocal shit

@EmilioPisanty Oh, I remember that one. That's part of the reason I protect questions after the second or third stinker of an answer.

What operator is the vacuum cyclic to in free QFT, is it the ladder operators

The states $(n,n,n,...)$ are not reachable by those operators, how does one prove that the vacuum + the ladder operators are dense in the Fock space

Help there are too many algebras

The $*$-algebra, the $C^*$-algebra, the von Neumann algebra, the unbounded algebra, the $W$-algebra

Al-Khwarizmi what have you done

@EmilioPisanty my MMA question blew up!

Oh no