The h Bar

Although there's this, lol

this is hilarious though

@EmilioPisanty lol

@EmilioPisanty Idk, I read the font page of reddit every few hours, and didn't see it.

2:08 PM

@0celouvskyopoulo7 why would news about ransomware be on a typesetting section?

=P

wot

I'm guessing /r/fonts is a thing and that's what you're talking about

Ok, there's a megathread on r/technology

@EmilioPisanty Oh, lol

I meant front page :P

bizarre thing to check every few hours but everyone has their pet peeves

@0celouvskyopoulo7 anyways - I'm not really sure what paclets are, to be honest with you

ACuriousMind

@EmilioPisanty You're not exactly digging for gold there :P

2:10 PM

@ACuriousMind ah, it's an alternative definition of gold

Cocaine?

I mean, look at this marvellous rant

user228700

The Abandoned Ferris Wheel Spins Anyway

> Are not reproducible results fundamental to science? [closed]

user228700

(Jesus Christ is that creepy or what?)

2:11 PM

Guys! I finally have a feel for what curl means. Can I please post my geometrical interpretation here? (I spent two days breaking my head over this)

(i.e., would anyone be interested:P or kind enough to read it)

@ShaVuklia curl

aha:P

I'm gonna drop it here. sorry, no f's given:

It's easy to show that the $k$ component of the curl $\nabla\times F$ at a given point $(X,Y)$ equals $\int \vec F\cdot d\vec r$ divided by $dx\,dy$, where our curve is an infinitesimal rectangle parallel to the $x$-$y$ plane, starting from the point $(X,Y)$, and then (in counter-clockwise direction) $(X+dX,Y)$, $(X+dX)$, $(Y+dY)$, $(X,Y+dY)$.
So for the $k$ component of the total curl of a surface $A$, we can approximate this by considering the curl of $N$ patches of the surface - which boils down to integrating when we consider the limit $N\to\infty$ and $dxdy\to 0$, so we have $\int_A(\n…

just look at it:P it's a perfect explanation

@Kaumudi.H that's a bummer, if I'd seen that video last year I'd totally have gone

that's creepy as hell, yes

@EmilioPisanty cogold

gold*

nope, just alternative gold

it perfectly fits the new definition of 'alternative'

user228700

2:15 PM

@EmilioPisanty Ah, were u in Germany last year? :-) I too have a strange desire to visit it and freak the heck out.

@Kaumudi.H I lived in Berlin up until January

user228700

Ah, I see.

But seriously, though, there's plenty of more interesting stuff to see & do in Berlin

though if abandoned places are your thing, Berlin is probably the best place for it

cf. abandonedberlin.com

user228700

@EmilioPisanty Oh wow.

user228700

Haha, no, abandoned places aren't really my thing although I do have a little fascination with them. It's a moot point anyway; I don't know when I'll get to visit Germany, if ever. Thanks for the link though!

2:18 PM

@Kaumudi.H yeah, it's a good read

@ShaVuklia yeah, that's pretty much it

user228700

Omg some guy had the guts to ride the thing

see Purcell's EM textbook for a more formalized description of that understanding

@Emilio it took me two days though to finally see it:P like, I understood it for the $k$ component, but I couldn't see exactly what happened when we got a wobbly surface, without getting too vague when describing the loops

and I'll see what Purcell says on curl

@ShaVuklia and if you haven't got Div Grad Curl And All That, now might be a good time

user228700

2:24 PM

@Kaumudi.H not surprised

it's in a pretty urban area

at least by Berlin standards

user228700

Ohh, I see...

@ACuriousMind or, say, this gem

@Emilio nice. I think I might actually buy that book for the summer. Right now, I have to focus on my finals, but I really love the geometry behind calculus. So thanks for the recommendation

> Taking rep points away from me is not an insult , it makes me continue my unauthadox view on on this unanswered confusion

wait, why would I buy a book when I can just find it online:P

ACuriousMind

2:26 PM

@EmilioPisanty I'll grant you it's amusing.

> Imagine a world that is watching ours, theory is theory , what if theory was real, why do we breath

Hey @0celouvskyopoulo7

dropbox.com/s/eganwlpq5c2zii2/book.pdf?dl=0

If u want to do a book

Do you see things to put in there

@ACuriousMind thank you

=P

I'm guessing Cauchy problem and Sobolev spaces

pretty close to my old commute, too

user228700

Ah, thanks! I watched some drone footage of the same and saw all of the urban places surrounding it.

2:29 PM

@ACuriousMind heck, even Kyle fell for it

> Add our stands, this is merely an odd statement.

@Slereah I would like to write a thing on Sobolev spaces....

There's a lot of stuff to write about though

Kau mu di!

Most probably

@Slereah I would structure it differently.

How would u

2:31 PM

I would put the basic geometry in an appendix like Straumann does.

user228700

@Avantgarde :-P Yeah?

Start the main book with Lorentzian vector spaces.

@Kaumudi.H that was just a hi

user228700

Hey :-)

which would you consider "basic geometry"

There is a lot of geometry

Is it basically all of chapter 1

Hm, what else could I put in

pp-wave spacetimes

for gravitational waves

Linearized gravity

I guess the first test of GR I should put in the experimental test is "a ball falls down"

2:40 PM

Well...why do you need Ehresmann connections

To define the connection on the bundle?

They offer no physical insight for vector bundles, and presumably you don't need principal $G$-connections

Don't I

What if I want to do gauge fields

@Slereah Connections on vector bundles are very straightforward

@Slereah You don't...

Don't I

Also I might want to explore torsion and non-metricity

2:43 PM

Then you still only need connections on vector bundles

You really don't want to have to deal with the tangent bundle of a principal bundle

$U(1)$ isn't a tensor bundle

there's too many details there

you'd need a whole chapter on Lie groups

MAYBE I WILL

I'm not talking about representation theory either

Or I will write "See book"

2:44 PM

I thought the whole point was to make this self-contained

We will see

I mean I still need to write it

Should I try writing it

I can probably get enough pages from that CTC book for a good start

So, if I were to write a GR book (which I have been thinking about, but it would be ridiculous for me to try), I would have many appendices.

Kind of like Wald and Yvonne

There's no need to bog down the main text with a bunch of basic topology and geometry

Basic topology, yeah

Differential geometry, tho

I dunno

Also, I think I would put the physical foundations in an appendix

No need for that to be in the main book

What do you call the physical foundations

2:48 PM

Derivation of the field equations, why we use $-+++$, etc

why particles follow geodesics

I think one could motivate all of that in ~20 pages or less

Depends what you mean by derivation of the field equations

Do you mean from the Lagrangian or from Newtonian gravity

"ELLIPTIC SYSTEMS IN SPACES $H_{s,\delta}$ ON MANIFOLDS WHICH ARE EUCLIDEAN AT INFINITY "

Oh Yvonne

what a nice title

in all caps too

What I definately don't want to do is the usual historic text

@Slereah Newtonian.

Where the progression of the first chapters is done along the historical procedure

2:51 PM

I don't know any book that actually does that, except for Zee

There's too many books like that and I'm sick of reading about the history

and he puts a spin on the whole thing

Straumann talks about the history a bit but he never follows a "historical path" or whatever

I want everything rigorous from the get go, and only then do specific cases for applications

@dmckee If I have a #define in a parser.h, when I #include "parser.h in my main.c will the define be "seen" by main?

@BernardoMeurer Yes. The effect of #include is as of the text of the included file was inserted in place of the preprocessor directive.

Thus, the targets of #includes in header files are also effectively reproduced at that location.

2:53 PM

@dmckee That's what I thought! However it doesn't work well. I want to use strptime() from time.h, but for that I apparently need a #define _GNU_SOURCE 1 for reasons I don't understand.

I've even written a very simple unit testing framework in c-preprocessor

AH

Because I'm importing the parser AFTER time.h

Yep. Order matters.

Bingo

@0celouvskyopoulo7 First two chapter of Straumann are kind of the historical path

2:54 PM

Why do I need #define _GNU_SOURCE 1 to use strptime()?

Hell he even does scalar gravity

It's part of POSIX ffs

@Slereah yeah we can forget about that...

So I would also do Basic Sobolev Spaces in an appendix

And then Weighted Sobolev Spaces should be a main chapter

Well I might bring up scalar gravity

But only in the chapter on alternative theories

And maybe the experimental one

No alternative theories

It's not a physics book

2:59 PM

@BernardoMeurer At a guess I would suspect it of being part of (or a residual of) of an effort to make the gnu library compatible across a bunch of different systems.

There is a lot of under-the-hood wonkiness in that library and much of it is about compatibility.

@dmckee What a PITA

pain-in-the-ass bread

@0celouvskyopoulo7 I'm afraid it is!

If I put the differential geometry in the appendix I'm afraid it would be more appendix than book

I don't see you getting away under 800 pages, honestly.

A solid GR book has a lot of stuff.

@Slereah Humor me, I will write a short outline...

dammit

3:11 PM

@EmilioPisanty what

overdupehammered again

@EmilioPisanty help

Q: If Quantum Mechanics and Special Relativity tend to "Classical" physics at everyday scales, how do magnets exist?

0

I'm aware that ferromagnetism, para-magnetism, etc. in materials could be properly explained only when a relativistic, quantum mechanical theory for the electron was developed, in which spin was a naturally arising, fundamental quantity. I suppose one could say all effects of QM and SR except th...

can folks peer-review that closure?

@AccidentalFourierTransform with what?

@EmilioPisanty a differential equation

@AccidentalFourierTransform fine, go ahead

3:12 PM

so, spherical harmonics, right

two differential equations

if it's $f''(x)+f(x)=0$ I can help

@Kaumudi.H I'll be around as usual, from about six a.m. to midday British summer time, and variable times in the afternoon/evening.

$Y=f(\theta)\Phi(\phi)$

fancier stuff, I dunno

@AccidentalFourierTransform Are you looking for existence/uniqueness or an explicit thing

3:12 PM

@AccidentalFourierTransform yeah

so, $L_zY=mY$ means $\partial_\phi \Phi=im\Phi$, so $\Phi=e^{im\phi}$

yes

and $m\in\mathbb Z$ for univaluedness

I'm trying to agree but chat won't let me

yes

Is that link to show/hide the icons for inactive users new? Or have I only just noticed it?

3:14 PM

there we go

now, the other one, something like $(1-x^2)f''+\text{stuff}+\left[\frac{m^2}{1-x^2}+\ell(\ell+1)\right]f=0$

... yes?

it's pretty dangerous to just $+\mathrm{stuff}+$ stuff

with solution $f=c_1P_\ell^m+c_2Q_\ell^m$

yes

Yeah it would be fairly hefty

3:15 PM

and $c_2=0$ for finiteness

bc $Q$ diverges at $x=\pm1$

yes yes yes

damn you, chat

but where does the quantisation condition on $\ell$ come from?

$P_\ell^m$ is finite for all $\ell\in\mathbb C$

@AccidentalFourierTransform from finiteness at the other pole

which other pole?

the Legendre function can be written as a series

which terminates for $\ell\in\mathbb N$

the one that is not the other other one

3:16 PM

for real $\ell$ it does not terminate, but the series converges and is finite in the interval (-1,1)

@AccidentalFourierTransform false

has a divergence at $x=-1$

let me see if MMA agrees :-P

dlmf.nist.gov/14.4

Limit[LegendreP[\[Pi], 1, x], x -> -1]

(* \infty *)

so yes, that's it

user228700

@JohnRennie Nah, it wasn't to ask about how often you'd be around here.

3:18 PM

noice

;-) there you go

thanks!

for some reason, I thought that $P_\ell^m$ was symmetric around $x=0$

and I did check that it was finite at $x=1$

so I did what any physicists would do

user228700

@JohnRennie ...I was wondering about ur book collection (and if you could recommend a few books to me) but on second thought, never mind; I suspect I'm going to have too many books to read as it is.

I assumed that it must hold at $x=-1$ as well

@Kaumudi.H ah, OK, my business levels should be back to normal from tomorrow i.e. not very busy.

3:20 PM

@AccidentalFourierTransform it has specific symmetry for integer $\ell$

mislead by a false symmetry!

but not for arbitrary order

well now I know

=P

user228700

@JohnRennie Wokay :-) Good to know.

3:21 PM

From more physical grounds, though, you've reduced the eigenvalue DE to some kind of wave problem in a 1D interval

you expect to need to impose finiteness at both ends

I'll be referencing this one next time anyone asks something about rotational motion

1. The Causal Theory of Lorentzian Manifolds
2. The Wave Equation
3. The Einstein Equations
4. The Cauchy Problem and Constraints
5. The Positive Mass Theorem
6. Spherically Symmetric Spacetimes
7. Singularity Theorems
8. Global Lorentzian Geometry
9. Cosed Timelike Curves

Appendices.

A. Topology
B. Smooth Manifolds
C. Semi-Riemannian Manifolds
D. Lorentz Metrics on a Compact Manifold
E. Riemannian Geometry
F. Natural Sobolev Spaces in R^n
G. Weighted Sobolev Spaces
H. Partial Differential Equations

@Slereah idk, that's a first thought

@AccidentalFourierTransform nice one

doesn't beat this one

amazon.com/Complete-Calvin-Hobbes-Bill-Watterson/dp/1449433251/…

but it's nice anyways

> Shipping Weight: 13 pounds

sounds about right

3:23 PM

oh I want it :-(

@Slereah That should fill roughly 2,000 pages.

user228700

@AccidentalFourierTransform How come he knows this stuff? His dad's usually not very smart...

user228700

@AccidentalFourierTransform I don't get it

@AccidentalFourierTransform you seen Hobbes & Bacon, btw?

imgur.com/gallery/tUzAL

3:25 PM

> you cant believe anything you read, I'm afraid

I HAVE A QUOTE FOR MY THESIS

lol

@AccidentalFourierTransform which you're going to attribute to ... ?

"Dad"?

"C&H."

Sir Cumference

-3

Q: The Sun and Earth

If the Sun can hold at least a million or more Earths,how are we able to look up at the sky and see a perfect ball which is the Sun?

earth

user228700

@JohnR: Anything especially great though?

3:28 PM

@SirCumference I created an account just to +1.

2

Sir Cumference

@0celouvskyopoulo7 wha...

user228700

Ah, didn't check my mail...

@Kaumudi.H :-)

> how are we able to look up at the sky and see a perfect ball which is the Sun?

user228700

@JohnRennie Ya know, the whole point of gchat was so we could avoid this mail delay thing :-P

3:30 PM

@Kaumudi.H yes, but gchat needs a call setting up whereas I can fling out an e-mail and you'll get it eventually whether you're around at the time or not.

@0celouvskyopoulo7 could use a bit more physics!

and more appendices

good books have more appendices than actual chapters

user228700

@JohnRennie Same with gchat. I check hangouts as many times as I do Gmail :-)

@Kaumudi.H after all you have three days (2½ now!) before the mail would become relevant.

@JohnRennie strptime() is amazing

user228700

3:32 PM

@JohnRennie 2.5 days?! A little more than 24 hours to go!!

No, two days, I keep forgetting the time difference.

user228700

The exam may be in 1.5 days but I only have to study for one more day.

OK :-)

user228700

Those extra 12 hours I'll spend sleeping and eating and etc.

@Slereah Ideally this would be published as a Graduate Text in Mathematics...

3:33 PM

@BernardoMeurer it looked pretty cool. Time conversions are the bane of every programmer's life. Why can't the world agree on a single format for writing times?

user228700

@JohnR: Anyhoo, thank you! :-) I was gonna ask if you have anything to recommend, having some idea of my taste but OK...

@Kaumudi.H what? you should never study in the last 48h before an exam!

sleep and eat and dance and play, but dont study

I mean, cmon!

It should combine the best bits of Yvonne, Straumann, HE, BEE, SW, Wald, my spherical work (assuming that goes anywhere), and it should put the Cantor/Yvonne/Bartle/Christodoulou theory of weighted Sobolev spaces/elliptic systems into book form.

user228700

@AccidentalFourierTransform x'D Says who? Had I followed that rule, I'd have failed. Consistently. (I have a thing for last-minute revision)

@AccidentalFourierTransform I'd be cautious about making judgements. The Indian examination system is a strange beast and rather different to the examination style we're used to in the West.

3:35 PM

@Kaumudi.H says someone who has failed many exams too, but is older than you >:| (:-P)

user228700

:-P OK, Sir.

user228700

> A strange beast

@JohnRennie ¯\_(ツ)_/¯

user228700

Fitting.

:)

3:37 PM

@Kaumudi.H In the next update of Fabulous beasts and where to find them the Indian examination system is included in Appendix A.

user228700

Hahaha x'D

lol

@Slereah chat.stackexchange.com/transcript/message/37410560#37410560

dunno if you saw this

damn Indian testing taking over the chat again

@Kaumudi.H anyway, I have lots of books I think are truly wonderful but it's always a risky business recommending books to other people as tastes vary so widely..

@Kaumudi.H I can recommend books to you

3:39 PM

bourbaki

This is one of the most amazing books I read in a while:

user228700

@JohnRennie Hmm, OK.

amazon.co.uk/Minotaur-Takes-Cigarette-Break/dp/184195487X

the one book

I didn't think we were talking about math books

@BalarkaSen wanna write a Lorentzian geometric analysis book with me

3:40 PM

no

wtf why

user228700

@JohnRennie Looks interesting! :-)

It was one of those books I couldn't put down. The Minotaur is such a wonderful character in the book.

my favorite Minotaur story is House of Asterion. It's like 2 pages

user228700

@JohnRennie I see. I am yet to update my to-read list but once I do, I'll show you.

3:42 PM

is that a novel?

@Avantgarde yes. Well, it's a mixture of prose and poetry. But mostly prose.

@JohnRennie That's a good question, the UNIX time format is not very nice

seconds since 1970, lol

@BernardoMeurer it works. Though the rollover in 2038 may be an entertaining time :-)

That's going to be a mess

There's a party scheduled in Copenhagen for the rollover date

@JohnRennie Okay

3:46 PM

I want to be there

@JohnRennie If you go you'll have outlived the UNIX time standard! :P

Excel uses days since 01/01/1900. Windows uses seconds since the beginning of the Gregorian calendar or something ...

Mess

@BernardoMeurer another 21 years - that seems a reasonably modest ambition :-)

My favorite minotaur story is the original one

@JohnRennie We go, I buy you a beer :P

3:48 PM

When they put the monster bull baby in a maze

@Slereah Well it's ok

House of Asterion is a reinterpretation from the monster's perspective, really

Is the entire book just gibberisg

RAAAARGH

user228700

@JohnRennie 77. Really?

I always felt a bit sorry for the Minotaur. It wasn't his fault his Mum got a bit experimental one evening.

MOOOOO

3:49 PM

what book

The book written from the minotaur perspective

it's not a book, it's 2 pages. no it's not gibberish

@JohnRennie they should have made hip into hamburgers

it's the most beautiful 2 pages i have ever read

@Kaumudi.H 77 seems a fairly modest age to aim for.

3:50 PM

"Would you believe it, Ariadne? The Minotaur scarcely defended himself."

@Slereah hip?

Him

I am on the phone

bad typo

brings disturbing visions

user228700

@JohnRennie Reasonably modest. Ah. Yes. I misread your message.

@BalarkaSen aha, it's available online

3:55 PM

That's the translation I read. Weirded out by the links though

@BalarkaSen yes, I'm not sure the links add anything to the story ...

I mean it's an interesting idea to illustrate the story, but the images used there are plain weird

@Kaumudi.H I plan to live 900 years as well

user228700

:-) That's a nice little poem.