The h Bar

danielunderwood

00:17

@enumaris distributed queues are where it's at!

1. Put things in queue
2. Pull from queue on N machines for processing

Need stateful processing? Stick events in a new queue!

enumaris

00:40

seems like MPI doesn't really do queues..

I dunno

danielunderwood

00:52

Well I was thinking of other queue implementations, but wouldn't a queue be a uh...interface to pass a message?

I've never actually used MPI though

enumaris

well MPI sends and receives messages, I don't know if it maintains some centralized queue

I don't think so

at least maybe not "normally"

danielunderwood

Yeah looks like it may be pretty low level. But it walks like a queue and quacks like a queue, so good enough for me

enumaris

o.o

danielunderwood

01:11

If it walks like a duck and quacks like a duck, then it must be a duck!

enumaris

01:35

maybe...

danielunderwood

01:52

has duck typing ever been wrong?

enumaris

not to my knowledge

I dunno what you could call a queue from what I saw of MPI tho lol

it's just processes sending messages to each other

danielunderwood

It was mostly a joke, but if something can send and receive messages, I'd just call whatever is between them a queue!

Send someone a text message? The phones and cell network must be a queue!

It seems like a slightly flawed way of thinking of it with that example

tpg2114

02:36

@enumaris MPI doesn't have a built-in queue. You could build one, if you wanted, using the MPI commands though.

I've done it for a master-slave system, where a central server determined the work needed and farmed out chunks to it as nodes became available. The nodes asked for more work, got the next part, sent the result back, then asked for the next chunk to work on

enumaris

yeah I'm sure you could build a queue with it..

tpg2114

The queue bit was just using the Queue module in Python

And then the MPI part just had the master pull the next bit from the queue and fire it off to the slave

enumaris

right

hmmm

enumaris

03:00

anyone here familiar with photogrammetry?

Ultradark

03:44

Is the cross product of space and time equal to infoentropy?

John Rennie

04:22

@Ultradark no

enumaris

that's like...a lot of buzz words in one

maybe the cross product of space and time is equal to quantum shannon infoentropy

Slereah

08:29

Hm

I guess that whole automorphism business makes more sense if I consider first the map between the bundles, then apply this to the original section

ie x(t) becomes f(x(t)), g(t)

And then replace $t$ by g(t) through some inversion process

As Mark said

that sounds like the thing indeed

After all it's just a graph in the bundle, no need to think of it as a function just yet

Slereah

09:06

Apparently the autmorphisms associated with coordinate changes are called external automorphisms

arxiv.org/pdf/gr-qc/0505138.pdf

Fairly good article on the topic, btw

Although a bit rushed

But I assume it's for people who already know most of this

Slereah

10:48

@ACuriousMind help

Slereah

11:37

0

Q: Extended objects, bundle description and transformations

In the hope of trying to come up with a clear mental picture for what a transformation is in physics, I encounter some difficulties due to the variety of objects that appear in physics. While no picnic, the notion of transformations in field theory seems fairly straightforward, with the automorph...

Semiclassical

13:22

@EmilioPisanty Got any clever ideas for this one? It's a weird integral which G&R doesn't cite a source for

1

Q: Evaluate $\int_{-\infty }^{\infty } \left(\cos \left(\sqrt{x^2-1}\right)-\cos \left(\sqrt{x^2+1}\right)\right) \, dx$

From Gradshteyn&Ryzhik 3.692.6 we know that $$\int_{-\infty }^{\infty } \left(\cos \left(\sqrt{x^2-1}\right)-\cos \left(\sqrt{x^2+1}\right)\right) \ dx=\pi (J_1(1)+I_1(1))$$ Where two special functions on RHS is the original/modified Bessel function of the first kind (see here and here for their...

integration definite-integrals special-functions

Slereah

@Semiclassical The source was drinking vodka in a cabin in Siberia

as you well know

Semiclassical

right, how foolish of me to forget

Slereah

I suggest you do so until you find wisdom

Semiclassical

i'd say it's too early in the day for that, but i guess that reveals me as insufficiently russian

Slereah

13:40

the internet doesn't seem to have photos of Gradshteyn and Ryzhik

They remain a mystery

ancient soviet wizards

Semiclassical

lol

Slereah

there's apparently a math professor called Lenya Ryzhik

I wonder if he's his son

Slereah

13:56

Does the space of all embeddings have a name

ie for a manifold $\Sigma$ and $M$ with embedding $\iota : \Sigma \to M$, is there a space such that $\iota \in C(\Sigma, M)$

Obviously a subset of smooth functions between the two but not quite just that

Ryan Unger

14:55

@Slereah topological embeddings or smmooth ones

Slereah

Let's say smooth

Ryan Unger

@Slereah Hirsch writes $\mathrm{Emb}^r(M,N)$ for $C^r$ embeddings $M\to N$

Student404Mus

Could someone figure out how a spirit has two indices like this identity?

$\left( \sigma ^ { \mu } \right) _ { \alpha \beta } \left( \sigma _ { \mu } \right) _ { \gamma \delta } = 2 \epsilon _ { \alpha \gamma } \epsilon _ { \beta \delta }$

Slereah

@RyanUnger thx

Good old Hirsch

Student404Mus

15:12

I mean spinor not spirit

Slereah

It's too late

Now we all think of the spirit of the Pauli matrices

Student404Mus

My phone is rendering my words to mindless

Slereah

Also there are no spinors there

Those are pauli matrices

Student404Mus

@Slereah I mean I cannot think of these matrices $\sigma^{\mu}_{\alpha \beta}$ as spinor components?

Slereah

They're not

It's a map

Student404Mus

15:20

In P&S identified these two indices as spinor components

Slereah

$$\sigma : S \times S \to TM$$

It maps spinors to vectors

It's essentially a solder form

Well it is a linear map on spinors, yes

so it does have spinor components

Student404Mus

For example if look at sigma zero which his the identity matrix. What is the component one two of sigma zero? Is it zero?

Slereah

yes

Student404Mus

Ok

M y phone could not recognise latex

ACuriousMind

15:44

@Slereah I'll look at it tomorrow, I'm playing cards this evening and won't have time

Slereah

16:05

@ACuriousMind What game

ACuriousMind

16:15

Skat

Loong

18

Abcd

taken from here: hyperphysics.phy-astr.gsu.edu/hbase/Atomic/lcoup.html

Does anyone know why they simply use j = l+s without considering the direction of the vectors?

Semiclassical

because what they're really 'adding' are just the vertical components

so $J_z=L_z+S_z$

the rest is just a visual way of depicting that the length of $\vec{S}$ would be $s(s+1)$ etc

bolbteppa