# /sqt/ - Stupid Questions Thread

RumChicken

Tips!
give context
describe your thought process if you're stuck
try wolframalpha.com and stackexchange.com
How To Ask Questions The Smart Way: http://www.catb.org/~esr/faqs/smart-questions.html

Ignoramus

Let $f:\mathbb{R}\to\mathbb{R}$ and consider the statement:
$\exists \hspace{0.2cm} \delta > 0 \hspace{0.2cm} \text{such that} \hspace{0.2cm} \forall \hspace{0.2cm} \epsilon > 0 \hspace{0.2cm} \forall \hspace{0.2cm} x , \hspace{0.2cm} (|x| < \delta \implies |f(x)| < \epsilon)$

So to translate this to English, there exists delta greater than zero such that for all epsilon greater than zero, for all x, if the absolute value of x less than zero then the absolute value of f(x) is less than epsilon.

What exactly is going on in this statement and how would I characterize the functions for which this statement is true. I think the former is more important, since I'm struggling to see what's going on here.

whereismyname

@Ignoramus
Let
$f:\mathbb{R}\to\mathbb{R}$ and consider the statement: [eqn]\exists \hspace{0.2cm} \delta > 0 \hspace{0.2cm} \text{such that} \hspace{0.2cm} \forall \hspace{0.2cm} \epsilon > 0 \hspace{0.2cm} \forall \hspace{0.2cm} x , \hspace{0.2cm} (|x| < \delta \implies |f(x)| < \epsilon)[/eqn]

Spazyfool

@whereismyname
Last try.

$f:\mathbb{R}\to\mathbb{R}$

[eqn]\exists \hspace{0.2cm} \delta > 0 \hspace{0.2cm} \text{such that} \hspace{0.2cm} \forall \hspace{0.2cm} \epsilon > 0 \hspace{0.2cm} \forall \hspace{0.2cm} x , \hspace{0.2cm} (|x| < \delta \implies |f(x)| < \epsilon) [/eqn]

Dreamworx

@Spazyfool
oh well, at least the most important part is displaying correctly in my first post.

Lunatick

@Ignoramus
The satement says that for any delta surrounding of 0 (i.e when the absolute value of x is less than delta). There exists another number (epsilon) such that the function value is less than epsilon.

So for instance if you let epsilon be 0.00001 then there is a delta surrounding of zero such that the absolute value of the function is not greater than 0.00001 in that surrounding.

Now imagine that delta = 1 works for every epsilon. That means that in for x in (-1,1) the absolute value of the function is less than any number, i.e 0.

I'm assuming you know it means that f is zero at zero and that f is continuous at zero.

Crazy_Nice

@Lunatick
I see, this makes it a bit clearer. what's given is what is in my post. We haven't discussed continuity yet.

haveahappyday

@Lunatick
is this an accurate picture of the problem?

5mileys

@Crazy_Nice
Np, have you discussed limits?

cum2soon

@5mileys
Nope. Next week probably.

iluvmen

@cum2soon
Oke, what you wrote is the definiton that the limit of f(x) is 0 when x goes to 0 (from both directions).

idontknow

@iluvmen
really? hate it when they pull this shit. Thanks.

massdebater

Someone explain the intuition behind orthogonal functions to me. How is it that the inner product of <f1,f2> can be orthogonal to each other?

likme

How do you find P(A^B) if the events have different probabilities of happening?

Nojokur

Help Veeky Forums, stuck on matlab

Trying to set initial conditions along a length (L) =1

e(x,0) = 0 for 0.00<= x < 0.25
e(x,0) = a for 0.25<= x <= 0.5
e(x,0) = a for 0.5< x <= 1

Pic is where I'm at so far, I can't get the syntax right

Illusionz

Would it be

m-->(e v p)

Spazyfool

@Illusionz
Forgot image

idontknow

@Nojokur
I'm close but I can't seem to get the step function

Methshot

@Spazyfool
m iff (e or p)

WebTool

Chemistry Question.
I need to write a sentence for the reaction (complete word equation - sentence without abbreviations or symbols) for the double replacement reaction.
Ammonium Carbonate and Calcium chloride is mixed. I'm sure this is a double replacement reaction.
I don't know how to write this sentence. What do I write?

AwesomeTucker

@Methshot
I'm confused because the textbook says you can express a conditional statement (p-->q) as: "p only if q"

For the statement posted to be a biconditional, wouldn't it have to be: Movie if and only if... ? Seeing as that's not the case, I don't see how it's a biconditional.

Nude_Bikergirl

Is there anything known about the integral:

[eqn] - \oint_{\partial \Omega} \Delta v \nabla u \cdot \mathbf{n} \ \mathrm{d} S [/eqn]

In $\mathbb{R}^2$? We also know that $\nabla u \cdot \mathbf{n} = - \nabla u \cdot \mathbf{t}$.

New_Cliche

I know this is a mega-brainlet question but what trig identities did they use to simplify $\sec \theta - \tan \theta = \frac{1-\sin \theta}{\cos\theta}\cdot\frac{1+\sin\theta}{1+\sin\theta}$?

King_Martha

$\sec \theta - \tan \theta = \frac{1}{\cos\theta}-\frac{\sin\theta}{\cos\theta} = \frac{1-\sin\theta}{\cos\theta}\cdot$

Spamalot

How do I make a linear regression graph with error bars in Mathematica?

girlDog

Also note that the right term in the final product is just one.

@King_Martha
@King_Martha
oh, i'm an idiot, it's just the definitions multiplied by the conjugate.

thanks man

Burnblaze

I make a bet with you that ill flip a coin 10 times and it will be tails every single time. That would only be a probability of 0.09% right? So you would take the bet.
Now if I already had flipped tails 9 times you would have to look at the situation from a different perspective. Wouldnt the probability of me getting tails be 50% again, since each flip is completely independent?
So why has every teacher since middle school refuted this and taught me that the probability still would be only 0.09%? I am confident that my solution is the right one, because thats the only thing that makes sense.
Are my teachers wrong or am I just retarded?

RavySnake

@Nude_Bikergirl
Stokes Theorem?

LuckyDusty

@Burnblaze
I suspect you misunderstand your teachers (or your teachers are retarded, its not uncommon). You are correct the coin tosses are independent and no matter the previous results the next one will be 50/50.

StonedTime

@LuckyDusty
You are correct the coin tosses are independent
Prove it.

Fried_Sushi

@StonedTime
Prove they're not.

idontknow

@massdebater
The inner product is just a function that satisfies a bunch of axioms. So strictly speaking the you have to even talk about two functions being orthogonal you would have to say in what sense. It's most of the time in the sense of L^2. In that sense it just means that the integral is zero. But i guess that doesn't really say anything intuitively. Intuitively it's harder. But for instance two functions which have disjoint support are orthogonal. But they can be orthogonal even if they don't have disjoint support. As you've certainly seen examples of (otherwise $e^{inx}$ for different natural n:s are an example). The most common (that i've seen) reason to care for orthogonality in L2 is because you want a basis. There is this nice theorem that says that such a thing exists in any Hilbert space (if i'm not misremebering).

hairygrape

What's the "point" of unit vectors? Is it to be able to write a vector instead of $\vec{r}=r_x+r_y$, ie. as a sum of its components, but to be able to show its size? Or waht.

Playboyize

Fellas, how can I prove sin(n + (1/n)) is divergent???

Snarelure

@hairygrape
Also when calculating the $\vec{i}$ component of a vector $\vec{AB}$ for example, why is it $(x_B-x_A)\vec{i}$ instead of $(x_A-x_B)\vec{i}$?

TalkBomber

There exist arbitrarily large n such that sin(n + 1/n) > .5 and sin(n + 1/n) < .5

Indeed the fractional parts of $(\pi/2 \pm \pi) m$ are dense in [0,1].

iluvmen

How do I raise my logic stat?

Inmate

Why does skin get darker as you go to hotter/sunnier places? Don't darker colors attract sunlight or heat or something, making dark skin worse in these climates? I would just google this but no one seems to actually explain it further than "the sun is involved" and many just get offended at the question for whatever reason.

Methshot

@Inmate
Melanin protects your cells from UV rays which cause thymine dimer forming which are bad (cancer etc). Melanin is dark. Well, not really, there's dark and light melanin, but the dark one does a better job at protecting the skin.

PackManBrainlure

Spivak Calculus is a little bit harder than I was anticipating. I'm in chapter 2 right now and it takes me a long time to solve just one problem. I'm in calculus 3 right now so it's not like I just bought this without any background. I'm wondering if the problem is solely because I need to get better at proofs, and if so, should I just keep going and hope that I'll eventually catch on, or should I complete a proof book?

Sharpcharm

@Methshot
Shoulda probably mentioned skin (melanocytes in it) produces melanin the higher the exposure to sunlight is too.

RavySnake

@WebTool
I'm not sure I understood what you need.
A sentence to explain what's happening in the reaction?
Like "when ammonium carbonate and calcium chloride are mixed, ions are swapped, forming ammonium chloride and calcium carbonate"?

StonedTime

@TalkBomber
Can you explain that last line a bit?

DeathDog

@Burnblaze
@LuckyDusty
But 50% is the probability of THAT toss, not the probability of getting 10 tail straight, isn't it?

TechHater

@PackManBrainlure
Velleman's how to prove it is pretty solid and not that long.

PurpleCharger

How many dimensions does the number 0 have?

@PurpleCharger
it depends which space you are working in

Spamalot

@Nojokur
e( (e>.25).*(e<.5) ) = a;
that's all you need after initialising it as a zero vector

idontknow

Is that it?

How about real and/or imaginary dimensions?

RavySnake

@idontknow
"0" is defined based on the space you are working in and has the same number of dimensions as said space. e.g.

on
$\mathbb{R}^3$
$0 := (0,0,0)$

TalkBomber

@Spamalot
@Nojokur
it's so pretty!

Emberburn

@TalkBomber
Indeed

Emberburn

@RavySnake
And if you are in a module over a non-commutative ring the idea of dimension doesn't really make sense all the time so there are spaces (that are quite similiar to vector spaces) that have zeroes and you can't really tell what the dimension is.

TurtleCat

@idontknow
Parmenides

My fucking man.

w8t4u

Why aren't electrons accelerated by their own field? I'm a physics major, and this has never been answered to me. I asked my e&m proffessor and he gave me a really hand-wavy explanation that I didn't understand.

Bidwell

I rewrote this problem in paint, because my handwriting is shit.

WebTool

@TalkBomber
Can someone make sense of the last line for me?

@RumChicken
How do I stop being a complete brainlet. I don't remember shit from the 12 years of life spent in schooling such as basic mathematics. What should I do besides kill myself?

girlDog

read textbooks and do problems. if u dont use it u looze it

Bidwell

Calculating the area of the parallelogram made by these points.
The sides are equal vectors so the cross product is always zero.

Am I forgetting some fuckery about positives and negatives or something? Google has a billion results for how to find the area, but none for what to do when your area is zero even when it doesn't seem like it should be.

likme

@w8t4u
An accelerated electron does in fact interact with its own field - the energy of the interaction is represented as an "electromagnetic mass" (which is only nonzero for point particles in QFT).

Here is a relevant paper: https://arxiv.org/abs/0905.2391

JunkTop

I think you had the equality flipped.

PurpleCharger

Has anyone actually tested their IQ before and after uni?

Boy_vs_Girl

@JunkTop
Thanks, user. I'll check if I had it backwards.

TreeEater

@Bidwell
The area of a parallelogram is the length of one edge multiplied by the perpendicular distance from the opposite edge. A parallelogram is a sheared rectangle, and a shear preserves area.

So given the two vectors corresponding to two non-parallel edges, the area is just the magnitude of their cross-product (the product of their lengths multiplied by the sine of the angle between them).

If you're getting zero, it's because you're using two parallel edges.

Garbage Can Lid

Why do so many more bad things happen on Friday the 13th than on any other day of the year?

Spamalot

is geometry just subset of algebra?

or like, is geometry study of "how humans perceive vector spaces"?

cum2soon

@RumChicken
In a Diffraction Barrier, can you have different wavelengths at fringes of confluence? We did an Experiment at Lab III, and we had light of Hg, which we tracked the 1st & 2nd fringes, and found the angles. Then we have to find the wavelengths, but one thing's bugging me.

We tracked the blue color, but through the equations, I get different wavelengths for fringe 1 and fringe 2. Why does that happen? My book doesn't have anything about it.

Best I can come up with, is that you know how in a Barrier of Difration it's Diffraction plus Confluence, right? So at each Confluence, the light fades a bit every time. So, technically, the wavelength changes as well. So you start with blue, then you get... light blue, then "almost gone blue",etc, etc. Kinda like how ie, Red & Green have different wavelengths.

Am I close?

Methshot

@RumChicken
Given a plane centered on the origin (normal unitary vector n), and a vector v, find a linear transformation (Tensor T) such that T*v = w is the reflection of this vector respect to the plane. Describe what this transformation does to a vector belonging to the plane. Explain and draw the algebraic steps done to build this transformation.

Emberfire

What punishment should a person receive for selling secrets to another countrie?

VisualMaster

@Nojokur
anyone ever used the Runge Kutta method?

I can't see where I'm going wrong

TechHater

@Methshot
let's say your plane is spanned by 2 orthonormal vectors $a,b$.
The projection of a vector v onto the plane can be written as $Pv = aa^Tv+bb^Tv$.
The vector that is pointing from $v$ to it's projection $Pv$ is $Pv-v$.
Therefore we can reach the reflection point by adding $2(Pv-v)$ to $v$.
The transformation you are looking for is then
$Tv = v+2(Pv-v) = [2(aa^T+bb^T) + I]v$

BinaryMan

@VisualMaster
what's the problem here? your solution looks like standard diffusive behaviour

BunnyJinx

@BinaryMan
ideally fig2 should look like fig1

I've tried changing most of the variables and cant seem to get it to generate anything past t+1

Ignoramus

@TechHater
I made a little mistake. It should be
$T = 2(aa^T+bb^T) - I$
for example the x-y plane is spanned by $a = (1,0,0)^T, b = (0,1,0)^T$, so [eqn] T = 2\left (\begin{pmatrix}
1 & 0 &0 \\
0 & 0 &0 \\
0&0 & 0
\end{pmatrix}+\begin{pmatrix}
0 & 0 &0 \\
0 & 1 &0 \\
0&0 & 0
\end{pmatrix} \right ) - \begin{pmatrix}
1 & 0 & 0\\
0& 1 &0 \\
0&0 & 1
\end{pmatrix}=\begin{pmatrix}
1 & 0 &0 \\
0 & 1 &0 \\
0&0 & -1
\end{pmatrix} [/eqn]

Nojokur

@TechHater
@Ignoramus
thanks bro!!

Need_TLC

@BunnyJinx
so I'm trying to figure out your program
your differential equation is e_t = k*e_xx - u*e_x.
fig 1 is generated by a spatial discretization with central finite differences and integrated in time with explicit euler?
fig 2 is supposed to be the same spatial discretization but this time integrated by rk4 and you don't seem to get any deviation from your initial condition?

If you zip your program i can check it for mistakes if you want

Lord_Tryzalot

What's the point in using Bayes theorem when you could just swap the way the original formula to find A|B?

Like A|B = A^B/B
So why couldn't you just do:
B|A= B^A/A?
Or is it assuming you don't know A?

Bidwell

The inverse of $f(x) = x^3-2$ is $f^{-1}(x)=(x+2)^{1/3}$.
Now, the domain of $f^{-1}(x)$ should be the range of $f(x)$, which is R. But $f^{-1}(x)$ is not defined for $x<-2$. Then, what to do?!

idontknow

@Bidwell
not defined for x<-2
why wouldnt it be?

likme

@Need_TLC
@VisualMaster
you should really check your Derivative function.
It will constantly return a zero vecor the way it's implemented now.
you should also consider implementing the finite differences not with for loops but through matrix multiplications with
[eqn] \frac{\partial }{\partial x} = \frac{1}{2h} \begin{pmatrix}
0 & 1 & 0 & \cdots \\
-1& 0 & 1 & \cdots \\
0 & -1 & 0 & \ddots \\
\vdots& \vdots & \ddots & \ddots
\end{pmatrix} [/eqn] and
[eqn]
\frac{\partial^2 }{\partial x^2} = \frac{1}{h^2} \begin{pmatrix}
2 & -1 & 0 & 0&\cdots \\
-1& 2 & -1 &0 &\cdots \\
0 & -1 & 2 & -1& \cdots \\
\vdots& \vdots & \ddots & \ddots & \ddots
\end{pmatrix}
[/eqn]
this runs faster than a loop and is easier on the eyes

another fuckup is, that you forgot to correctly bracket the 2dx part in the explicit euler loop.
the solution should move in positive x-direction with speed k, if I'm interpreting it correctly

@idontknow
Forgot to say that $f: R \rightarrow R$

hairygrape

@Bidwell
not defined for x<−2
?

Poker_Star

the third root can be well defined for all real numbers, for example (-8)^(1/3) = -2

Gigastrength

@Poker_Star
@hairygrape
Yeah, I see. Thanks, mates.

(-1)^(1/3) = -1, a real number. (there are also two complex numbers that solve the equation x^3 = -1 but I assume you're only interested in the (unique) real solution)

TechHater

@likme
I'm just going to stick to Euler,

I see the mistake with (2*dx) but when I parenthesis this I get this:

AwesomeTucker

@TechHater
@likme
This is the actual problem, I've used the spatial discretisation for Forward Central Central as on excel it provides the best model (much better than FFC,FBC), everything that starts with Central or Backwards doesn't form a suitable equation to model

Supergrass

@TechHater
yeah, that's the convection term working.
rk4 isn't worth it anyways, because your spatial discretization will at most be of order 2, so the temporal order 4 from rk4 won't be visible.
you could try implementing heun's method
@AwesomeTucker
you should definately check out the effects of different spatial discretisations for the first derivative when the diffusion coefficient k is small.

forward and backward differences will have dramatically different solutions depending on the sign of u.

Lord_Tryzalot

@Supergrass
I'll check them out, thanks.

So do you think @TechHater
is the correct representation for FCC here?

SomethingNew

@Lord_Tryzalot
sure I'm not the biggest fan of the mesh representation though.
I would plot it like

for tdx = 1:length(t)
plot(x,e(:,tdx))
pause(.01)
end

so you can see the solution evolving

5mileys

Does the position you sleep in affect your dreams?/sleep state?

Ie. left,right and on your back.

cum2soon

@RumChicken
If pic related is my velocity-distance graph( distance and velocity are proportional) what will my velocity-time graph look like and what will be it's function, I think I know the solution but I can't be sure if someone doesn't check on me

w8t4u

@SomethingNew
Woah thanks that's really cool

Spamalot

Is there a simple way to "prove" that all types of subatomic particles are exactly, completely identical to each other, i.e. every proton is just like every other proton, etc.

LuckyDusty

@Spamalot
Same mass, same composites, same charge in any experiment.

@cum2soon
parabola

LuckyDusty

Isn't "Gauss Criterion" a sick name for a band?

Nude_Bikergirl

@RumChicken
How do I higgs my own boson?

TalkBomber

definitely not parabola

@TreeEater
A) The professor told us which edges to use.
B) I also tried different edges defined by these vertices and still got 0.

Burnblaze

@TreeEater
Fuck. I get what you're saying now.
I should have repeated a point and used it as the corner of the two edge vectors.

My fucking dipshit professor told us otherwise.
The man has done this kind of thing countless times this semester.

He tells us the wrong shit, like "x=r*sin(theta)" all lecture, then we get to the homework and find he mixed shit up.

He taught us that an ellipse was a degenerate circle.

Every class it's some new bullshit.

fml

Firespawn

Given a scalar field, I'm supposed to find the x component of the unit vector in which I have to move for the variation of the scalar function to be minimal, given that we're initially in a point P

Do I have to find the gradient, then use it to build a unit vector by substituting the values of point P? Or maybe the opposite of the gradient since the function is supposed to be minimal.

takes2long

Hey guys, EMT here with 1 quarter of community college chemistry years ago. Trying to understand this:

"Activated charcoal is produced to have a very small particle size to increas its adsorptive properties. To better illustrate this, consider that a standard 50-gram dose of activated charcoal has roughly the same surface area as ten football fields."

wut? How?

iluvmen

@takes2long

@cum2soon
$\frac{dv}{dx} \cdot \frac{dx}{dt} = \frac{dv}{dt} \\\ av=\frac{dv}{dt} \\\ \int adt = \int v^{-1}dv \\\ at+c=ln(v) \\\ v=v_0e^{at}$

@RumChicken
Assuming C and B are independent, if P(A|C) is 0.25 and if P(A|B) is 0.3 , what are P(A|B∩C) and P(A|(B')∩(C')) ?

Everything I've been getting makes no fucking sense.

Crazy_Nice

how do I begin to factor trinomials of form [eqn]ax^2+bxy+cy^2[/eqn]
for example: [eqn]14a^2+41ab+15b^2[/eqn]

@Crazy_Nice
14a2+41ab+15b2 =
(wa+xb)(ya+zb) =
wya^2+(wz+xy)ab+xzb^2
so you take w=7,y=2,x=3,z=5
14a2+41ab+15b2 =
(7a+3b)(2a+5b)

Bidwell

When people count forward, they tend to start at 1.
When they count backwards, they end at 0.
Why?

hairygrape

@Bidwell
when you say on 3 do you mean the third second or the second after that?
same thing applies here

Playboyize

@Bidwell
When people count
When they count
Speak for yourself.

PackManBrainlure

How do you generate a calibration curve to normalize observed values with literature data in Vernier lab software?

t. brainlet

w8t4u

why does my head hurt when i drink really hoppy beers? do the hops caused a similar sensation to benzos like a really small high

RumChicken

Where can I find problem sets introductory Algebra? (Groups, Rings, Modules etc.)
Googling for individual topics gives results more advanced than done in class, and "introductory algebra" gives middle school stuff.
I've done the relevant problems in Artin already.

Nojokur

@Bidwell
Computers are better ocunters:
Start at 0 always
Negative numbers do not exist
NEGATVE NUMBERS DO NOT EXIST
if a number is larger than the number of bits in your bytes, it doesn't exist either.

Dreamworx

@takes2long
Volume is proportional to the cube of the radius. Surface area is proportional to the square of the radius. So surface area per unit volume is inversely proportional to the radius.

If you halve the particle's radius, its surface area goes down by a factor of 2^2=4 but you get 2^3=8 times as many for a given volume or mass, so the total surface area doubles.

Need_TLC

@Snarelure
because of notation you cuck, the vector is pointing toward B(x0,y0)

Crazy_Nice

@w8t4u
how can an electron push itself away?

cum2soon

$ds^{2}=\frac{L^2}{z^2}\left(dz^{2}+\eta_{\mu\nu}dx^{\mu|dx{\nu}\right)[\math] calculate the Ricci Tensor. I've started with compting the Christoffel symbols by I can't seem to calculat the Riemann tensor which is needed for the Ricci tensor. Can any one help with calculate the Riemann tensor? I have that [mah]\Gamma^{t}_{tz}=\frac{L^{2}}{z^{2}}, \Gamma^{x}_{xz}, \Gamma^{y}_{yz}, \Gamma^{z}_{zz} = -frac{L^{2}}{z^{2}}[\math] Thanks in advance likme The questions are: 1. Select the correct expression for the relationship between the accelerations of blocks 1 and 2 2. Select the correct expression that describes how the magnitude of the acceleration of block 1, a1, depends on the tension in the rope, T 3. Select the correct expression that describes how the magnitude of the acceleration of block 2, a2, depends on the tension in the rope, T My attempts: 1. Block one accelerates left and Block 2 downward. Thus, they both have the same sign. Also, for every unit of length Block 1 moves, Block 1 only moves half as far. Therefore, the relationship is [math]a_{1x} = 2a_{2y}$.

2. The balance of forces on Block 1 is
$-T=m_{1}a_{1x}$
$\therefore \lvert a_{1x}\lvert = \frac{T}{m_1}$

3. The balance of forces on Block 2 is
$2T-m_2g = m_2 a_{1y}$
$\therefore \lvert a_{1y} \lvert = \frac{2T}{m_2}-g$

I have to submit the answers in batch, so I'm not even sure which of these I'm going full brainlet on.

@likme
$a_{2y}$ in question 3, not $a_{2y}$

whereismyname

I'm struggling with something really stupid in MatLab.
I basically have a function
f = x;
I want to get all the coeffs for it, so I type
coeffs(f,x,'All')
Yet I get an error about "Too many input arguments."
I just want to get the [1 0] vector out of it, what am I doing wrong?

Methnerd

stupid question:
In index notation, what are the rules for partial derivative operators? I know position is relevant, because it indicates what it applies to, but imagine I have
[eqn]rot(\omega\times(\omega\times r))[/eqn]
which I then simplify to
[eqn]\epsilon_{ijk} \partial_{i}\omega_o\omega_jr_o - \epsilon_{ijk} \partial_{i}\omega_n\omega_n r_j[/eqn]

now what I don't get is what the partial derivatives apply to and by what rules

Snarelure

@cum2soon

$ds^{2} = \frac{L^2}{z^2} ( dz^{2} + \eta_{\mu\nu} dx^{\mu} dx_{\nu} )$

calculate the Ricci Tensor.
I've started with compting the Christoffel symbols by I can't seem to calculat the Riemann tensor which is needed for the Ricci tensor.
Can any one help with calculate the Riemann tensor?
I have that
[eqn]\Gamma^{t}_{tz}=\frac{L^{2}}{z^{2}}, \Gamma^{x}_{xz}, \Gamma^{y}_{yz}, \Gamma^{z}_{zz} = -\frac{L^{2}}{z^{2}}[/eqn]

Dreamworx

Any of you big bois good at PDE?

I just determined a general, radially symmetric function $\Phi \in C^\infty)(\mathbb{R}^n \setminus \{0\})$ that satisfies $\Delta^2 \Phi = 0$ on $\mathbb{R}^n \setminus \{0\}$. Now the teacherdude wants me to eliminate all the integration constants by saying that $D\Phi$, $D^2 \Phi$ and $D^3 \Phi$ need to be integrable on compact sets. The function $\Phi$ in question has a singularity at $x = 0$.

However this makes no sense to me, since all the vectors $D^n \Phi$ will again consist of $C^\infty(\mathbb{R}^n \setminus \{0\}$ functions, these functions are then bounded on all compact sets and thus integrable. Obviously we need to do something with the singularity but I don't know how to tackle this since we can't just look at an interval like $(0,1]$ for $r$. How do sneak my singularity into a compact set?

Spamalot

@Dreamworx
Okay what I meant to say with this clusterfuck of a post:

The equation $\Delta^2 u = 0$ gives rise to a Green's function for the operator $\Delta^2$. How to determine the integration constants for this function?

farquit

@whereismyname
The problem is that you have too many input arguments. I hope that helped

Look at the signature of the coeffs function

eGremlin

What class is this?

It's more complex than the trig or geometry I took, but I'm getting into Euclid's work and loving it and want more.

AwesomeTucker

@Bidwell
"1" is the span from 0 to 1.

If you count forwards, you say, "1" to go from 0 to 1.

If you count backwards, "1" is just the span from 2 to 1. You must go to 0 to capture the interval from 1 to 0.

@Nojokur
Negative numbers do not exist
NEGATVE NUMBERS DO NOT EXIST
Bullshit. Negative numbers in computers are EXACTLY as they are defined in math:
-x + x = 0

haveahappyday

Can someone help me with the bonding that goes on in the Primary, Secondary, Tertiary and Quaternary structures of Proteins?

I know that in the primary structure, we just have peptide bonding in the polypeptide chain. In the secondary structure, we have predominantly hydrogen bonding (but I have a textbook which says that there are other things like VDW's interactions, ionic bonding?). Tertiary structure is VDW, Ionic, Hydrogen. And Quaternary structure is again the same but with also disulfide bridges. Have I got this right?

Also, can someone explain hydrophobic interactions in a chain? I'm confused on what they mean by this.

Fuzzy_Logic

@Snarelure
I am absolutely brain dead for GR, and also lazy, but hopefully someone will correct what I did at least and give you the correct answer after all.

Your metric tensor seems to be
$g= -( \frac{L}{z})^{2} \left[ \begin{array}{cccc} -1 & & & \\ & 1 & \\ & & 1 \\ & & & 2 \end{array} \right]$

So putting it in a computer, it tells me that the Ricci tensor R is the matrix in the pic. Your Christoffel Symbol seems to be $\Gamma^t_{tz} = \frac{1}{z}$, which is the only one I've done by hand to confirm it.

You should absolutely not trust anything I've written here since I've only taken GR online as a hobby and this is the first exercise I've done any actual calculation.

Raving_Cute

@Snarelure
Notice that when computing the Christoffel Symbol, you have $\Gamma^a_{bc} = \frac{1}{2}g^{as}( \partial_c g_{sb}+\partial_b g_{sc} - \partial_s g_{bc})$, meaning you have g with both upper and lower indexes, so the L^2 cancels out, since g_ab = 1/g^(ab).

takes2long

sin(npi)=0 for all integer n, and cos(npi)= (-1)^n for all integer n, right?

iluvmen

@takes2long
yes

likme

Uhhh, why is [1, 2] not an open set? Any epsilon neighborhood around 1 is nonempty. It has nothing to the left it, but that's fine, there's infinite points to its right.

kizzmybutt

@likme
[1,2] is open in [1,2]
[1,2] is not open in [0,3]

Playboyize

@kizzmybutt
I guess I'm asking why [1, 2] isn't open in R

Methnerd

@Playboyize
look at the definition of open

PackManBrainlure

@iluvmen
thanks,

Carnalpleasure

@Playboyize
consider the point 1 in [1,2]
then a>0
now 1-(a/2) is in the a-ball around 1
but 1-(a/2) is not in [1,2]
so the a-ball around 1 is not contained in [1,2]

Evilember

@Methnerd
From my textbook:

Set O is open iff given any x in in O, there exists an epsilon neighborhood about x that is a subset of O.

Oh, so the problem is that this epsilon neighborhood about 1 has to include points to its left which makes this set NOT a subset of [1, 2]? Can you just be a dick and say let epsilon = 0?

Flameblow

@Carnalpleasure
Gotcha. Thanks

RavySnake

@RumChicken
What the fuck does this dude on /vp/ mean? >>33812801

FastChef

@Evilember
Does your book define neighbourhood? If not then it kinda should for that definition to make sense. Anyway a neighbourhood of a point is a set that contains an open set containing that point.

Taking epsilon to be zero would give a point. If points are open then you are a strange place.

Take a unit cube. Clearly the cube has 6 faces with an area of 1 each, so the cube has a total surface area of 6. Now slice that cube in half and calculate the combined area of the two new cubes. You just gained some more area, from nothing! It's magic. If you keep slicing it like cheese, you can get infinite area!

I've tried with viette's theorem but really couldn't figure it out, can Veeky Forums help a brainlet out ?

Emberfire

A set O is open if for every x in O you can find an open ball B(x,r), r > 0, s.t. B(x,r) is a subset of O. If you're not familiar with balls, in R an open ball for x would be the interval (x-r,x+r).

Clearly at point 1 the interval (1-r,1+r) would not be a subset of interval [1,2] with any r>0.

Flameblow

x^3+ax^2+bx+c =
(x-a1)(x-a2)(x-a3) =
x^3-(a1+a2+a3)x^2+(a1a2+a2a3+a1a3)x+a1a2a3

so a=-(a1+a2+a3)=-2, c=a1a2a3=3

x^3+cx^2+bx+a =
(x-b1)(x-b2)(x-b3) =
x^3-(b1+b2+b3)x^2+(b1b2+b2b3+b1b3)x+b1b2b3

so c = -(b1+b2+b3) = 3, a = b1b2b3 = -2

so -c/a = (b1+b2+b3)/b1b2b3 = 1/(b2b3)+1/(b1b3)+1/(b1b2) = -3/(-2) = 3/2

LuckyDusty

@Flameblow
whoops, sign mistake

x^3+ax^2+bx+c =
(x-a1)(x-a2)(x-a3) =
x^3-(a1+a2+a3)x^2+(a1a2+a2a3+a1a3)x-a1a2a3

so a = -(a1+a2+a3) = -2, c = -a1a2a3 = -3

x^3+cx^2+bx+a =
(x-b1)(x-b2)(x-b3) =
x^3-(b1+b2+b3)x^2+(b1b2+b2b3+b1b3)x-b1b2b3

so c = -(b1+b2+b3) = -3, a = -b1b2b3 = -2

so c/a = (b1+b2+b3)/b1b2b3 = 1/(b2b3)+1/(b1b3)+1/(b1b2) = 3/2

RumChicken

@Flameblow
To find the component of F parallel to the ramp, surely you don't just subtract 5 from 34 degree, right?

I also tried finding the unit vector for the ramp and finding the projection of the force onto that vector. No luck.

StonedTime

It's implied by the excercise that both of this polynomials have 3 roots.
the first one can be written as f(x)=(x-a1)(x-a2)(x-a3) and the second one as g(x)=(x-b1)(x-b2)(x-b3).
The constant term of f(x) is c=(-a1)(-a2)(-a3)=-a1a2a3=-3.
The coefficient of x^2 of f(x) is a=-a1-a2-a3 = -2.
Therefore,
g(x) = x^3 -2x^2+bx-3 = (x-b1)(x-b2)(x-b3)
The coefficient of x in g(x) is b=(-b1)(-b2) + (-b1)(-b3) + (-b2)(-b3) = b1b2 + b1b3 + b2b3.
And in f(x) it is b = a1a2 + a1a3 + a2a3 = -3 (1/a3 + 1/a2 + 1/a1).

I dunno man....

Skullbone

You have 6 people, A,B,C,D,E,F, and you want to put 3 of them inside a group, thing is, you have to put either A OR B inside the group, how can i solve this other than calculating all the possibilies and subtracting the possibilities where they are together and then subtracting possibilies where A and B arent in the 3man group? answer is 12 btw

Flameblow

@Skullbone
You want one person out of (A, B), and two people out of (C, D, E, F). That gives you (2 choose 1) * (4 choose 2) = 2 * 6 = 12 options.

TurtleCat

@RumChicken
To find the component of F parallel to the ramp, surely you don't just subtract 5 from 34 degree, right?

that's exactly what you do the dot product is ABcos(theta), theta is the angle between the vectors

I also tried finding the unit vector for the ramp and finding the projection of the force onto that vector. No luck.

$F \begin{bmatrix} cos34° \\ sin34° \end{bmatrix} \cdot \begin{bmatrix} cos5° \\ sin5° \end{bmatrix} = F(cos34° cos5° +sin34° sin5°) \\\ Fcos(34° - 5°)$

If I eat piping hot lasagne then immediately eat cold ice cream, and do it regularly, will this process have the same effect on my tongue that quench hardening has on steel? Can you strengthen your tongue this way? Will it become brittle at some point?

Spazyfool

@Flameblow
Thanks man, im retarded

Stark_Naked

Thank you.

I tried that approach repeatedly when I first learned the subject, but got it wrong over and over again, so I internalized that this couldn't be the right way to do it.

Even looking at it spelled out, I can't accept it yet.

Garbage Can Lid

I'm confused on the definition of subgraph.
We define a graph as G=(V,E) with V being a finite set and E satisfying $E\subseteq [V]^{2}$. We can allow E to be the empty set, but V will always be not empty.
G'=(V',E') is a subgraph of G if $V'\subseteq V$ and $E'\subseteq E$.
So, if in this picture we use the red points to form V' and E' only has the diagonal, that would be a valid subgraph, even though by itself it would not be a graph since $E'\nsubseteq [V']^{2}$. Am I wrong somewhere? Because if I'm not, I don't see the point of such a subgraph.

Harmless_Venom

What is the best alternative to Wolfram were I don't have to pay for the step-by-step solutions?

@Garbage Can Lid
https://en.wikipedia.org/wiki/Glossary_of_graph_theory_terms#subgraph
The vertex subset must include all endpoints of the edge subset

MPmaster

Veeky Forums

Evil_kitten

@Harmless_Venom

farquit

I was going by Diestel definition and this was not clear in the book, but that other definition is what I expected, thanks.

@farquit
they're assuming that G' is a graph here, so if G' is a graph and V' < V and E' < E then G' is a subgraph of G. your example in the first post wouldn't be an exclusion since it wasn't a graph to begin with

Poker_Star

You're right, earlier in the text it even explicitly says G' is a graph. I got too confused with my false example of subgraph and only realized G' is assumed to be a graph afterwards.

Burnblaze

@Harmless_Venom
use mathematica (maybe your school computer labs have it).

DeathDog

Is it possible to bookmark articles on elsevier (sciencedirect) directly or do you have to keep links somewhere else?

TreeEater

@5mileys
Anyone for this?

w8t4u

can some one explain to me how you end up with 6?

massdebater

@w8t4u
multiply both sides by x^2-16

hairygrape

umm, i'm pretty sure you multiply both sides by (x+4)(x-4)

Techpill

@hairygrape
umm, i'm pretty sure you multiply both sides by (x+4)(x-4)
That's what I said

StonedTime

3 tough exams next week
planned to study intensely the entire weekend
wake up this morning with a fever and agonizing nasal drip, hurts to breathe , swallow, move
can't focus at all, mentally foggy

My question is do I drop out of all my classes now or later

JunkTop

holy shit, im retarded

i just can't remember how to multiply these together, i completely forgot everything from spring semester...

Illusionz

@JunkTop
Division and multiplication are communities a la subtraction and addition

Put the shit in parenthesis in the numerator

Supergrass

@Illusionz
*communitive

Autocorrection sucks

okay,

now what?

TreeEater

Do we have a pulse in our brain? If not, why do I always feel/hear a pulse in that area? It's actually really distracting and scary sometimes.

TalkBomber

clue

47*53=(50-3)(50+3)

TurtleCat

you cannot feel anything inside your head because their aren't any nerves there

its most likely blood pulsing around your sinuses

i think i figured it out, you subtract all the stuff on the right over to the left and then it sets it up for a quadratic equation?

Need_TLC

@TurtleCat
its most likely blood pulsing around your sinuses

Interesting. But, why do I seem to hear it so much? I feel like I am sensitive to hearing the pulse in my sinuses. Is this just something I have to live with, or does it sound indiciatve of a problem? My doctors all told me "whatever", but it seems like most people dont live with this constantl pulsing feeling in their brain

King_Martha

that's just might be how you are, there's a chance your sinus is naturally more constricted than other peoples, so the pulsing is louder. try drinking more water maybe?

you might have some mental condition that somehow makes you notice things you're not supposed to. Technically you hear feel smell and taste everything around you right now, but you're brain sifts through all that stuff and only makes you notice the thing you're currently focused on, or else you'd go completely insane if you were constantly perceiving everything at once.

StonedTime

@Harmless_Venom
You can pirate Mathematica and you can use wolframalpha from it with all the features.

TechHater

Anyone know why this 'end' isn't going blue?

Ignoramus

Anyone got tips for getting higher points for the ACT like any specific things to study?

girlDog

Again...interesting. My doctor told me that I may have something called Sensory Processing Disorder, in which I sense things too heavily and its a detriment to my life. I constantly hear the pulse in my brain, just like I constantly feel the clothes on my body

Evil_kitten

@RumChicken
Are space elevators valid or nonsense?

Are orbital rings connected to space elevators valid or nonsense?

Firespawn

I am on Prozac. I asked my doctor if I can still drink, and she said "yes, SSRIs and alcohol affect totally different parts of the brain." Is this true? Do they really have no interaction at all in the brain? Sounds hard to believe

BlogWobbles

@TechHater
have you tried supressing the lines first (i.e. adding a semicolon at the end of command lines)

FastChef

I left an assignment until the last minute and I don't think I'll get it finished to a satisfactory standard in time - it's due tomorrow morning. I can finish it tomorrow so it's ready for the next day without any difficulties, but I'm scared of going to hand it into my professor as I have been a day late on the previous 2 assignments as well. I know I need to do them earlier, but for now what should I do? Find a time when he's out of his office and slip it under his door?

PurpleCharger

I need the integral of a continuous function, but it has no closed form. I thought about doing a fourier transform, is that possible? The function is periodical and composed mostly of trigonometric functions, but as I said, there's no closed form integral of it.
Or do I have to use numerical integration?

StrangeWizard

@Ignoramus
practice. do a ton of practice exams over and over. You'll find the specific things you need to study and get better at by doing them and seeing what you get stuck on.

whereismyname

Do I need to read baby Rudin as a right of passage? I glanced over the chapters, and the stuff it covers, I already know about from other books, and desu it's not hard stuff/concepts. The only reason I can see why baby Rudin would be considered hard is it condenses a lot of stuff in 300 pages, but that doesn't feel like "real" hardness. The book I read that covered all the same stuff was 900 pages long and it was a breeze.

Supergrass

@whereismyname
Do I need to read baby Rudin as a right of passage?
No, Rudin is a meme.

Dreamworx

Anyone good with probability and game theory?
If you have a game (player versus House) where there are strategies for the player, but you don't know the optimal strategy, how do you calculate the House Edge?

For example, let this be the game:
The dealer is dealt 4 cards to make his best 3/4 cards hand.
The player is dealt 3 cards plus an additional Power card (so 3+1, 4 in total) to make his best 3/4 cards hand. If the player uses his original 3 cards and beats the dealer, he gets paid 3:2. But, if he uses the Power card to make his best 3/4 cards hand and beats the dealer, he gets paid 1:2.
All ties push.

There is obviously an optimal strategy here, but if you don't know it, how do you calculate the House Egde?
In this case it's around 2.15%, but I'm not sure how to get there...

Emberburn

@whereismyname
Analysis is fucking garbage anyway so it doesn't matter.

TalkBomber

I need to show that for 2-cycles $(1a)(1b)(1a) = (ab)$.

To start with I don't know if what I'm doing is the correct way to arrive to what I want to show (I'm a beginner with Group Theory).

I did this:

$(1a)(1b)(1a) = (1a)(b1)(1a) = (1a)(b1a) = (1a)(ab1) = (1ab1) = (a11b)$

Then I don't know what to do... Tbh I don't know if I'm doing the correct thing here, lol. Somebody can help me?

Nude_Bikergirl

@TechHater
you forgot to close a square bracket just before that end

girlDog

@Nude_Bikergirl
he has such a shit life that he manually parsed a screenshot of a programming code in a text editor to get praise from some anonymous guy online
that is seriously sad, my friend

farquit

@girlDog
what exactly do you think the pupose of a stupid question thread is?
looking through 3 lines of code is also not exactly the greatest amount of time i've spent helping other people with their problems

@TalkBomber
I don't know what to do
do you even fucking know what cycle notation is?
https://math.stackexchange.com/questions/31763/multiplication-in-permutation-groups-written-in-cyclic-notation
https://en.wikipedia.org/wiki/Permutation#Cycle_notation
http://mathworld.wolfram.com/PermutationCycle.html

haveahappyday

@cum2soon

SomethingNew

@farquit
in all honesty Im just jealous you found the syntax error before I did.
I didnt get much sleep and spent the last our staring blankly at that screenshot, trying to find the missing semicolon.

upvoted

w8t4u

@whereismyname
right of passage
*rite

and fucking no. you should have no shame in going for the easiest dumbest exposition available, if you put the work in.
these terse books like lang's algebra are often only praised because they are slightly encyclopedic and contain many of the most-elegant proofs.

doesnt make them good introductory study material.

@SomethingNew
missing semicolons don't cause errors in matlab
if you get errors involving loops, the cause is in the loop most of the time so you just got to check those rather then the whole code

idontknow

Im trying to make fun of you here, stop being calm and informative

Carnalpleasure

How many ants could you eat in an hour?

Firespawn

@Carnalpleasure
bout 3k maybe

CouchChiller

@Carnalpleasure
@Firespawn
So I thought the question was interesting since I believe in the future insects will be used as foodstuff in most countries. There's research about this, so these are estimates.
I found the average person eats 1.8 kg a day, the average american 2.8 kg a day and there's a record of eating 4.75kg in an hour. An ant weights about 3mg, which means a million ants weights about 3k. Knowing this, the absolute most ants a person could eat in an hour would be 1583333 ants. Assuming the average person is trying to eat as much as he can, which I measure as half of his daily intake, would eat 600000 ants, 933333 if american. If we assume a normal meal is 30% of the daily food intake, an average meal of ants would be 180000 ants, 280000 for americans. For this calculations, it's reasonable to expect for the meal not to take over an hour.

DeathDog

@CouchChiller
Also forgot to say, I don't believe there'll be entire meals out of only ants, so the number of ant for an actual meal would be a lot lower.

SomethingNew

@CouchChiller
600000 ants, 933333 if american.
lol

Illusionz

@FastChef
GO and see him. You fucked up, it's your fault so you've got to face the consequences.

Firespawn

Let $(A, <)$ be a partially ordered set and $f: A \rightarrow B$ a function. Consider then the relation $R$ in $B$ given by:
$zRw \iff \forall x,y \in A$ with $f(x)=z$ and $f(y)=w$ , $x < y$.
Now, prove or disprove that if $f$ is bijective $\implies R$ is a partial order relation.

I don't really know how to use the fact that f is bijective to show that the statement is true or false. Should I use it to show the existence of an element in the codomain that is related to an element in the domain and then proceed to show the properties of a partial order relation?

LuckyDusty

@PurpleCharger
What do you need it for? If you just want to state your result using it. Just write it as in integral (look up the errorfunction on wikipedia for what i mean).

VisualMaster

@Firespawn
definition of partial order

eGremlin

Here is a really stupid question: I don't understand the concept of repeating numbers. How can a number be repeating? Let's say you divide a pizza into 3 equal sized slices....How can one slice be .333333333~ of a pizza? Is this just a "flaw" in our number system?

BinaryMan

@Firespawn
It obviously is. It an isomorphism. The structures are exactly the same except in name.

@Firespawn
That f is bijective means that for any z,w in B there exists unique x,y in A such that f(x)=z, f(y)=w. Now compare with the definition of partial order.

StrangeWizard

At what point does science become metaphyics? If, for instance, I ask "Does the fact that, when you take drugs, it changes your perception of reality, prove that there is no such thing as an objective perception of reality?". Is this a scientific, or a metaphysical question, or both?

Nojokur

@VisualMaster
@BinaryMan
Is the following right (the idea to prove this)?
Reflexivity
Take z in A such that z = f(x), then show that x is related to x by the fact that < is a partial order in A and conclude that zRz.

Transitivity
Take z, w, k in A and assume that zRw and wRk, then use the properties of < in a to show that zRk

Antisymmetry
Take z, w in A, assume that zRw and wRz and use the fact that f is injective to show that z = w.

Stark_Naked

@eGremlin
Is this just a "flaw" in our number system?
Yes, 0.333.... is the limit of the series $3 \sum_{i=1}^{\infty} (\frac{1}{10}) ^{i} = 3 ( \frac{1}{1-\frac{1}{10}} -1) = \frac{1}{3}$ .
Base 3 it would be 0.1. You can't have a base where this don't happens at all though.

Lunatick

I'm coding my first neural network. A simple classifier.

I have a training set with 3.000 x,y input points and each has the respective output class index (1, 2 or 3)

I convert the output to vector form, so class 1 is [1, 0, 0], class 2 is [0, 1, 0] and class 3 [0, 0, 3] therefore I have 3 output neurons with range (0,1).

Now, if input coordinates x and y are in range (-1.6, 1.2) then should I normalize them to (0, 1)?

Booteefool

@Nojokur
Yes, it's correct.

Harmless_Venom

@Nojokur
except you want to take your things in B. I.e take x in B and ask is xRx? Well for x in B there is a unique y in A such that f(y)=x and obv y<y and the definition of R then gives us exactly that xRx. Since we can do this for every x in B. R has the reflexive property.

And then do kinda the same thing for the other properties

girlDog

@Harmless_Venom
Ops, I meant z, w, k in B, not A.
Thanks, man.

@Booteefool
Thanks!

5mileys

@eGremlin
it's not a flaw with the numbers themselves, it's just a "flaw" in the way we are used to represent the numbers, this is a completely different thing. the decimal representation is not perfect and this is just one of the examples for why is it so, the quantity you're considering is better described as "1/3".

Bidwell

@TechHater
Missing a square bracket on preceding line my dear

Skullbone

@Lunatick
pls respond

DeathDog

@5mileys
And if we want to get fancy we could look at one of the definitions of real numbers and say that 1/3 represents the equivalent class of sequences of rational numbers that goes to 1/3.
This is if we look at 1/3 as a real number.

Even if we look at just the rational number the number 1/3 actually represents a whole equivalent class of numbers. The numbers 1/3 and 2/6 and 700/2100 are all in this equivalent class. And they are all (in some regards atleast) equally good representations.

eGremlin

@DeathDog
(the definition of the real numbers as the completion of the rationals under the absolute value as norm).

BunnyJinx

@Skullbone
Sure why not. It's not gonna make much difference. It's only bad if your features have a range difference that is too large, like x1=(-100,100) and x2=(0,1).

RumChicken

Thanks guys!

Need_TLC

@StonedTime
Do what you have to do, faggot.

Lunatick

I am not asking this for medical advice, I am asking out of scientific curiosity. Last night I drank a lot and ate a lot. Today, my stomach hurts terribly, as if my organs are aching and being squeezed. What exactly causes this feeling?

Lord_Tryzalot

@Need_TLC
faggot
Why the homophobia?

massdebater

@haveahappyday
Also, can someone explain hydrophobic interactions in a chain? I'm confused on what they mean by this.

Simple: a protein, in a cell, is in an aqueous environment. So a protein generally folds in such a manner that the hydrophobic amino acid residues are facing inwards and the hydrophilic AA residues are facing outward to the proteins surface.

w8t4u

Why is in $\vec{F}=G\dfrac{m_1m_2}{r^2}$ mass multiplied, instead of added? What difference does it make?

I read that multiplication and division in physics' formulas don't have quite the same implications/meaning as in mathematics, that is, its the relation between the variables that matters, take $\vec{F}=m\cdot\vec{a}$ for example, leave the mass as it is, but increase acceleration, the force increases accordingly. That's all what multiplication here means, the phenomena of $\vec{F}$ can be described with the components in which the change also results in a change of what is being described.

idontknow

@Ignoramus
is this supposed to be the definition of a limit or something else? def of limit is for all epsilon there exists a delta, not the other way around.

likme

Why is in $\vec{F}=G\dfrac{m_1m_2}{r^2}$ mass multiplied, instead of added? What difference does it make?I read that multiplication and division in physics′ formulas don′t have quite the same implications/meaning as in mathematics, that is, its the relation between the variables that matters, take $\vec{F}=m\cdot\vec{a}$ for example, leave the mass as it is, but increase acceleration, the force increases accordingly. That's all what multiplication here means, the phenomena of $\vec{F}$ can be described with the components in which the change also results in a change of what is being described.

not sure why it fucked up the first time...

Gigastrength

bump

Ignoramus

Can someone help me with the second part of this question, showing that J is minimized when p!=0 if you pick the quadratic plus/minus that's the same sign as p? Is it just a matter of doing a ton of algebra?

Stark_Naked

What is the difference between frequency and periodicity? I kind of thought they were the same until I came across this formula F = 1/P

Evil_kitten

@Ignoramus
It is quadratic in $c_2$. It is asking "what is the vertex of the parabola?".

5mileys

@Stark_Naked
The formula tells you everything. They're inverses of one another.

A frequency of 60 cycles per minute has a period of 1 minute per 60 cycles.

Period is how long it takes for a wave to repeat itself.
Frequency is the speed at which it repeats itself.

You can define a period as the distance from one peak of a wave to the next, or from one trough to the next, or from one midpoint to the next, etc. One full cycle of a wave.

Frequency is how close together those peaks/troughs/midpoints/etc are compared to the period.

You know frequency to have units of cycles per second, and that cycles aren't really a unit, so you can write frequency units as inverse seconds or hz. So the inverse of that is just seconds.

This unit analysis is how I got the two straight. Frequency is inverse time, and period is time, so period measures the time between waves and frequency measures how many waves per unit time.

cum2soon

@Stark_Naked
Period is the time it takes to complete a loop. Frequency is how many loops it completes per time.
Period: it takes 2 seconds to reach the end and start again
Frequency: In 1 second, it makes half a cycle.

Emberburn

How are you guys typing in that fancy font and using those fancy symbols?

TalkBomber

@Emberburn
This thread has been going for four days? Fuck! Nobody will ever answer me. I just want those fancy symbols.

StonedTime

@TalkBomber
You have to sell your soul to satan and pay in newborn baby's blood. Worked for me.
$Hail$ $Satan!$

VisualMaster

@Emberburn
you put text between a $[math]$ tag and a  [/math] tag

eGremlin

@VisualMaster
$Weiner$

CodeBuns

@eGremlin
$a^2 + b^2 = c^2$

$c2$

$(a/2)^2$

Stupidasole

@CodeBuns
You know, There is a TeX link in the reply box so you don't have to post to test stuff.

Illusionz

${{{I^{love}}^{to}}^{eat}}^{ass...}$

Crazy_Nice

@Ignoramus
Think about what happens to the term $-2\rho s_a s_b c_2$ when $\rho$ and $c_2$ have the same sign.

Booteefool

@Stupidasole
$HolyFuck$

Evil_kitten

@Crazy_Nice
Oh I see thanks. I'm a dumb cunt, I thought it was asking you to write out the quadratic equation and then show that if you took the sign of p for the sign of the discriminant that J is minimized, which I tried doing for a while but it was working out to be a ton of algebra that didn't go anywhere. Thanks a ton for helping me stop being a retard

StonedTime

@Lunatick
should I normalize them to (0, 1)?
Mathematically, it doesn't matter. Training will just scale the weights of the first layer accordingly.

However: from an implementation perspective, there may be some advantage to using a fixed-point representation, which would mean that all signals would be in the range [0,1] or [-1,1].

BunnyJinx

@likme
Because that's how gravity works. It's proportional to the product of the masses; if you double either mass, the force doubles.

With regard to physical models: quantities have dimensions (distance, time, etc), and it's meaningless to add (or subtract) quantities with different dimensions (you can't e.g. add a force to a distance).

If you find yourself adding dissimilar quantities, something has gone wrong; either there's an error in the formula or a constant has the wrong units. A common example of the latter is confusing weight and mass; particularly when force is sometimes stated in kilograms; in which case, you need to multiply by g=9.81m/s^2 to get newtons.

Nojokur

Is it too late to get into the "data science" meme? I understand that the term is a little ambiguous
Graduating with a CS B.S. this spring and my school recently started a masters program for Data Science. Is it still a lucrative field or am I just going into something that will be obsolete/automated in a few years? I'm already a research assistant doing data analytics/machine learning stuff so I kinda know the ropes

I'm a brainlet that's going to community college because my parents want me to (Thankfully my GPA was high enough for a free ride).
If I don't do good on calc 1 should I just retake it?

@Nojokur
Three words:

Topological
Data
Analysis

SomethingNew

What is the "smallest" algebraic closure of the rational numbers? Obviously the complex number is an algebraic closure but is there anything smaller?

What about Q extended with all the nth roots of every prime number and the square root of -1. Would that be enough?

massdebater

@SomethingNew
What is the "smallest" algebraic closure of the rational numbers?
$\bar\mathbb{Q}$
https://en.wikipedia.org/wiki/Algebraic_number#The_field_of_algebraic_numbers

Obviously the complex number is an algebraic closure
No, it's obviously not (it's not algebraic).

What about Q extended with all the nth roots of every prime number and the square root of -1. Would that be enough?
Probably not, $\bar\mathbb{Q}$ is notoriously hard to work with.

idontknow

@massdebater
should be $\bar{\mathbb{Q}}$

hairygrape

@idontknow
the field of algebraic numbers
I guess this makes sense. But what IS this field. What does this field contain? I assume it contains all the nth roots of every prime number and the square root of -1. But is there anything else it contains beyond that? (Besides obviously rational numbers themselves and combinations of all the previously mentioned objects)

@SomethingNew
What about Q extended with all the nth roots of every prime number and the square root of -1. Would that be enough?
this is enough for polynomials of degree less than 5, explained by a very nice galois theory result

@hairygrape
bring radicals are algebraic numbers that cannot be represented by combinations of nth roots of integers
while the set of rationals is a proper subset of algebraic numbers, they are both countable i.e. same cardinality

PackManBrainlure

>>9235699
yes it can
what does this have to do with A\Q ?

Carnalpleasure

bring radicals are algebraic numbers that cannot be represented by combinations of nth roots of integers

This makes me angry. Why are polynomials such assholes?

RumChicken

@Carnalpleasure
its more the properties of Q than polynomials

sqrt(2) is only shorthand notation for "the positive real solution for x^2 - 2 = 0"
we can express any algebraic number in a similar way, and approximate them by as many decimal places as you can be bothered to

Fuzzy_Logic

@RumChicken
If you took the potential energy of a human and perfectly converted it to energy a bomb would use, how big would the explosion be?

I mean a person can kill a few people with their bare hands but that's like concentrated on one point buuut it takes a long time for someone to get completely exhausted dead

Lunatick

I got a question like this.

f(x)= x^2/3-x
g(t)=(t^2+4)/3t

Find f(3)/g((2)+1)

I can't work it out because f(3)=9/0. Its not a very high level workbook though so I think I'm doing something wrong, I don't think they'd be throwing out trick questions or infinities at this point.

@Lunatick
is f(x)=x^2/(3-x) or f(x)=(x^2/3)-x?

SomethingNew

@Lunatick
As you have it written, order of operations says that it's $\frac{x^2}{3}-x$. So $f(3)=\frac{3^2}{3}-3=0$. But if it's $\frac{x^2}{3-x}$ then it's undefined.

Gigastrength

No idea because the tutor hasn't specified. He's not put brackets on any part of of the function, so I guess I'll assume its x^2/3 subtract x.

DeathDog

is it possible to become smarter if you're naturally retarded?

LuckyDusty

I want to get a double major.

I'm studiying cs right now, and i'm planning to do math too, as it's interesting as fuck and it will help me with machine learning and data sciences, the topics i want to be the expert of.

I'm a freshman in this uni, but studied two calculus, linear algebra and intr physics at a garage community college before and it went pretty well. Right now, the differential calculus i'm taking here it's pancake, but i can foresee this college being more demanding.

It's a public university here at /notamerica/, so it's basically free (Universidad del Valle, Colombia)

Is it worth it? Any tips? Will i burn and drop out?

FastChef

why is time dependent on light?

ZeroReborn

@LuckyDusty
hey same here! join me in my double major struggle, it's worth it if u like math

Stark_Naked

Can someone explain to me how to derive a Mean Square Error(MSE) using Residual Sum of Squares(RSS) through Ordinary Least Squares(OLS)? I've coded an OLS estimator that outputs my beta. I don't know how to get MSE. Any help would be greatly appreciated, thanks.

w8t4u

Why do people vomit when they become dizzy or disoriented? I understand the relation between the inner ear and balance but cannot find any connection between that and why it would cause you to throw up the contents of your stomach

@Lunatick
You ingested poison and your body is letting you know

Emberburn

I was looking at math books and the American version was $100+ The international version (printed in English) with the exact same contents was$12 and had a warning on it saying "illegal for sale in united states/canada"

Can someone explain this bullshit to me? Why do Americucks put up with this fuckery?

RavySnake

@Emberburn
freedom aint free

iluvmen

how do i determine interatomic bond length and stiffness from the density of a metal and its grams per mol?

farquit

Chemistry question:
I heated bees wax with olive oil and honey. I accidently heated it to the point that the sugar in it caramelized.

Could I boil everything in water and hope that the burnt sugar mixes with the water while the oil and wax floats to the top? It's still usable as it is, but smells nasty. I want to get rid of the burnt sugar smell.

Carnalpleasure

@Emberburn
America is a corporatocracy.

Sharpcharm

because textbook publishers are assholes and universities don't care

BlogWobbles

@farquit
You could try even without boiling, maybe just heating a little.
Try on a small amount first.

*Results may differ.

SomethingNew

I know how to compute and represent convolutions integrals, but I don't get what they suppose to mean, what's their purpose.
Can someone explain that to me?

cum2soon

@farquit
@BlogWobbles
So I've tried gently boiling it.
The burnt sugar particles settled at the bottom in the water, but the smell stuck to the oil. Oh well.

TalkBomber

Simple statistics & probability question:

Let's say that I'm playing a dice game. I have to choose a number between 0 and 99 and bet on the lucky number being higher or lower than the number I chose.

If I lose, the next bet will be increased by a percentage until I either win or run out of balance.

How do I calculate the probability of running out of balance (the amount that I would have to bet is higher than the balance itself)?

e.g

Betting that the lucky number is going to be less than 5. What's the probability of losing the total balance of $3.000.000, if my initial bet is$50 and each time I lose the bet gets increased by 5.84%

Betting that the lucky number is going to be more than 24. What's the probability of losing the total balance of $3.000.000, if my initial bet is$250 and each time I lose the bet gets increased by 300%

Which is more risky? This one I can tell just by calculating the probability of running out of balance.

Any help will be appreciated. I got stuck trying to make a probability-of-getting-rekt calculator and Google isn't helping. I can calculate the number of times that it takes to run out of balance but got stuck on getting the probability itself, I know it's a very basic question.

Garbage Can Lid

Let's say I have an excited state with life time $\tau=26$ns, and I know the frequency of the absorption/emission for this excitation.

How do I find the full width at half maximum (FWHM) for the spectral line?

I have Heisenbergs uncertainty principle
$\Delta E\tau \geq \frac{\hbar}{2}$

Is the FWHM $=\Delta E$
Or do I have to do something more?

kizzmybutt
VisualMaster

Is there a better way of doing this?

Like Plot every 1000 between 0-10000 without putting each one individually

ZeroReborn

@VisualMaster
This is what I get, but I want to see what it would look like with 10 or even 20 points

SniperWish

@LuckyDusty
univalle
CS
Systems engineering?

I want to double major here at the unal too, but it will take a few more years to graduate and I already got into college a bit old. I don't know if its worth it either.

BlogWobbles

@SniperWish
I'm probably just getting a stats grad degree desu

Crazy_Nice

Which reagents would you use to get the upper chemical to recieve the lower one?

Emberburn

I am having a brainfart. I already have a partition coefficient and I am trying to find the molar concentration that would be in the octanol [Co] and aqueous [Cw] phases.

How do I do this? P = 38.905

@Harmless_Venom
Use the mobile version of the site to get free step-by-step solutions. Works for me for some reason at any rate, might not for others.

Harmless_Venom

I forgot to add, you need to open the "step-by-step solution" page in another tab to get the solutions.

Poker_Star

@VisualMaster
You could do this with a loop:
hold on
for u=1:1000:10001
plot(xx,Xi(:,u),'-')
end

Instead of having an x amount of lines, you could also 'animate' it with hold off and having a pause in a loop like in
@SomethingNew

Raving_Cute

Sharpcharm

@Nude_Bikergirl

Garbage Can Lid

I'm struggling to prove this identity following my professor's instructions.

Here's what i've done so far.

Sin(alpha), gives us that BD=Sin(alpha)

From these results,

Cos(Beta), gives us that AC=Cos(alpha)/ Cos(Beta)

Sin(Beta) gives us that Sin(Beta)*Cos(alpha)/Cos(Beta)=CD

But i'm not sure how to find the area of the triangle in two ways to get our desired result.

Am I even on the right track?

girlDog

What is a good strategy for writing out lewis structures in alternative ways contrary to their most common states ie non zero formal charges?

massdebater

@Garbage Can Lid
Figured it out. Didn't realize there was an area formula using SAS triangles.

Raving_Cute

A metal requires a photon of wavelength 250. nm to just eject an electron with no kinetic energy. If a photon of wavelength 200. nm strikes the metal, what will be the velocity of the electron that is ejected?
Question 2 options:
6.6 x 105 m/s
3.2 x 105 m/s
4.7 x 104 m/s
8.2 x 106 m/s
2.1 x 106 m/s
Determine the INCORRECT statement regarding ionization energies [ IE(1) means first ionization energy, IE(2) is second ionization energy, etc....]
Question 3 options:
IE(2) P > IE(1) P
IE(2) Na > IE(2) Mg
IE(2) Na > IE(2) K
IE(1) S > IE(1) Te
IE(1) Cl > IE(1) F

Which one electron configuration corresponds to an excited state of a neutral halogen atom (Group 17 or 7A)?
Question 1 options:
[Ne]3s24s2
[Ne]3s13p6
[Ar]4s23d24p4
[Ar]3d5
[Kr]5s24d105p5

LuckyDusty

@RumChicken
simple noob physics question
if a certain force is required to shear a metal sheet with a flat blade

does having the blade at an angle so that at any given moment the blade is in contact with only a part of the sheet
require a constant force only enough to sheer that part
will it eventually cut the whole sheet this way?

do scisors and guilotines work on this principle?

WebTool

I take too long to solve the most basic problems. How do I fix this?

Flameblow

@massdebater
It's orthogonal when you consider a proper interval and weight. The orthogonality bit becomes useful in physics when dealing with quantum mechanics problems like the harmonic oscillator, where they form the eigenstates of the system.

I'm guessing it will have little effect besides burning your tongue regularly.

@WebTool
Practice.

CouchChiller

I've got questions on linear equations and I was doing perfectly fine until this one, I cannot for the life of me figure out why this won't work.

A companies sales obey a straight line law and fit the following figures:

1983: 22000
1992: 36000

So of course I did y2-y1/x2-x1 and found the slope to be 9/14. I put 1983 in x and 22000 in y, and subtracted the x value to get c, and got 20725. But then do the sum with 1992 as the x value and it doesn't work, it gives you 22005.

So then I thought "maybe its the years" and tried 1 and 9 as the x value since maybe the graph started at 1983, still nothing.

Can someone explain to me what I'm doing wrong, because its really bugging me.

JunkTop

Is it theoretically possible to charge a Li-SOCl2 battery under the right conditions?

They can rupture from heat right? So what if you kept it cold, like extremely cold? (CO2, Liquid Nitrogen, etc) and charged it?

I notice there are some 3.6v/2200mah batteries with a 560 wh/kg density, but not chargeable.

Lunatick

@CouchChiller
So of course I did y2-y1/x2-x1 and found the slope to be 9/14

do it again

kizzmybutt

I saw my mistake and got the equation:

y=1555.5x+20444.5 and if you put 1 as the x value you get 22000, but if you put 9 (for 1992) you only get 34000. But then if you put 10 as the x value you get 36000 almost perfect. How is 1992 10 on the x axis?

Carnalpleasure

@kizzmybutt
you started at 1, and delta x is 9
so x2 is delta x + x1
10 = 9 + 1

Flameblow

Ahhh, thank you. This is why I should take breaks.