/sqt/ - Stupid Questions Thread

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

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

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

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

Help Veeky Forums, stuck on matlab

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

e(x,0) = 0 for 0.00

Would it be

>m-->(e v p)

I wish these textooks had answers for even number question mane.

Forgot image

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

m iff (e or p)

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?

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.

Is there anything known about the integral:

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

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

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

[math]\sec \theta - \tan \theta = \frac{1}{\cos\theta}-\frac{\sin\theta}{\cos\theta} = \frac{1-\sin\theta}{\cos\theta}\cdot[/math]

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

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

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

thanks man

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?

Stokes Theorem?

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.

>You are correct the coin tosses are independent
Prove it.

Prove they're not.

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 [math] e^{inx} [/math] 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).

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

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

Also when calculating the [math]\vec{i}[/math] component of a vector [math]\vec{AB}[/math] for example, why is it [math](x_B-x_A)\vec{i}[/math] instead of [math](x_A-x_B)\vec{i}[/math]?

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

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

How do I raise my logic stat?

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.

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.

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?

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

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"?

Can you explain that last line a bit?

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

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

How many dimensions does the number 0 have?

I'm high, please overstand.

it depends which space you are working in

e( (e>.25).*(e

Is that it?

How about real and/or imaginary dimensions?

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

on
[math]\mathbb{R}^3[/math]
[math]0 := (0,0,0)[/math]

it's so pretty!

>Indeed

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.

>Parmenides

My fucking man.

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.

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

Can someone make sense of the last line for me?

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?

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

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.

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: arxiv.org/abs/0905.2391

#

I think you had the equality flipped.

Has anyone actually tested their IQ before and after uni?

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

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.

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

is geometry just subset of algebra?

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

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?

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.

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

anyone ever used the Runge Kutta method?

I can't see where I'm going wrong

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

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

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

I made a little mistake. It should be
[math] T = 2(aa^T+bb^T) - I [/math]
for example the x-y plane is spanned by [math] a = (1,0,0)^T, b = (0,1,0)^T [/math], 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]

thanks bro!!

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

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?

The inverse of [math]f(x) = x^3-2[/math] is [math]f^{-1}(x)=(x+2)^{1/3}[/math].
Now, the domain of [math]f^{-1}(x)[/math] should be the range of [math]f(x)[/math], which is R. But [math]f^{-1}(x)[/math] is not defined for [math]x

>not defined for x

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.
this leads to the advection part of your differential equation to get swallowed.
the solution should move in positive x-direction with speed k, if I'm interpreting it correctly

Forgot to say that [math]f: R \rightarrow R[/math]

>not defined for x

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

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)

I'm just going to stick to Euler,

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

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

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
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.

I'll check them out, thanks.

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

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

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

Ie. left,right and on your back.

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

Woah thanks that's really cool

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.

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

parabola

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

How do I higgs my own boson?