/sqt/ Stupid Questions Thread

how do i study everyday for more then an hour and not feel tired

why is the GRE so expensive? any way to get waivers like the SAT?

I'm self-studying Abstract Algebra (Topics in Algebra by Herstein) and when I read the chapters I feel like I'm understanding the material, but when I get to the problems I can usually only manage like a third of them. Should I put it down and get How to Prove It or something?

The flow is tangential to the circles and it takes 1 arbitrary unit of time to cross from one circle to the next.

nice one brother. too much fuckery on this board. I was going to make a QTDNTOT thread but saw this. hopefully all they brainlets placing garbage, time wasting threads all over the board will stop and post here. lets make this a regular thing. QTDNTOT sci edition soon. the board space will hopefully free up for more beneficial threads for the people on here. the mods should be ashamed.

As you do algebra before simplifying it.

>I'm self-studying Abstract Algebra (Topics in Algebra by Herstein) and when I read the chapters I feel like I'm understanding the material, but when I get to the problems I can usually only manage like a third of them

That's normal. Just keep working on the problems.

>Should I put it down and get How to Prove It or something

You don't know how to do proofs?

I understand how to do them, I'm just... bad at them it seems. I'm an EE so I didn't take a lot of proof-based math courses.

What's the intuition behind inference with gaussian processes? I wrote a program for it but I've got no idea where the formula for [math]\mu[/math] and [math]\Sigma[/math] come from.

That or I'm just underestimating how hard the problems are, since the ones I can solve I can usually do in my head.

getting good at proofs goes like this:
0. understanding what constitutes proof. things like implications, necessary and sufficient conditions, and logic are key.
1. understanding common proof techniques e.g. direct proof, proof by contradiction, induction, etc.
2. being familiar enough with common tricks and the material you're working with to employ them in a proof.

there is more of a learning curve because you are not being taught some method of computation which you can apply to different kinds of problems. you're constructing an argument.

>Zip-Line Into River. Work out the mathematics of the situation from the provided videos. First carefully watch all videos and approximate all relevant distances. Find the slope of the zip-line and the speed of the person using the zip-line. Note that the person will undergo projectile motion once he lets go of the rope. Find how far from the shore he landed (on either side). Videos can be found at: youtube.com/user/piano4life18
This meme problem is really confusing me , isn't the slope of the zip line the distance of the zip line over the time it tooks to glide on it how is that diffrent from the speed (besides the sign)
Anyone has an idea how i tackle this problem and what i should calculate

>tangential

Is there an official name for the space that is perceived to exist on the other side of a mirror? I'm doing a project involving mirrors and I've been referring to it as "Virtual Space" since the image you see in a flat mirror is called a virtual image. This diagram calls it "the mirror world" which just sounds silly. Is there actually a name for this?

Thak Scrotungus.

Context: Physics, Dimensions.
Hypothesis: Velocity equals Motion over Resonance plus Vibrancy.
Explanation: The speed at which an object can be taken from one dimension and brought into another is solely up to the decision-making of the individual taking it.

Question: Having factored in simple stuff like Uncertainty Principle and Planck Length; what other maths exist to assist if proving this can be done? I'm happy to come up with my own, but that kind of thing does go over well in the community. I'm not gonna go expressing stuff like this in public until I've got filmable evidence.

pls helb

Is this a homework problem? How are you supposed to approximate anything remotely close to accurate from those videos? Also video 3 is hilarious.

Let [math]A=\mathscr{P}(\mathbb{R})[/math] and let [math]f:\mathbb{R}\to A[/math] be defined as [math]f(x)=\{y\in \mathbb{R}|y^2

>Is f onto (a surjection)?
obviously not

If I have a multivariate continuous probability distribution, and want to know the distribution of one of the variables at a fixed value of the other variables, how would I go about that?

I have a wavefunction of three variables, and I want to fix one at its expectation value, then vary another through a number of given values, and at each given value find the probability distribution of the third variable. Is there a physical way of doing this? I thought it could be as simple as just setting the other two as constant and then treating it as a single variable wavefunction.

That's what I thought, but this person thinks otherwise.
inchmeal.io/htpi/ch-5/sec-5.2.html
I thought maybe I was missing something.

Which solution is it supposed to be?

Sorry, it's solution 6 part b.

I can't tell what that solution is supposed to be saying with {q in R}, but an easy way to tell it's not onto is that every set in the image is 'negative symmetric' in the sense that if y is in the image then -y is in the image. So any subset of the real numbers that contains a number N but does not contain its negative -N can not be in the image of f

>an easy way to tell it's not onto is that every set in the image is 'negative symmetric' in the sense that if y is in the image then -y is in the image
should be: an easy way to tell it's not onto is that every set S in the image is 'negative symmetric' in the sense that if y is in S then -y is in S

My reasoning was that [math]f[/math] is not onto because there are subsets of [math]\mathbb{R}[/math] that are not connected intervals that can't be derived from the formula for [math]f(x)[/math], which are all connected intervals.

>My reasoning was that f is not onto because there are subsets of R that are not connected intervals that can't be derived from the formula for f(x), which are all connected intervals.
That's fine too

Many you should practice with easier math courses like combinatorics or graph theory.

>proof-based math courses
That's the only kind.

>Is f onto (a surjection)?
No, it doesn't even matter how f is defined. There is no surjection [math]f : A \to \mathscr{P}(A)[/math] for any set [math]A[/math]. See en.wikipedia.org/wiki/Cantor's_theorem

But neither of those are math courses.

except they are

It's rather obvious that a vulgar peasant such as yourself wouldn't be able to see the truth.

I suppose to you symplectic geometry isn't math either..

This post is pure retardation.

Yeah, Im juts gonna guess the distances and angles, just a bit confused ofbwhat should i do

What's the best resource to learn the Wolfram Language?

I know some elementary programming (really basic stuff with C, Fortran and Assembly, but never used any of these outside school) and I often use Mathematica for simple calculations, but that's about it.

It's ok. Abstract Algebra is a great place to start getting used on proving stuff.

>If I have a multivariate continuous probability distribution, and want to know the distribution of one of the variables at a fixed value of the other variables, how would I go about that?
The other variables having a fixed value is a 0 probability event.
P(A|B) "is not defined" when B has zero probability.
Look at this though:
en.wikipedia.org/wiki/Conditional_probability#Measure-theoretic_definition

>but when I get to the problems
Just treat the theorems themselves as problems. Try proving as much as you can without looking at the proof given in the book.

meant for

I haven't found a single best resource for it. There's things like the introduction Stephen Wolfram wrote (wolfram.com/language/elementary-introduction/2nd-ed/) but it's rather basic.

What helped me the most is looking at semi-complex examples and taking them apart piece by piece to figure out how exactly they worked. There are some good candidates for this reverse engineering in the feature showcases for new versions of the language (e.g. wolfram.com/mathematica/new-in-11/) and the "Neat Examples" sections of the documentation for various functions (e.g. reference.wolfram.com/language/ref/GradientOrientationFilter.html).

en.wikipedia.org/wiki/Conditional_probability_distribution#Continuous_distributions

Brainlet here.
If a key has 50% chance of opening a chest that has a key in it and that key in the chest has 50% chance of opening the second chest. What is the chance of opening the second chest?
Also if a study says that 1 out of ten people is homosexual. What are the chances that a group of ten men has a gay man?

And say how you solved both. Thanks.

1/3 for the first question

Second question is poorly worded. Could either be 1 or 1 - 0.9^10

Not telling you why tho ;^)

why not?How do you even come to a response like that?
brainlet too

0.5 chance of opening the chest, so 0.5^2 of opening both chests.
0.1 chance of a faggot, so 0.1^10 chance of one of them a faggot

Hey Veeky Forums
I feel like a total fucking retard.
Whenever I try to solve most math or math-based programming problems, my brain just shuts down.
Take for example something I was just working on:
Write a program to find the 1000th prime number.
I have no goddamn clue where to start when solving something like this! Can someone point me in a direction to increase my ability here?

First question map out the possibilities: either the first key doesn't open, either the first key opens and the second key doesn't open, either the first key opens and the second key opens. That's one valid scenario out of 3 scenarios, hence 1/3.

Second one, the probability that there's a faggot in the group is equal to 1 minus the probability that there are no faggots in the group. There probability of being a faggot is .1, therefore the probability of not being a faggot is .9, so that the probability of all 10 people not being fags is .9^10, giving the result 1 - .9^10

Implement the sieve of erasthothenes.

lol

Thanks for the answer, but what I am looking for is more like how do I figure these things out myself? More specifically instead of feeling completely lost when approaching a problem, how can I figure out what steps to take to begin to solve it?

Well I mean I kinda knew that algorithm from experience, I guess the question supposes that you know it too.

Break the problem down into manageable parts. You might start out with something like[eqn]
\texttt{i = 0;} \\
\texttt{n = 2;} \\
\texttt{while i < 1000:} \\
\ \ \texttt{if is_prime(n):} \\
\ \ \ \ \texttt{i++;} \\
\ \ \ \ \texttt{n++;} \\
\texttt{print(n);}
[/eqn]without actually knowing how to implement [math]\texttt{is_prime}[/math], but now you've broken the problem down from "how do I find the 1000th prime" to "how do I decide if a number is prime" which is a less difficult sub-problem of your original problem. Eventually you'll have accrued enough knowledge to know that there is a standard and more efficient algorithm for this like pointed out, but when you're starting out it's okay not to be focused on the absolute best solution if it means you can get started.

Oops, n++ should not be double-indented.

Let V be a finite dimensional normed vector space over the Complex numbers or the Real numbers.
Let e1, ... ,en be linearly independent elements of V with ||ei||=1 for all i.
Consider a linear combination v := c1 e1 + ... + cn en

Does |ci|

You can express it is a conditional multivariate using Bayes'.

I was working through some problems on the Challenging problems in algebra book to kill some time. For this one, the solution given in the book for the challenge is 8, but I get 24 (1243,1342,2134,2431,3124,3421,4213,4312,12034,12430,13024,13420,21043,21340,24013,24310,31042,31240,34012,34210,42031,42130,43021,43120)
Did I not understand something or is the book wrong?

wait lemme get this straight, that's just a linear combination of unit vectors right?

What's a bibliography database used for? Is it supposed to contain only all the works you've ever read or also the pages you find interesting?

I'm setting mine up right now, using JabRef, but noticed that there doesn't seem to be a way for a BibTex-key to contain multiple page ranges. If I say:

pages = {5--49}, {90--118}, {200-202},

It'll delete the latter two ranges:

pages = {5--49},

Am I supposed to put interesting pages into the keywords or comments instead? But then I can't cite it. Might as well not use JabRef then.
How am I supposed to know 6 years later where that noteworthy quote or table is? Having to go through the entire book/paper/... again just to find what could be appended to pages = ... seems clumsy.
Am I supposed to have an one entry per citeworthy passage? But then the entire thing becomes huuuuge and filled with countless similar-looking entries.

consider an arbitrary [math]u[/math] of unit length and put [math]v = -u[/math] but rotated by an arbitrarily small angle so that [math]u[/math] and [math]v[/math] are linearly independent. does your conjecture hold for this set of vectors ?

What I meant was what are the chances of the group having a faggot not a person in the group being a faggot.
If it's 1 out of 10 then the chances of the group of 10 people having a faggot in it is 100% but that's inaccurate.
Excuse my Englishletry

So how do i calculate arc length and why is it 10*x?

pic related

rtheta

>solving a problem
>my brain literally starts to hurt
is this normal

Why do structures such as the Solar System and galaxies form in disks rather than a general sphere? Obviously a single orbit will be in a plane, but why do they all line up?

Yes.

Oh right, if it held, then it would be 1 = |1|

I don't think so.

[math]
r(t):=10(\cos t, \sin t) \\
\frac{d r}{dt} = 10(-\sin t , \cos t) \\
\lVert \frac{d r}{dt} \rVert = 10 \\ \\

\text{Arc Length } = \int_{0}^{x} \lVert d r \rVert = \int_{0}^{x} \lVert \frac{d r}{dt} \rVert dt = \int_{0}^{x} 10 dt = 10x
[/math]

How do you simplify the [math]E_{period}
[/math] equation in pic related? I'm assuming j stands for [math] \sqrt{-1}[/math], but even then, I don't know how the expression simplifies to one.

t. engineer

e^(j x) always has absolute value 1 when x is real (it's on the unit circle), and in your case w_0 and t are real

I mean... You're not wrong.
I just have no idea how they got it

Okay. That makes sense.
For some reason, despite seeing the whole e^ipi thing many times over, I still can't wrap my head around complex exponentials...

This has to do with the way in which the solar system formed. In short, a large cloud of gas collapses spinning faster and faster as it does (think conservation of angular momentum) and centripetal forces warp it into a disk

reread, the actual question asks for 1234, 12034 is a seperate Challenge question (which you correct btw)

>le centripetal jew

I'm old and brainlet and trying to remember quantum mechanics I had a lecture series on from more than a decade ago. Please tell me if I am remembering this correctly.

If you have a quantum mechanical system then that system has a wave function.
If you want to calculate a physical quantity of that system then you need to apply the correct operator to that system, e.g. the momentum operator.
When you apply that operator, you will only get a deterministic value if the wavefunction is an eigenfunction of that operator, otherwise you will get a probability distribution of the possible values that physical quantity could take.
My questions:
1. is what I have said correct so far?
2. for a wave function to be valid, does it need to be an eigenfunction of the hermitian operators of physical observables? i.e. is it impossible to have a quantum system whose wave function is not an eigenfunction of say the momentum operator? Or the hamiltonian operator?
3. if the operators of physical quantities can be applied to valid wave functions that are not eigenfunctions of those operators, then how do you obtain the probability distribution of possible measurements of that physical quantity? Is it simply the result of applying the operator to the wave function or do you need to integrate over it and multiply it by its complex conjugate or something?

>thinking I give a shit about idiotic semantics arguments

If 2 hydrogen atoms are bonded together, what does that visually look like? Is it like 2 planets which both orbit 2 suns in a figure 8 pattern?

>It usually represents an arbitrarily small width of some square.

physicist kys

Those names are as good as any.

no, it's quantum mechanical shit bruh

Really? How do you know if you mind be asking?

Also I believe that an OH molecule is able to exist for more than 10 minutes: just 1 oxygen bonded with 1 hydrogen. I might be wrong.

Time dilation. Time perception remains the same for two different observers moving at different speeds right right? . Entertain this absurd scenario involving two people: somehow we're able to live stream footage, say through Skype, ( let's also assume the delay is a negligible) of Person A on earth in a room and Person B on a ship . Person B is moving at the speed of light. Person A experiences less time right? Would they be able to perceive each others different age rates? Let's also assume they've got nothing to do but watch each other till either one dies. How's that for a stupid question..

what are some good biostatistics books?

|z|^2 is short hand for z* times z where z* is the complex conjugate.

(e^ix)* (e^ix) = (cos(x) - isin(x))(cos(x) + isin(x)) = cos^2(x) + sin^2(x) = 1

Thanks man!!
After I got the answer initially, I re-convinced myself that e^(ix) = i(sin x) + cos x, and then it hit me what taking the "absolute value" of an imaginary number was and then felt like a moron.

Oh well. Better now than later, I guess

>Person B is moving at the speed of light
this is impossible in a way you can't just hand wave away, lets assume you mean just a significant fraction of c, enough to cause noticeable time dilation. You have to consider the fact that it takes time for the video stream information to travel from one person to another, and this again this isn't something that you can ignore, the limit on the speed of information is a very fundamentally important one and is one way to explain that that Person A does indeed perceive Person B to age slower while Person B perceives Person A to age slower (from either one's perspective the other is the one with a velocity near c)

>1.
No. Sounds like what people would gather from skimming Griffith. Go read an actual QM book like Townsend, Sakurai or Landau-Lifshitz.
>2.
Eigenfunctions are representation of eigenstates of observables on [math]L^2[/math]. Eigenstates exist for any diagonalizable operator you want, since Hilbert spaces have [math]\mathbb{C}[/math]-linear representations. Now Hermitian operators have the nice property that if the Hilbert space is separable, then the eigenstates of the Hermitian operator spans the Hilbert space. This means that your space of states can be characterized by eigenstates of the Hermitian operator, which is very useful as it tells you immediately what "sector" of the Hilbert space corresponds to what measurement.
>3.
As I've said before, the eigenstates of an observable spans the Hilbert space, hence any vector in it can be written as a linear combination of them. The probability distribution is just the squared sum of all the coefficients. Namely if [math]\{|\psi_n\rangle\}[/math] are the eigenstates of an observable [math]A[/math] acting on the Hilbert space [math]\mathcal{H}[/math] such that [math]\mathcal{H} = \operatorname{Span}_n\left(\{|\psi_n\rangle\}\right)[/math], then for any state [math]|\phi\rangle \in \mathcal{H}[/math] we can write it as [math]|\phi\rangle = \sum_n c_n |\psi_n\rangle[/math], then the probability distribution of the results of the measurements of [math]A[/math] on the state [math]|\phi\rangle[/math] is [math]\sum_n |c_n|^2[/math].
Of course all of this is true only for single-particle systems in a pure state. Mixed states are characterized by the density matrix [math]\rho[/math] which mixes different eigenstates of an observable in different factors [math]\mathcal{H}_i[/math] of the total Hilbert space [math]\mathcal{H} = \bigotimes_i \mathcal{H}_i[/math].