/sci/ - Science & Math

Mods should make /sqt/ an infinite sticky with rolling answers, it would be awesome. How could we propose this?

>>12777403
Personally, I wouldn't support it. Would bring unwanted attention. Wouldn't want this thread to become a tourist thread.

How viable of an option is it to take premed courses at 28 years old with a BFA in graphic design to go to medschool and pursue a career in psychiatry? Is the 10 year commitment of time and money worth it in your opinion at this point in life? Doing design and art direction is decent money but consumer marketing is at the pinnacle of things I hate about humanity.

>>12777403
>How could we propose this?
Make a thread on >>>/qa/ and harass the mods on IRC.
There's also >>>/feedback/

first dumb question. are there any proteins where the only difference between two proteins is a single nucleotide?
to word it differently let's say i have nucleotide that's different than what it's supposed to be, given that it will result in a different amino, is the protein going to misfold no matter what or is it possible that there a different protein whose only difference is that single nucleotide? i kind of can't wrap my head how all vital stuff seem to code properly 99.99% of the time when there is so much room for it to go wrong, you never see misfolded hemoglobin

>>12777391
Is there any way to reduce the probability of pair production in a system? For example, as I understand it, every photon of a sufficiently high energy has some probability of pair producing an electron-positron pair. Is it possible to change the environment in some way that impairs this, such that you can have many high energy photons that have low or zero probability of pair producing?

I have a question about the Monty Hall problem. If I make a mistake I hope you can still understand the question.

Your odds are 1/3, host reveals empty door, switching increases odds to 2/3 is my understanding. What if another person, uninvolved until this point, is allowed to select a door of the two remaining? Is their chances of being right 50/50 while yours are still 2/3? Is the problem only a problem because the host knows what's behind each door?

>>12777391
future proof academia careers ?

or any flexible jobs that wont be replaced by bots ?

/ is E.E a future proof job

if protons attract electrons, why do electrons hover outside of the nucleus like betas rather than merging with the protons to enter a lower energy state by fusing into a neutron? it should tend in that direction, and yet it doesn’t
I’m fucking retarded, please explain this with as little quantum physics as possible

>>12777391
I'm reposting from another thread:

>Apply to graduate math program
>Get an email from a professor there
>Next day I receive an email from admissions officer: Congrats, you've been accepted anon. Response required.
>Respond a few hours later
>Never hear back from admissions officer
>1 week has passed
>Send another email yesterday, this time to the professor
>Still haven't heard back

Wtf is going on? Is this normal? I'm sure they're pretty busy, and I don't want to pester, but it seems really strange that they would be ignoring me, and my online application still says that it's awaiting a decision. There are supposed to be 1 on 1 meetings between faculty and new student next week, and I still haven't heard anything.

>>12777871
the easiest explanation is that it's a balance of energy.
imagine a planet orbiting a star. it has a gravitational potential energy (analogous to the attraction between protons and electrons) and a kinetic energy as it orbits. in general, these two effects are opposites: if you move a planet closer to the star, lowering its potential energy, it will want to orbit faster and more erratically, increasing its kinetic energy.

the analogy breaks down because with macroscopic objects, there are ways to continually dissipate energy until the orbiting objects collide. the small amount of quantum you need to understand for why this doesn't happen with protons and electrons is that 1) energy is quantized, meaning you can only have proton+electron configurations with certain energies, and 2) the uncertainty principle implies that trying to localize a particle (bringing the electron closer to the proton) will give it a massive momentum spread (greatly increasing its average kinetic energy, which depends on momentum squared). so, states where the electron is closer to the proton than in the ground state actually have a higher TOTAL energy than the ground state, because the effect on the kinetic energy is much higher than the decrease you get from the potential energy.
since nature tends towards the states of lowest energy, you get a stable configuration of electron+proton.

also of note is that slamming electrons and protons together doesn't create neutrons in the traditional sense. it's a more complex interaction where often times it's more energetically favorable to create a stable atom than for the two to interact and produce a neutron.

>>12777910
If they said you've been accepted then you've been accepted and shouldn't worry too much.
Try calling them tho, it might have gone to their spambox.

>>12777910
usually these "response required" emails should link to the application page for you to accept/decline admission (at least in the US).
I would call admissions tomorrow if you don't hear back.

>>12777965
this explains it perfectly, bless your kind heart

>>12777848
"the bots will replace us" has been going on since the 1800s. what really will happen is that the skill ceiling is going slightly more up every year. a generation or two ago unpaid internships at shitty companies were ridiculous now it's a must. you want money go to whatever sector is booming, spoilers it's entertainment, health and CS

This was in my culture which has been contaminated, and I have no clue what it is. Do any of you know?

what is this called and how do I write it in LaTeX?

>>12778162
\partial

>>12778168
domo

>>12778162
https://detexify.kirelabs.org/classify.html

>>12777828
It depends upon what you mean by "uninvolved until this point". If they don't know which door is the original choice and which is the "switch" choice, it's 50:50.

To evaluate the probabilities, you need to model the host's behaviour. The standard assumptions are that:
a) the host will always open a door
b) the host will never open the door the player picked
c) the host will never open the door hiding the car

So when there are two closed doors and one open door, each closed door can be closed because:
a) it is the door selected by the player
b) it is the door hiding the car
c) both of the above
Knowing whether the player initially chose the door affects the probability of it being the door hiding the car.

Do gravity wells more closely resemble the top-left or top-right, in pic related?
black = vaccuum potential due to gravity
yellow = massive object (SMBH, star)

Wouldn't it make more sense if the deviation from the vaccuum due to an object's gravity well "integrated" (averaged?) to zero? It doesn't make sense that something unipolar would only ever be negative WRT vaccuum potential.

Would the top-right diagram explain how gravity waves work - rises and falls is gravitational potential?
And for the bottom diagram, would the discrepancies between the two curves explain the "dark matter" (here the object on the red curve appears less massive than the blue curve) of orbiting objects?

Is Khan Academy good? I find myself frustrated by the format of their tests sometimes

to clarify: on the bottom diagram, I am comparing the curves of the top-left (blue) and top-right (red) diagrams.
The galactic center goes below the page; and only the right half of the gravity well is zoomed in on.

Should I go to grad school? BS in Mech Eng, I am interested in going to do further research in aero or mechanical.

Is paying for it myself stupid? If crypto does well this spring I could cash out and pay for it pretty easily.

>>12778777
>To evaluate the probabilities, you need to model the host's behaviour. The standard assumptions are that:
>a) the host will always open a door
>b) the host will never open the door the player picked
>c) the host will never open the door hiding the car
lolno

Best book on probability, descriptive and inferential statistics ? I want to master probs.

I do not understand the parts underlined. How is the f(y) seen to the left derived from the first x=... equation? Furthermore, upon substituting f(y) into the integral why does it then resemble something new altogether? I understand everything else, just not where the definitions of f(y) come from.
Thank you.

>>12778805
Those are the standard assumptions. Without them, you aren't talking about the "Monty Hall" problem.

>>12778856
I now see that just using [math]\frac{1+y^2}{y}[/math] as f(y) works, and why it reduces to just [math]1+y^2[/math]. I still am confused about the expression to the left but I guess it does not matter.

>>12778900
> I still am confused about the expression to the left
As you should be. It's gibberish since f(y) = x

>>12778787
you're thinking about it too hard. the right side would imply repulsive gravity.
the gravitational interaction is a simple 1/r^2 potential. do you have these qualms when thinking about positive and negative charges too?

>>12779126
1/r potential, my bad

I'm stuck on this. I solve for X and Y with cramers rule, but my answers are REALLY ugly. for x I got this:
[math]
\frac{\frac{s}{4} - \frac{3}{4}}{s^2 - 2 s - 1} + \frac{\frac{1}{4} - \frac{s}{4}}{s^2 + 1}
[/math]

and Y
[math]
\frac{\frac{3}{4} s - \frac{7}{4}}{s^2 - 2 s - 1} + \frac{\frac{1}{4} - \frac{3}{4} s}{s^2 + 1}
[/math]

i have no idea how to find the inverse L of those

>>12779189
oops , forgot pic

>>12779189
i put the same X and Y, one or the other should be
[math]
\frac{\frac{s}{4} - \frac{3}{4}}{s^2 - 2 s - 1} + \frac{\frac{1}{4} - \frac{s}{4}}{s^2 + 1}
[/math]

>>12779189
>>12779195
Equations for ants? Use \dfrac, not \frac to make those fractions readable.

>>12778856
That pic is incoherent gibberish.

If you're rotating about the y axis, the surface needs to be defined by y=f(x) and the volume is
[eqn]2\pi \int _{x_0}^{x_1}xf(x)\,dx[/eqn]

If you have x=f(y) for y in some interval, you'd use disc integration, not shell integration. For disc integration, the volume is
[eqn]\pi \int_{y_0}^{y_1} f(y)^2 \, dy[/eqn]
In this case:
[eqn]\pi \int_1^4 \left( \frac {y^2+1} y \right)^2 \, dy = \frac {111 \pi} 4[/eqn]

The function given is invertible in that region, x=(y^2+1)/y => y=(√(x^2-4)+x)/2, x∈[2,17/4]. Accounting for the y bounds, shell integration gives
[eqn]2 \pi \int_0^2 x (4-1) \, dx + 2 \pi \int_2^{17 \over 4} x \left( 4 - \frac {\sqrt {x^2 - 4} + x} 2 \right) \, dx = \frac {111 \pi} 4[/eqn]

None of which seems to correspond to anything in the pic.

>>12779197
ok, here it is. I deleted cuz i found a dumb algebra error and thought I was gonna make it, but I'm stuck again.

Starting with pic rel, I apply L and then rearrange for
[math]
(s-2)X+Y = 0, -X+sY = -\frac{1}{s^2+1}
[/math]

I then apply Cramer's rule, resulting in, after doing partial fractions on my calculator:
[math]
X = \frac{s + \frac{1}{2}}{s^2 + 1} + \frac{1}{1 - s}
+ \frac{\frac{1}{2}}{(s - 1)^2}
Y = \frac{s}{2 s^2 + 2} + \frac{\frac{1}{2}}{1 - s}
+ \frac{\frac{1}{2}}{(s - 1)^2}
[/math]

I do NOT understand how to find their inverses here, are these reasonable or did i fuck up?

>>12779238
ugh, forgot pic!!!

>>12779220
If it actually meant rotating around the x-axis it would make more sense

>>12779244
>>12779238
ok, i used a calculator to find each one. I think i can see most of them now.

is there a calculator where I enter the original problem and check my solution?

>>12779238
1. You have X and Y swapped.
2. All of those can be found in a table of standard transforms, except:
3. (s+1/2)/(s^2+1) should be expanded to s/(s^2+1)+(1/2)/(s^2+1) => cos(t)+(1/2)sin(t)

The final result should be
x = cos(t)/2+te^t/2-e^t/2
y = cos(t)+sin(t)/2+te^t/2-e^t

>>12777810
well the first thing is to look at what you do with the coupling constant for QED

>>12779258
Right. It would still be wrong in 3 places (stating rotation about the y axis, twice, and the definition of f(y)), but not complete nonsense (and shell integration would be appropriate). In that case, f(y) should be f(y)=(y^2+1)/y so yf(y)=y^2+1. Everything from there on is fine.

Is scientific progress an inevitability?

How could one "discover" Cauchy's integral formula? Namely, if [math]\gamma[/math] is a positively-oriented circle contained in an open set [math]U \subset \mathbb{C}[/math], and [math]f: U \to \mathbb{C}[/math] is holomorphic, then for every [math]a[/math] in the interior of the disk bounded by [math]\gamma[/math] we have [math]f(a) = \frac{1}{2 \pi i} \oint_{\gamma} \frac{f(z)}{z-a} dz[/math].

How could one conjecture this formula in the first place? Specifically, what makes the function [math]\frac{f(z)}{z-a}[/math] a natural choice to consider?

>>12779420
logical positivism is dead, and it's the atheist rationalists themselves who killed it

Is it possible to build lofstrom loop?

>>12779420
Considering that there are still a few isolated neolithic communities, I'd say nope.

>>12779626
They could be progressing just very slowly.

this is a stupid question from a stupid thread. Please click the quote link >>12779699
Can someone please help a brainlet out? I dropped out of college and I only took precalc in highschool, so I don't understand the notation that the other anons are using in that thread.

What are some careers that will hire anybody with a Bachelor's Degree regardless of the major?
t. Applied Math major who is graduatong from a 3 year BSc. instead of a 4 year Bsc. Honours.

>>12779738
if you have good grades and lots of teacher recommendations you can get a good job in any virtually any STEM field.
people say STEM is crowded, yeah, it's crowded with idiots because everybody's mum told them they could be a scientist. there is still a lot of room for smart people.

>>12779703
The expected number of heads and tails are equal.
Flipping the coin until it lands on tails does not matter.

Technically the third scenario is the least likely partially due to the fact that you're flipping 30 times, so there are a lot of possibilities. The specific (T=10,H=20) result is only a very small fraction of those possibilities and therefore very unlikely.
You really shouldn't be comparing scenarios where you have a different number of flips.

But regardless, the expected number of heads and tails are equal, which is what I think you're asking.

Is it just or have there been less math questions recently?

>>12779940
It's almost as if there was another general thread specifically for math questions.

>>12779950
It's almost as if you chose the reply that would anger me the most.
You collaborated in making /mg/ the garbage it is today.

>>12779940
I haven't started my math homework yet... I've been avoiding it...

>>12779404
Can you elaborate? I've been looking at this wikipedia page
https://en.wikipedia.org/wiki/Coupling_constant
but I'm not sure exactly what you mean. I haven't studied QED yet.

>>12779126
>do you have these qualms when thinking about positive and negative charges too?
...yes? every positive charge has a negative somewhere, right? Why wouldn't gravity be universally symmetric too? though charged particles are weird.
A better example would be magnetism, maybe
>would imply repulsive gravity.
doesn't gravity being in waves imply that the "peaks" are relatively repulsive, compared to the mean?

I've been meaning to ask: what does a manifold actually mean? Recently, I bought a linear algebra and differential equations dover book (shilov and tenebraum) and both of them used the word manifold. I've been taught to just "plug and chug" so I have no idea what manifolds actually mean (though I do want to know more).

Is it acceptable to 'ditch' studying for a quiz today vs an exam tomorrow? My lowest 3 quizzes would be dropped but my exam for that different class would not be. I know that the more rational choice would be to ditch my quiz if it would be dropped and if the exam was important, however, I have this 'can't let go / not willing to sacrifice' mindset. Any opinions sqtbros?

>Taking Differential Equations
>Taking Linear System Analysis
>Linear System assumes I know how to do differential equations + fourier/laplace
Why did my college not put differential equations as a prerequisite?
Can I learn fourier/laplace ahead of time only just knowing integrating factors? This is where I'm at in my diff eq class.

>>12777391
>>12771755
help bros what does vector*nabla even mean?? i dont get why the 4rth term is supposed to be 0 given that v is cosntant (i do get why the 1st is 0 though).

>>12780447
A manifold is a "space" which looks locally like euclidean space.
The traditional analogy is how the earth is a "sphere", but locally it looks flat (like an euclidean bidimensional place), so spheres are manifolds.
>>12780620
It's just abuse of notation.
[math]G = (G_x, G_y, G_z)[/math], [math]v = (v_x, v_y, v_z)[/math] and [math]\nabla = (\partial _x , \partial _y, \partial _z)[/math]. Then [math]G \cdot \nabla = G_x \partial _x + G_y \partial _y + G_z \partial _z[/math]. The [math]\partial _i v[/math] are all just evaluated coordinatewise.

>>12780661
yes i know that. i was asking what is [math]\nabla \cdot G[/math] supposed to mean

How the fuck do I do this AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

>>12780661
so how the fuck is v(N.G) different form (v.N)G

>>12780730
Expand them out and look at the terms.

>>12780417
well read this
https://en.wikipedia.org/wiki/Pair_production#Cross_section

it's the cross section which matters. the cross section is just the coupling times parameters tied to the target stuff.

Easiest way to show convergence of the sum from n=6 to infinity of 1/|(n^2)-25|?

>>12781033
comparison

>>12781033
[math]\displaystyle \sum _{n = 6}^{\infty} \dfrac{1}{n^2 - 25} = \sum _{n = 6}^{\infty} \dfrac{1}{(n + 5)(n-5)} \leq \sum _{n = 6}^{\infty} \dfrac{1}{(n-5)^2} =\sum _{n = 1}^{\infty} \dfrac{1}{n^2} < \infty[/math]

>>12780609
Solving linear ODEs with Laplace is pretty straightforward. Apply Laplace transform to ODE, solve for F(s), get rational function (ratio of polynomials) in s, use partial fraction decomposition to get a bunch of k/(s-α)^n terms, apply inverse transform to each term to get a bunch of k.t^(n-1).e^αt terms, sum to get f(t). It's all rather plug-and-chug.

Most of it boils down to L{df/dt}=sL{f}-f(0) and L{t^n.e^αt}=n!/(s-α)^(n+1) (mostly the n=0 case; n>0 only happens if the denominator has repeated roots). The transforms for sin/cos can be derived from the above via e^jx=cos(x)+j.sin(x).

The Fourier transform is just the Laplace transform with s=jω (purely imaginary, no real component, so the function is periodic).

>>12778787
as a follow up, I found a guy named Dragan Hajdukovic (sick name) who claims something similar.
>https://phys.org/news/2012-01-repulsive-gravity-alternative-dark-energy_1.html
>https://arxiv.org/abs/1106.0847

My instincts say "crank," as I can't find any follow-up and he seems to have a persecution complex. But can any physicists confirm whether this is legit?

how do i fix my sleep?
>mattress is 8 years old
>when I'm not sleeping with my back on the mattress I get back pain in the morning
>sometimes wake up with dry mouth
could this be sleep apnea ? I barely had relaxing sleep in the last years (22 y/o rn)

>>12781231
buy a new mattress?

helping a friend and i've got no idea how to interpret one of the tasks:
>"we're using the english alphabet. to "code" a word, 11 is used as a "code key". what's the "decode key"?
what the fuck is 11? is this just a retard's explanation of the caesar cipher?

>>12781248
im looking into that, unfortunately i cant try new mattresses because muh covid lockdown

>>12781256
>is this just a retard's explanation of the caesar cipher?
That's also what I thought, so probably.

>>12781294
look into getting a memory foam slab and topping your current mattress with it

>>12781294
Not sure where you are but I know in my country many mattress sellers offer a trial for 28 days and return for free if you don't like.

>>12781304
yeah, it's what i'll just assume then, thanks. can't believe the person who wrote that is tasked with preparing like 50 future mathematics teachers at high schools all around this country. can't believe my friend is gonna become a mathematics teacher either. she's struggling with basic linear congruences and shit like that. whatever man, rant over.

>>12780711
KVL, KCL and lin.alg.
At t=0^-, vc=1, ix=1/3 (set V=5, i0=0, solve)
At t=0^+, i0=1-vc/3, IOW Thevenin is 1V in series with 3Ω (set V=15, solve with vc as a parameter).
ODE is (1/12)*vc'=1-vc/3, vc(0)=1 => vc(t)=3-2e^-4t => i0(t)=(2/3)e^-4t.

>>12780432
>...yes? every positive charge has a negative somewhere, right?
yes, but what you said is that every positive charge has a spot where it would repel a negative charge.
>doesn't gravity being in waves imply that the "peaks" are relatively repulsive, compared to the mean?
so if your idea predicts something that has never once been observed, do you think it makes sense? at the very least you can test it.

However how I should have answered your question is this: absolute values of potential don't have any meaning. you can add a constant value to shift the position of the potential along the y-axis without affecting any of the physics (since all that matters is differences in potential at different points). so you can always rescale the potential such that it's below the "vacuum potential" that you wrote.
plus the humps would imply a more complicated shape than 1/r which is not what we observe

>>12779461
well, for starters z-a scales your numerator by the distance relative to point a, so if you're trying to find the contribution of the function at point a then you want to pay less attention to parts of the curves that are very far away from the point, as they contribute less.

Practicing for my diode test next Tuesday, just trying to get some hands-on work here.
My question is how I would go about solving this problem because I have only learned the theory which is why I'm a bit lost but here's what I'm thinking:
Since V(in) is a AC input (10sin (omega * t)), the highest it will go is 10 volts.
When it is in the positive cycle, it'll go from Vin -> R1 -> D2 -> ground, and then I'm supposed to find the voltage across the diode D2 as that'll be my Vout right?
In the negative cycle, it'll go from Vin -> D1 -> R2 -> ground, and I do the same thing I did for the positive cycle but with the voltage across resistor 2?

>>12781134
>assuming that a particle and its antiparticle have the gravitational charge of the opposite sign
this is a big assumption in sentence 1. our current theories believe that antiparticles experience gravity in the same way as particles, although this is being tested.

non-american here
if i know how to differentiate, integrate, and can do limits, do i basically know calc 1?

>>12777391
Why isn't Li-7 used as fusion fuel in thermonuclear weapons? It worked in Castle Bravo.

>>12781417
> When it is in the positive cycle, it'll go from Vin -> R1 -> D2 -> ground, and then I'm supposed to find the voltage across the diode D2 as that'll be my Vout right?
They're ideal diodes; when forward-biased, the voltage is zero. If the anode voltage tries to exceed the cathode voltage, the diode conducts and forces them to be equal.

So, D1 requires Vout<=Vin while D2 requires Vout<=0. So Vin>0 => Vout=0 while Vin<0 => Vout=Vin. IOW, Vout is either 0 or Vin, whichever is less, i.e. Vout is the negative half of the waveform.

id like some coin flip data tables
lots of trials, real flips none of that generated rng stuff

>>12781679
you think that data would give a different result??

>>12781231
i changed my thirty [30] yo matress back in june and it is the best


density of foam is 100% latex 85 kg/m3
height of latex core is 14cm
total height is 20 cm

price was 400 euros

>>12781750
yeah
Im looking into gel matresses

>>12781596
Uwah... I'm trying to wrap my head around it for a second. Okay, so I think I got the same thing as you! I think so at least?
I'm not sure if I drew the output waveform correctly though!

>>12781596
right
tl;dr
have-wave rectifier passing negative values.

>>12781596
>They're ideal diodes; when forward-biased, the voltage is zero
retarded

How do I find the inverse of 100 modulo 1001? Using Euclid's algorithm gives me 1 = 1001 - 10*100 which isn't exactly what I'm looking for.

>>12781447
his antiparticle-antigravity stuff sounds schizo, I'll give you that. (has the antihydrogen drop happened yet? I saw error bars of like +/- 7500% on wiki)
But more importantly, is the gravity well stuff/repulsive gravity legit?

>>12780464
Sure.

what are the first two non zero terms of the taylor series expansion of sinz/(z^2+5) centered at 0?

>>12782425
https://www.wolframalpha.com/input/?i=taylor+expand+sin%28x%29+%2F+%28x%5E2+%2B+5%29

>>12782432
it doesnt change anything that its complex?

>>12782441
Nah.

>>12782476
thx boss

I don't understand anything about functions. Is there any good videos or books that will help me understand?

>>12782925
What is there to understand about functions?

Is it possible to brush up on math from maybe algebra onwards by yourself?

>>12782990
Is it possible? Of course. Would it be easy? Hell no.

However there are plenty of online courses and content on youtube to get you quite far.

>>12783027
Yeah I meant using tools like books, videos, online courses, etc lol

>>12783043
In my experience most math books suck to learn from as nothing beats visual explanations by a human teacher. But that does not have to be true for everyone. Good luck though anon, I've been doing something similar since the first lockdown started.

Where can I find a good linear algebra problem sets? Some proof-based and computational-type questions are both welcome.

>>12782123
Take -10 modulo 1001, that's what you're looking for.

>>12782925
Usually functions are intended as operations ('mappings') between a set [math]X[/math] and [math]\mathbb{R}[/math]. The notation [math]f:X \to \mathbb{R}[/math] is quite clear in showing you symbolically take we take an element [math]x \in X[/math] and we assign to it a real number. It is important to notice that for every element [math]x \in X[/math] we assign ONE real number, so [math]x,y \in X[/math] can be mapped to the same number, but [math]x \in X[/math] CANNOT be mapped to two distinct real numbers. We distinguish some peculiar types of functions that are important: functions that are one-to-one (which means that if [math]f(x)=f(y)[/math] then [math]x=y[/math]) and functions that are onto/surjective. This last type means that the set of values that [math]f[/math] takes in [math]\mathbb{R}[/math] is equal to [math]\mathbb{R}[/math]. For example [math]g(x)=x^2,x \in \mathbb{R}[/math] is not surjective because it only takes values from 0 to infinity, so the negative real numbers are missing.

>>12783167
Do videos provide something closer to that?

>>12783516
For me they do. There are entire lecture series online from places like MIT or Stanford. Also YT series like the Essence of Calculus by 3Blue1Brown which when combined with text books taught me more than I ever did back in school.

>>12783198
check out hefferon's linear algebra book, it's free and has a full solution manual as well, just google it. every section of the book ends with a lot of exercises, both computational and proof-based. the material is somewhat basic, so depending on where you are, it might not be advanced enough. maybe take a look at halmos linear algebra problem book; the title sounds like it might work, although i haven't checked it out myself.

>>12783198
>>12783597
oh and i only used the website to look up some linear algebra proofs, but i think there are some problems on the website by yu tsumura, a quick google search should bring that up as well

I suck at breaking out something from an equation to solve it

ie, x=y/z*b, make an expression to solve for b.

but in this case how do I break out and make an expression for Cr from
VrCr,0=VrCr+Vg<Cb+Cr/2>+VbCb

>>12783442
That is what I'm looking for, thanks man.

>>12783617
What's up with the <>?

How are sperm containing X or Y chromosomes produced in equal proportion?
Fun fact, chromosome Xq28 and a gene on chromosome 8 seem to be found in higher prevalence in men who are gay. http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=9385646&fileId=S0033291714002451

anyone got any ideas for this?

>>12785411
[math]P(Y_n=(j,k))=P(X_{n+1} = j,X_n = k)=P(X_{n+1}=j|X_n=k)P(X_n=k)=p_{j \to k}p_k[/math]

>>12785496
sorry last one is [math]p_{k \to j}p_k[/math]

>>12785503
could you explain what that notation means? I haven't seen it before

>>12785512
I refer to [math]p_{k \to j}[/math] as the probability of transitioning to state j at time n given state k at time n-1. I assume it does not depend on n. [math]p_k[/math] is the marginal probability of finding X in state k.

What is the "plato.stanford.edu" of math and physics (a good online encyclopedia)

>>12785541
unironically wikipedia. Some articles have autism levels I have hardly seen anywhere else, and that's almost an understatement.

>>12785531
makes sense, though I'm not sure how you build this to show, for example, [math] P(Y_n = (j_n,k_n) | Y_{n-1} = (j_{n-1},k_{n-1}),\dots,Y_0 = (j_0,k_0)) = P(Y_n = (j_n,k_n) | Y_{n-1} = (j_{n-1},k_{n-1})) [/math]

>>12785561
can you share the most autistic one you've found?

>>12785597
Maybe this?
[math]P(Y_n=(j,k)|Y_{n-1}=(k,m),Y_{n-2}=(m,n),...)=[/math]
[math]=P(X_{n+1}=j,X_n=k|X_n=k,X_{n-1}=m,X_{n-2}=n,...)=[/math]
[math]=P(X_{n+1}=j|X_n=k,X_{n-1}=m,X_{n-2}=n,...)=[/math]
[math]=P(X_{n+1}=j|X_n=k)=p_{k \to j}[/math]

>>12785621
Just some examples
https://en.wikipedia.org/wiki/List_of_integrals_of_Gaussian_functions
https://en.wikipedia.org/wiki/List_of_nonlinear_partial_differential_equations
https://en.wikipedia.org/wiki/Fourier_transform#Tables_of_important_Fourier_transforms

Where is the universe?

Beginner Linux ln -s Symbolic link command.

I need to create two links at the same time. Is it possible or is my professor retarded.

ln -fs ../../pal pal and ../../..ka ka

I've tried to put them between ( ), separated by commas, { } and what not.

Basically like creating two files simultaneously with touch:
touch pal ka.

Thanks.

>>12785817
ln -fs ../pal pal && ln -fs ../ka ka
as a show case of true genius

How to stop wasting my time trying to solve a single exercise?. I can spend hours and hours obsessed with one proof, at the end I don't solve shit and end up watching the solutions manual. This shit is affecting my progress, please help!!

>>12785817
In general, Unix commands which require both source and destination don't allow you to specify multiple source/destination pairs. However, you typically can have multiple sources and a single destination, e.g.

ln -fs ../../pal ../../..ka .

The destination is just ".", the current directory. The link names will be the same as the source, i.e. pal and ka.

How can mosquitoes feed on blood? It has no carbohydrates, fat or proteins

Is there an equation that, given a fluid pressure, aperture radius/diameter, fluid density, etc, will yield the instantaneous fluid output of a hose? I guess fluid flux?

>>12786189
Assuming laminar flow then probably the Poiseuille Equation, if its turbulent then Bernoulli's Equation would be more applicable.

>>12786277
Shit I have no idea what to consider it. I was looking at Bernoulli's equation earlier but I wasn't sure.

I rigged up an electronic circuit for my friend that fired off fuel injectors for a car engine he's rebuilding. He wanted to test the flow rate for each injector
Two problems. He used an external pump which was at a different pressure than was intended, which is the main problem I'm trying to correct for. I need the equation to come up with some correcting function/proportionality constant.

The other problem is that it uses pulse-width modulation which I definitely wouldn't call laminar, but it's not turbulent either. So I have no fucking clue what to do there. Though, in EE, usually you just consider a PWM signal after it's been low-pass filtered into a continuous waveform... so I'll pretend it's laminar.

>>12786304
Why don't you download some books on mechanical engineering? Theres got to be something there
Maybe a book on injectors just use gen lib

Is studying mathematical engineering a viable way of specialising in cryptography?
I don't care much for continues mathematics, but I think discrete is interesting

>>12786304
No one has ever said fluid mechanics is easy.

Not sure how to put this question but heres we go. This is two currents of a coil surrounded by an square amperian loop. Obviously we can see that there are 2 turns of I and the enclosed amount of current is [math]-2\mu_0 I[/math] but how do I actually break up the line integral to show this? I would maybe assume there are four separate lines to the rectangle that all oppose the magnetic field from the currents so summing them up would be 4 or something. Very stupid question I think.

>>12778777
Thank you.

>>12786364
What are you trying to solve?
When doing ampere's law to calculate the magnetic field along an amperian loop, you have to pick a loop such that the field along the loop is constant, otherwise you can't pull the magnetic field out of the integral. clearly in your case you've picked a loop that has a non-constant field along it.

>>12786383
Ok so I shouldn't actually be able to "solve" this integral? The question was "Evaluate Closed integral of B * dl". I thought that maybe there was some way to reasoning it out thinking about the direction of the vectors, canceling or something. Similar to how my teacher used a square amoerian loop around a solenoid coil to show that a magnetic field is [math] B = N\mu_0 I [/math]. The field on the sides are zero because the magnetic field is parallel to the loop and the field on the top is zero because far out the magnetic field is zero. Thats along the lines I was thinking. Maybe I'm thinking too much into it.

>>12786410
the difference for a solenoid is that you assume there are infinite wires. in your diagram, that would mean there are extra wires on the outside of the loop whose contributions cancel except on the top and bottom. unless there's more to the problem, you aren't able to evaluate this

Linux Beginner CP command.

I need to copy a file to another folder(car). The thing is that in between I have to change the name of the file. Is it doable in with just CP once?

Prof is asking us to use one command only the thing is I'm not sure if I can use cp more than once.

cp -a ../../psword psword.txt ???? car

>>12786434
yeah you just specify the output file to be what you want the copied file to be named as

so, like
cp ~/Downloads/foo.txt ~/Documents/bar.txt
will copy "foo.txt" in the Downloads directory, to the Documents directory. It will also rename it to "bar.txt"

how do i find the inverse laplace of this nasty little devil?

[math]
\frac{5 s + 16}{s^2 + 16} - \frac{5 s + 26}{s^2 + 2 s + 26}
[/math]

>>12786447
Thanks mate.

>>12786453
I probably should've formatted it as
[math]
\frac{5 s + 16}{s^2 + 16} - \frac{5 s + 26}{(s+1)^2+ 5^2}
[/math]

I'm aware it's very much like cos(5t)/5, but the numerator for both are little shit heads I don't know how to resolve.

>>12786447
>>12786434
as an addendum, you can "rename" a file in linux the exact same way, but using "mv" instead of "cp"
mv copies the original file to the target file, and then deletes the original file, so you're just left with the target file
mv ~/Documents/foo.txt ~/Documents/bar.txt will rename foo.txt to bar.txt

>>12786457
no problem, I can't imagine what it's like having to learn linux in an academic setting

>>12786461
>>12786453
nevermind, i get it now. i see there's kind of a trick with summing cosine and sine to get the numerator you want, since they both have the same denominator

i have found the solution to x(t) via inverse laplace, but I'm drawing a blank on how I'm supposed to solve the IVP. am I just supposed to answer what x(0) is?

>>12786513
well i just realized x(0)=10 contradicts the givens. when i run the laplace on the function given and solve with the inverse laplace, the result is x(t) right? or is x''(t)?

>>12786466
>mv copies the original file to the target file, and then deletes the original file, so you're just left with the target file
that is only true if the destination is on a different device. otherwise it just updates the inode details and doesn't move any data.

>>12786563
"if it looks like a duck and it quacks like a duck"

>>12786572
well it makes a difference if you are mv'ing large files and wonder why sometimes its instant and other times it takes minutes.

>>12777391
What can we do about all the poltards who constantly raid /sci/? They are completely immature, and they have been shitting up Sci for months now. All they do is post trollish racial content and spread conspiracy theories.

>>12786606
It also makes a difference if the file has additional hard links. "mv" will simply move one of the links, leaving all links pointing to the same file. "cp+rm" will result in two distinct files. Also, extended attributes will only be preserved if cp knows how to do that.

god the linux pedant weenies are here like white on rice and flies on shit

How do I figure out the expected value of something that has (discrete) variable probability? Like, for instance, say I roll a 1000-sided die 50 times, then a 500-sided die 20 times, and then keep rolling a 400-sided die until I get a 1. I want to find out the expected amount of total rolls before I roll a 1.
With rolling a single die it's easy, since on average it'll take n rolls of an n sided die before you get a specific roll, but I'm lost as to how that formula would expand into a discrete variable context.
Pic related is the actual probability density I'm trying to figure out, i.e. the expected number of encounters before a shiny appears (ignore the middle column).

>>12786676
If I were to phrase my specific question like I did with the die:
>I roll a 4096-sided die 50 times, then a 2048-sided die 50 times, a 1365-sided die 100 times, a 1024-sided die 100 times, an 819-sided die 200 times, and then I'm given a 683-sided to roll. What's the expected number of rolls I will need to do before I get a 1?
I'm really just looking for a formula though.

>>12777391
How do I increase my breast size as a male? Not trans no hormones.

>>12786651
I've only ever seen one situation where someone used hardlinks. I'm honestly surprised when anyone knows that feature even exists.

>>12786513
>>12786522
mx''+cx'+kx=82cos4t
=> m(s^2X-sx(0)-x'(0))+c(sX-x(0))+kX = 82s/(s^2+16)
=> ms^2X+csX+kX = 82s/(s^2+16) (as x(0)=x'(0)=0)
=> (s^2+2s+26)X = 82s/(s^2+16) (as m=1, c=2, k=26)
=> X = 82s/((s^2+16)(s^2+2s+26))
= 82s/((s^2+16)((s+1)^2+5^2))
Partial fraction decomposition:
82s/((s^2+16)((s+1)^2+5^2)) = (As+B)/(s^2+16) + (Cs+D)/((s+1)^2+5^2)
=> A=5, B=16, C=-5, D=26 (can't be arsed writing out the expansion)
=> X = (5s+16)/(s^2+16) - (5s+26)/((s+1)^2+5^2)
= 5s/(s^2+16) + 16/(s^2+16) - 5(s+1)/((s+1)^2+5^2) - 21/((s+1)^2+5^2)
=> x(t) = 5cos(4t) + 4sin(4t) - 5e^-t.cos(5t) - (21/5)e^-t.sin(5t)

>>12786699
They've largely been obsoleted by symlinks. Nowadays, their main use is in atomic file update:
1. write to foo.tmp
2. hard link foo to foo.bak
3. rename foo.tmp to foo
The name "foo" atomically changes from the old version to the new version, while leaving the old version as a backup.

It has a secondary use for emulating O_EXCL on NFS prior to v3 (hard linking was one of the few operations which were actually atomic).

>>12785541
nlab

What in the human body determines accents? For example all french people who learn english have a similar french accent, is this because of physical development which learning a native language has on the tongue/vocal chords? The effect a language has on the way messages are sent from the brain to the tongue? Or possibly something else.

Abbott says the second term in the sequence is 7/2.
Am I retarded? Because I calculated 3 for the second term in the sequence.
[math]y_1 = 1, y_{n+1} = 4-1/y_n[/math], so [math]y_2 = 4 - 1/1 = 3[/math]

>>12786999
You're right

>>12787045
Thanks for clearing that up. I self-study and make a lot of dumb mistakes, so I usually assume the solution manual is correct.

Suppose there is a surjective group homomorphism from [math]G[/math] to [math]H[/math]. Is it true that [math]H[/math] embeds as a subgroup of [math]G[/math]?

>>12786925
Muscle memory. If you learn another language, you tend to build pronunciation from the set of sounds you're familiar with making (e.g. Japanese using R for L). It's possible to lose the accent, but it takes some effort so it normally only happens if it's really necessary. Actors often learn a specific accent for a role (apparently the producer of House was surprised to learn that Hugh Laurie wasn't American).

>>12787073
[math]\mathbb{Z} \to \mathbb{Z}/{2\mathbb{Z}}[/math]

If [math] \frac{A_{n+1}}{A_n} \to 3(n+\frac{1}{2})[/math] how can i show that [math]A_n \to 3^n \Gamma(n+ \frac{1}{2})[/math] asn [math]n\to \infty[/math]?

Bros, I'm about half way through Gen Chm II and this class fucking blows. Dissociation constants are boring as fuck. My question is the rest of the clalss going to be like this as well? We just finished chemical kinetics, acid base equilibria, titration calculations. It's not like the class is hard at all I've got an A, it's just so fucking painfully boring. This shit feels like working through a cook book

Would anyone care to recommend a textbook on linear algebra with a focus/target audience of engineers?
Likewise, any books like that for complex "analysis" (I don't need super heavy proofs and stuff, just enough to get me a working ability with complex functions, their calculus, and the intuition behind it all)

it's a miracle that my university doesn't require either class to get an undergrad degree in DSP

>>12783971
Read up on meiosis.

So it’s been a while so maybe I’m just rusty as fuck and missing something obvious, but how the fuck does 1.28 follow from 1.27?
Shouldn’t there be a Cos() term that stops whatever they did?

>>12786004
That's not true. If it were, all your cells would not have access to nutrients either.

>>12788053
Uhh I hate having to phone post, I hope that photo isn’t rotated on pc

>>12787743
Since [math]A_{n+1} = 3(n + \frac{1}{2}) A_n[/math] you can write

[math]
\begin{align}
A_2 &= 3(1 + \frac{1}{2}) A_1 \\
A_3 &= 3(2 + \frac{1}{2}) A_2 \\
&= 3^2(2 + \frac{1}{2})(1 + \frac{1}{2}) A_1 \\
... &\ \\
A_n &= 3^n A_1 \prod_{k=1}^{n-1}(k + \frac{1}{2}) \\
&= 3^n A_1\Gamma(n + \frac{1}{2})
\end {align}
[/math]

So in the limit as [math]n \to \infty[/math] then [math]A_n/A_1 \to A_n[/math]

>>12788064
>>12788053
you bet it is

>>12788075
i had a bruh momento there. Thanks

>>12786304
Poiseuille Laminar Flow, pressure driven tube as a function of radius (z-points down the center line of the tube)
[math]v_z = \frac{\Delta p R^2}{4\mu L} (1-\frac{r^2}{R^2})[/math]
p is change in pressure from inlet to outlet
L is length of the tube
R is the inner radius of the tube
r is the radius you're evaluating at
[math]\mu[/math] is the viscosity of the liquid


Integrate this shit for volumetric flow rate, Q
[math]Q = \frac{\Delta p \pi R^4 }{8\mu L}[/math]

Laminar flow condition
[math]L>L_e = 0.058D(Re)[/math]
[math]L_e[/math] distance from the entrace of the tube
D - inner diameter of tube
Re - Reynolds number

>>12779742
What if i dont have good grades?

>>12788090
Thank you kind anon

any quantum computer nerds here?

could grovers algorithm be used to perform collision detection of an object with n other objects in sqrt(n) checks, or is the idea of the search limited in what you can look at? particularly interested in the case of voxels as a new paradigm for 3D simulation, the main problem usually being that the number of objects scales up by orders of magnitude and it requires too much computing power to compare everything against everything else every cycle. Also are there any potential issues with using a quantum computer like this at a rate of many cycles per second? any like, timing issues with entangling particles?

>>12777391
We have two functions: [math]f(x) = 3x+7[/math] and [math]g(x) = x[/math]

We want to know if f(x) is big O of g(x), that is, we want to know if there are two "witnesses" C and k such that for all x>k, [math] |f(x)| ≤ C|g(x)|[/math].

Now obviously we know that f(x) is O(x). But my question is, what is the smallest value C we can use to prove this relationship, assuming we take k = 1? Or how about for k= 0?

[math] 3x+7 ≤ Cx [/math] We divide every term by x to get [math] 3+\frac{7}{x} ≤ C[/math] So basically we are asking if we can find some constant C that is always greater than the value of this function, and that should be the answer we are looking for, right? Well, for x = 0 obviously it is undefined, and approaching from the right we see the function approaches infinity. So we look at x = 1, and obviously we have 10. So does this mean that if we take k = 1, any value C > 10 should work for the original function 3x+7?

>>12788296
and further, if we wanted to show that f(x) = 3x+7 is O(x) and we add the constraint that for our witnesses C and k, k has to be 0, is there any value C that would work? I guess what I am asking is: what are the minimum values we can take for C and k to show that 3x+7 is O(x)?

>>12788053
More context? What is italic-r-dot? If it's the magnitude of the derivative of r, it doesn't make sense. If it's the derivative of the magnitude of r, it might.

I have a really stupid question, where is my mistake. If [math]v = (i,1)[/math] then: [eqn]|v| = \sqrt{ (i)^2 + (1)^2} = \sqrt{ (\sqrt{-1})^2 + (1)^2} = \sqrt{ -1 + 1} = 0 [/eqn] Now I know this is wrong, I know the magnitude of v is [math]\frac{1}{\sqrt{2}}[math] so where am i making my mistake?

>>12788296
>>12788312
> what is the smallest value C we can use to prove this relationship, assuming we take k = 1? Or how about for k= 0?

|f(x)| ≤ C|g(x)| => C ≥ |f(x)|/|g(x)| = (3x+7)/x = 3+7/x

So choose any k>0 and set C=3+7/k, then you have ∀x>k : |f(x)| ≤ C|g(x)|

k=0 won't work because there isn't any C s.t. 7<C*0. But any positive k will work; e.g. k=0.001 => C>=7003 as 3*0.001+7=7.003 <= 7003*0.001.

> So does this mean that if we take k = 1, any value C > 10 should work for the original function 3x+7?
Yes.

> k has to be 0, is there any value C that would work?
No.

> I guess what I am asking is: what are the minimum values we can take for C and k to show that 3x+7 is O(x)?
k>0, C>=3+7/k

It's only necessary to show that some suitable k,C pair exists. Typically, you can pick any k>0 and find a suitable C. Concerning yourself with minimum values is somewhat pointless, as big-O is about what happens for large x.

>>12788452
> where is my mistake
This:
[eqn]|v| = \sqrt{ (i)^2 + (1)^2}[/eqn]
should be:
[eqn]\Vert v \Vert = \sqrt{ \vert i \vert^2 + \vert 1 \vert^2}[/eqn]
The usual norm on C^n is √(|z1|^2+|z2|^2+...+|zn|^2) where |z| is the magnitude of z.

BTW, this is √2, not 1/√2.

>>12788452
A vector with a complex number as one of its components is pretty unusual.
I feel like the point or vector you are actually thinking about is (1,1), which corresponds to the complex number 1+i.
This has magnitude [math] \sqrt{2} [/math] .

>>12788518
Thank you, I was using it as a normalization but yes you're correct.
>>12788546
I'm doing problems from a textbook, those were the values given. thanks anyways for the attempt.

Do naturally acquired infections boost you INNATE immunity?
They obviously improve your adaptive immunity, but is there an effect on innate immunity?

Is QFT useful for cosmology? I need to choose a theoretical course in my career, but I don't know which and I was considering QFT. What do you guys think?

>>12788627
well if you do physics seriously, you need QFT

for baby cosmology you need babyGR +cosmology, and perhaps astrophysics if you really like this, QM for quantum approach to black hole

QFT is needed if you want to study near the bigbang with nucleosynthesis

>>12788653
Ok thank you. The alternative would be something like stat mech, or fundamental interactions, but the QFT professor is said to be very good.

>>12788653
In my experience most cosmologist have never studied QFT and have never needed to.

>>12788627
>Is QFT useful for cosmology?
Maybe in a few fringe cases

>>12788394
Yeah it’s the magnitude of the derivative of r

>>12788706
>>12788753
Mmm I see, thanks. Do you think there are some other areas in astrophysics where QFT is useful then? Maybe neutron stars or black holes?
Otherwise I'd need to choose among statistical mechanics, fundamental interactions or many body theory and I am quite undecided.

>>12788777
>statistical mechanics,
this one is important too in general physics, but not really for cosmology

>>12788777
Well I'm not saying QFT isn't interesting to learn but it's a very, very hard subject and generally not applicably to cosmology. It's the realm of theoretical & particle physicists. Only in the most extreme / edge situations would be where it may find some use - Hawking radiation, the first second of the big bang etc.

Fundamental interactions may be useful if that is a higher level overview of the Standard Model.

Yeah you do QFT full on or not all, otherwise it will fuck you grades lol

Stick to a wavy handed QFT, ie
Fundamental interactions, if you know that you are autistic about cosmology and don't care about anything else.

>>12788777
>many body theory
That does sound useful for cosmology. You'd want to simulate orbits and shit.
>statistical mechanics
That's something every physicist should know

>>12788825
at least at my uni many body theory had nothing to do with orbital mechanics. it was to do with bulk systems, i.e many particles. more to do with chemistry than cosmology.

>>12788838
Oh I'm retarded, I thought of n-body problems and other complex systems stuff.
In that case, some surface stuff like fermi gases should be covered by statistical mechanics as well.
Don't know much else about it.

>>12788817
Well, I'm don't know if I am autistic about it, but I am doing many astronomy and cosmology courses, and I am enjoying them.
>>12788825
>That's something every physicist should know
I know the very basics, but the mandatory curriculum at my uni is a bit lacking in this field.
>>12788838
That's the same at my uni.

Hey, I'm about to begin my first math semester but forgot most of the shit I learned during the admission course (6 months ago).
I'd like to go through the notes they (the uni) put up but seems their website is under maintenance or something.
So, what do you suggest as a refresher for calc 1? I'm mostly worried about theorems and stuff around derivatives/integrals and function analysis.

Thanks

Can I use a hydrofluoric acid dip to remove some accidental oxide deposition from aluminum, or will it eat the aluminum as well?

Cosmology doesn’t need anything like orbital mechanics, but the statistical mechanics and many body stuff both sound good. You’ll be treating galaxies like they’re tiny gas molecules forming a fluid. GR then tells you about how that fluid expands. Just like normal fluid mechanics there’s things like a continuity equation, it’s just unlike normal fluid mechanics when you’ve got to think about SR and GR.
Both statistical mechanics and many body sounds good to me, they actually sound very similar so it might be worth talking to profs to see which sounds best

>>12788462
Thank you anon
>It's only necessary to show that some suitable k,C pair exists. Typically, you can pick any k>0 and find a suitable C. Concerning yourself with minimum values is somewhat pointless, as big-O is about what happens for large x.
That makes sense. I won’t concern myself with such things, then. I see that it doesn’t matter to find a minimum pair, the end result still the same as long as we can find any 2 witnesses.

Any idea how I do this question? It's a sample for my finals and it's not looking so good chief.

>>12789271
It is third order because the highest order derivative in the equation is 3.
It is ordinary because the equations depends on a single variable (t).
It is nonlinear because of tan(t+y).
Therefore I, III and V are correct.

>>12789271
I is true because the maximum order of derivation of y is 3
II is false because the maximum order of derivation of y is not 5
III is true because the equation is non linear in y (there is [math]\tan(y+t)[/math]
IV is false because of what I have written before
V is true because y depends only on t (or at least, it is only derived in t and only t is present in the rest of the equation).

Therefore, only I, III and V are true. As an aside, this is a purely theoretical question, so if it is not clear you should probably review the theory before doing exercises.

Sorry to ask, but I am missing something basic here:
Why am I getting different results when integrating towards x
[math]a = 1/(b+x)^2[/math] and [math](b+x)^2*a = 1[/math]
The first one yields
[math]a*x = -1/(b+x) + C[/math], whilst the 2nd yields [math](b+x)^3/3*a = x + C[/math]
Am I forgetting something fundamental about integration?

>>12789293
>>12789296
THANK YOU TWO SO MUCH!!!!
I'm genuinely bad at differential equations and yes, my theory is really bad (my professor treats our class like a rap class) so I'll be trying to catch up over the weekend!

>>12789315
You haven't explicitly shown the dx and it matters when integrating.

[math]a = \int \dfrac{dx}{x^2}[/math] is not another way of writing [math]\dfrac{1}{a} = \int x^2 dx[/math]

>>>/g/80488586
Let [math] t_0 [/math] be the time in minutes at which the experiment started, then
[eqn]
\int^{t_0 + 120}_{t_0} g(t) \, dt = 0
[/eqn]

circuitretard here, still preparing for my exam next week.
How do I derive v_out?
So far, what I have is:

I (current through resistor R) = Iz (current through zener) + IL (current through load resistor)

I know that I can convert the zener diode into it's equivalent form which is a + -> - voltage source (Vz_0) and a resistor called Rz, the zener resistor or whatever it is called.

The current through that zener diode would then be Iz = (Vb + Vz_0 - 0)/(Rz).

After this part, I'm not sure what to do because of the load resistor on the right, otherwise, I would've just did a KVL around it to solve for the the Vout as it is the voltage drop across R_load (would V_out also be same as the voltage drop across the zener resistor? I'm not sure actually).

>>12789195
HF will eat fucking everything anon. How do you even get your hands on HF?

>>12789325
>my professor treats our class like a rap class
What?

>>12789768
I just he speaks way too fast for no actual reason other than "Hey, this course will cover a lot of material, so we need to go fast fast fast!!!"

>>12789773
I just mean he speaks way too fast for no actual reason other than "Hey, this course will cover a lot of material, so we need to go fast fast fast!!!"
Also missed a word so I had to repost...

>>12789768
Okay now my internet went down

>>12789706
>I would've just did a KVL around it to solve for the the Vout as it is the voltage drop across R_load
Seems about right
>would V_out also be same as the voltage drop across the zener resistor? I'm not sure actually
Yes

>>12789706
You can still use KVL, it's just that you now have two equations: one for the left loop, and one for the right loop (either the zener + R_load, or v_in + R + R_load), and two unknowns, I_z & I_L. Then you can just use ohm's law for R_load to get V_out.

>>12789814
>>12789828
I wasted a day on this.
>it's just that you now have two equations: one for the left loop, and one for the right loop
Okay I'll go try it out.
Thank you both so much.

Anyone here who is good at vector calculus that can confirm my calculation of [math] \vec{B} \cdot \nabla [/math] in spherical coordinates?

[eqn] \vec{B} \cdot \nabla = \vec{B} \cdot(\frac{\delta}{\delta r} \ \hat{r} + \frac{1}{r} \frac{\delta}{\delta \theta} \hat{\theta} + \frac{1}{\sin{\theta}} \frac{\delta}{\delta \lambda} \hat{\lambda}) [/eqn]

[eqn] \vec{B} \cdot \nabla = (B_{r} \ \hat{r} + B_{\theta} \ \hat{\theta} + B_{\lambda} \ \hat{\lambda}) \cdot(\frac{\delta}{\delta r} \ \hat{r} + \frac{1}{r} \frac{\delta}{\delta \theta} \hat{\theta} + \frac{1}{\sin{\theta}} \frac{\delta}{\delta \lambda} \hat{\lambda}) [/eqn]

[eqn] \vec{B} \cdot \nabla = \frac{B_r}{r^2} \ \frac{\delta}{\delta r} (r^2) + \frac{B_{\theta}}{r \ \sin{\theta}} \ \frac{\delta}{\delta \theta} \ (\sin{\theta}) + \frac{B_{\lambda}}{r \ \sin{\theta}} \ \frac{\delta}{\delta \ \lambda}[/eqn]

Note that I am finding an expression for [math] \vec{B} \cdot \nabla [/math] and NOT the divergence i.e. [math] \nabla \cdot \vec{B} [/math]

My logic was that for cartesian coordinates the solution is:

[eqn] \vec{A} \cdot \nabla = \vec{A} \ (\frac{\delta}{\delta x} \ \hat{x} + \frac{\delta}{\delta y} \ \hat{y} + \frac{\delta}{\delta z} \ \hat{z})[/eqn]

[eqn] \vec{A} \cdot \nabla = (A_x \ \hat{x} + A_y \ \hat{y} + A_z \ \hat{z}) \ (\frac{\delta}{\delta x} \ \hat{x} + \frac{\delta}{\delta y} \ \hat{y} + \frac{\delta}{\delta z} \ \hat{z}) [/eqn]

[eqn] \vec{A} \cdot \nabla = A_x \frac{\delta}{\delta x} + A_y \frac{\delta}{\delta y} + A_z \ \frac{\delta}{\delta z} [/eqn]

So I followed the same line of thought but remembered that the expression changes when we multiply the theta, r and lambda unit vectors with each other, while in Cartesian unit vector they just simplify to 1.

>>12789195
>accidental oxide deposition from aluminum
fucking wut? assuming you don't rot your bones and etch pits in the aluminum, they're just gonna reform new oxides immediately

>>12777391
What are the integral versions of the product and quotient rule?

>>12789853
why do you use [math]\delta [/math] instead of [math]\partial [/math]?

>>12789853
Looks incorrect to me tbqhwy.

>>12789881
>product
Integration by parts
>quotient
Varies with the form of the quotient, usually you'll be using partial fraction decomposition then integrating each element individually when the decomposition isn't too complicated.

>>12789853
It should still be an operator at the end.
The surefire way is to apply it to an unknown scalar function f(r,theta,phi), and then apply it. Then "factor out" the f on the right side at the end.

is there a reason to use \delta and not \partial?

>>12789367
I fully get it now, thanks! I mistakenly took [math]a[/math] as a constant, which makes no sense considering it's dependent on x

I'm hoping to both skip some prequisites and take a grad course so I cram more useful upper-div classes into my degree. Do any of you have experience talking professor's into this (and any tips if so)?

How likely do you rate the following for approval
>>skip "stats for engineers" to take class on randomized algos and probabilistic analysis
>skip intro to modern physics I, II to take classical, statistical and quantum mechanics
>taking a "computational geometry" grad level course (not offered at undergrad level)
>skipping group, field and ring theory intros to take Abstract Algebra
>skipping intro to analysis to take real analysis

I've already studied most the stuff i wanna skip on my own, aside from the modern physics stuff, and could potentially brush up in a reasonable timeframe to hopefully please some professor's sufficiently.

appreciate any advice

>>12789902
It is still an operator at the end, the r^2 and the sin theta are obviously abuse of notation for "multiply first then differentiate the product".

Given [math]| x >[/math] in some basis as: [eqn]| x > = a| a > + b| b > [/eqn] and the same quantity in a different basis as [eqn]| x > = c| c > + d| d >[/eqn] Is it then true that [eqn]a = < a | x > = < a| c > + < a| d >[/eqn] describes the constant [math] a [/math] in terms of the basis [math] | c > [/math] and [math] | d > [/math]?

Great question op. How do I sleep knowing?

Not asking for a friend.

>>12789885
>>12789902
>>12789921
>>12789887

Just me being a brainlet
thanks for pointing out that the correct notation is [math]\partial[/math].

Thanks I will try again from scratch and scrap this

>>12789932
[math]a = < a | x > = c < a | c > + d < a | d >[/math]

>>12777391
/sci/, Im about to do my thesis in AI, it is an undergraduate thesis so any shit project that barely works will do. My question is:
How do I start learning AI? I'm a cs undergraduate, I took two related classes but I didn't learn much. Show me the way, /sci/.

can u study organic chemistry and just repair your deficiencies as you go along?

Do I really need to use superposition for circuits if I can just kvl it?

>>12786466
How did you learn it btw, on your own?
And also what was the reason that made you learn it?

>>12786640
>>/sci/thread/12223161

Trying to figure out how to start this.
1) Assumption
2) Analysis
3) Verify

I'm going to try to assume that Diode 3 is off and then I'll try to find the voltage drop across R1, but since D1 and D2 are both 'On', wouldn't the voltage at the point after R1 be 0.7? So I_r1 = 5-0.7/5k?

Linux Beginner mv or grep command

I need to move many files starting with A# and B# +6 random charac, at the same time but they are in different depositories

Ive tried 'mv -T B#* Depot' but it only works if I specify the path and I have way too many files.

Please remind me how to make this jump

>>12790550
Multiply left term by [math]\dfrac{x^2+1}{x^2+1}[/math]

>>12790557
Thanks, I thought it was something like that but my brain wasn't working.

Is there a word to describe organisms that lack the ability to think/learn and get along by just reacting to external stimulus? Things like bacteria, plants, jellyfish, etc.

>>12790423
try
mv ~/someDir/**/{B#*, A#*} ~/destinationDir

i have no idea if it'll work

>>12790623
why yes anon,
>(you)

>>12790636
Lol it kinda worked, I'm amazed, what does ** mean btw?

I've been stuck on this problem for quite a few days now. Prove that there exists a map [math] p: E\to B[/math] and a homotopy [math] f: A\to B[/math] such that the map [math] E\times_B A \to E[/math] is not a homotopy equivalence. I know that it can't be a fibration but I don't really know that many non-examples of fibrations so it's been really stumping me.

>>12790671
it's kind of like a recursive wildcard

see this link:
https://stackoverflow.com/questions/28176590/what-do-double-asterisk-wildcards-mean

and scroll down to the answer beginning with "Globbing"

Linux Question

I'm supposed to remove lower and upper case letters from a text without using Sed or awk.

I feel like cut could work but don't know how to write It. I'm getting errors.

>>12791149
Found it nvm, with the -d 'lower' 'upp'

can you get a high paying job from studying game theory?

Any OR plebs here? Why is this an incorrect reduction using min cost flow to model maximizing revenue?

Linux

I have a long text and I need to reverse the characters.

I've tried rev but I want the characters to switch without changing lines.

Basically I want:
123 -> 321
456 -> 654

And not
654
321

Im out of ideas.

>>12791442
Nvm I'm a retard. I got it.

>>12791149
are you taking a course or something?

Ok. Now I'm going to insert that thing into your brain and stir it. Don't be afraid, trust the science bro. You see? I'm a scientist. I know what I'm doing.

>>12791565
Yes, I'm doing homework and its noted.

tr command:

tr 'z' 'k' ; tr '3' 'm'

Is it possible to do the tr command only once to do the replacements simultaneously?

I've tried && "," () {} and what not.

Is it doable or can I use another command?

Not allowed to use awk or sed.

In the molecular term symbol, for example [math] {}^2 \Sigma ^- [/math], could someone help me intuitively understand the reflection symmetry denoted by - or +?

I understand it in words: the symmetry properties upon reflection on a plane containing the internuclear axis. but I don't understand what this looks like or means. my best understanding of [math] \Sigma [/math] states is that they are the zero angular momentum projection states, but when I picture these states they are always symmetric about the internuclear plane. I'm essentially just thinking the shape of s orbitals

>>12790395
D2 is off, D1 and D3 are on.
The voltage at the cathode of D3 is -0.7V => IR3=4.3/5000=860μA.
On the left, IR1=ID1=IR2 = (15-0.7)/10k = 1.43mA => V=5-1.43*5=-2.15V.
D2 is reverse biased by 2.15-0.7=1.45V => ID2=0.

I started with the assumption that all were on, leading to:
The voltage at the cathode of D3 is -0.7V => IR3=4.3/5000=860μA
=> The voltage at the cathode of D2 is -0.7V
=> The voltage at the anode of D2 is 0V
=> IR1=5/5k=1mA, ID2=IR2=9.3/5K=1.86mA
=> ID2=1mA-1.86mA=-860μA (by KCL)
=> D2 must be off.

In hindsight, this should have been obvious. With all 3 diodes on, the anodes are all at 0V and the cathodes at -0.7V. But R2 has a ~10V drop while R1,R3 have ~5V. So IR2>IR1 => ID2 would be negative.

>>12791149
>>12791442
>>12790423
>linux shell programming
Why? Just use python and get with the times.

>>12789706
> How do I derive v_out?
Is the zener ideal or following some I-V curve?

If it's ideal, the first thing to do is to remove it, calculate Vout=Vin*R/(R+Rload), and check whether that's greater than the threshold voltage. If it isn't, you have Vout. If it is, Vout is simply the threshold voltage. Iz=(Vin-Vout)/R-IL=Iz=(Vin-Vout)/R-Vout/Rload (input current minus load current).

If it's non-ideal, for a given load current IL, find where Iz=(Vin-Vout)/R-IL intersects the I-V curve.

> would V_out also be same as the voltage drop across the zener resistor? I'm not sure actually.
Yes. V_out=Vb. For an ideal zener, it's either the threshold voltage or Vin*R/(R+Rload), whichever is lower.

>>12791683
Bourne shell (incl. variants) sucks as a programming language, but it's guaranteed to be present on any Unix system, functional from the moment the system boots, and largely immune to version and configuration issues.

>>12791727
i'd think the time spent learning bash would be greater than the grand total of time spent installing python on systems that don't have it. and obviously one is way more useful than the other.

i use bash all the time too though, by nature of using linux as my daily driver. but it's not something I'd 'learn', imo.

>>12791583
>Not allowed to use awk or sed.
lol unis are so dumb, can't make this shit up

>>12791740
Yes, I agree. I hate my fat bitch of professor.

>>12788546
A vector with complex values is perfectly fine and very common. A vector space is just a module over a field and [math]\mathbb{C}[/math] is a popular field to work with. If he's talking about the vector (i, 1) then that's what he's talking about.

Sorry to ask again about >>12789916 , but I feel like I am missing something very basic here:
When exactly can you integrate both sides of an equation? For example something like
[math]{\displaystyle {\frac {dN}{N}}=-\lambda dt}[/math] or [math]{\displaystyle {\frac {d^{n}f}{dx^{n}}} = 0} [/math] work, but something like
[math] x = a [/math]
doesn't.
I really am missing something very basic here… I feel like it is related to missing [math]dx[/math], but I cannot make sense of it

>>12792023
>x = a
I guess that's a bad example, since a isn't a constant here, but for example
[math]{\frac {df}{dx}}= C[/math]

>>12792023
You have to integrate with respect to some variable. A statement like [math]\int x[/math] is nonsense. It literally has no meaning. You have to write something like [math]\int x\ dx[/math] or [math]\int x\ dy[/math].

As for why you have to go back to the basic definitions of calculus and the fact that integration is the infinite sum of some infinitesimal. If there is no infinitesimal how can you perform the sum.

>>12792023
In general, you can't integrate a variable which depends upon the independent variable in some unspecified way.
[eqn]\int a(x)\,dx = \int a(x)\,dx[/eqn]
In order to get anything meaningful, you need to know the relationship between x and a(x). So:
> but something like x = a doesn't.
You can't treat "a" as a constant. So
[eqn]\int a \,dx \neq ax+C[/eqn]
As a = x:
[eqn]a = x \implies \int a \,dx = \int x \, dx = {x^2 \over 2}[/eqn]
If you have an equation defining a dependent variable as equal to some expression in the independent variable, to integrate the dependent variable you have to replace it with the expression. So given
[eqn]a = f(x)[/eqn]
then
[eqn]\int a \, dx = \int f(x) \, dx[/eqn]
becomes
[eqn]\int f(x) \, dx = \int f(x) \, dx[/eqn]
which is pointless.

Now, if you have an equation which equates two *different* expressions, then you can integrate both sides to get something meaningful.

E.g. given y=f(x), you have dy/dx=f'(x) and thus y dy/dx = f(x)f'(x). Integrating both sides gives you
[eqn]
y = f(x) \\ \implies \int y \, {dy \over dx} \, dx = \int f(x)f'(x) \, dx \\ \implies \int y \, dy = \int f(x)f'(x) \, dx \\ \implies {y^2 \over 2} = \int f(x)f'(x) \, dx \\ \implies y = \sqrt {2 \int f(x)f'(x) \, dx}
[/eqn]

>>12786333

>>12777391
How does this calculation work?
Prx=G.Ptx
I'm not sure what the period actually denotes, and it's not explained in the documentation

>>12792250
It could mean multiply, it could be a typo, it could depend on the context of which you have given none.

>>12777828
Not making a choice is a choice
It doesn't matter

>>12792294
Turns out it was multiply, and sorry for lack of context. It was in a bunch of slides about telecomms, and I didn't want to copy paste the context

>>12792313
>>12792294
But thanks a lot. You got me in the right direction

>>12792061
>>12792116
Thank you very much both of you! I think I fully get it now!

Sorry to ask one last question, but is there a name for that way of solving y=f(x)?

Sorry to ask, but am I getting something wrong with the exact model here? This is literally the first task of my ODE course…

>>12792863
You don't need to solve for r, just transform each of those equations into a first order system. This is done by introducing auxiliary variables (e.g. r_1) such that [math] r_1 = r' [/math]. Then for a general second order system, instead of writing [math] r'' = ar' + br + c [/math], you set up the following system of first order equations:
[math] r_1' = ar_1 + br + c [/math] (from your 2nd order equation)
[math] r' = r_1 [/math] (by definition)
Is it clear how to apply this method to your problem?

>>12792881
Very clear, thank you very much!

>>12791908
my bad, I shouldn't be assuming stuff

if a is a integer and x is a prime and x is never greater than y and y is not divisible by x. use bezout's theorem to prove there exists a integer b such that for every integer o: x|a + yox-aby

If you combine the following

[math]
-1x^-2 * e^x + x^-1 * e^x
[/math]
Is the result e^x(x^-3)? If not, how else do you sum it up?

>>12793135
Wait it messed up
-x^-2 * e^x + x^-1 * e^x

Applied Differential Equations exam today, it's really easy but one thing I've been neglecting is how to graph and classify stuff properly. Here is a prime example:
>https://www.slader.com/textbook/9780321796981-differential-equations-and-boundary-value-problems-computing-and-modeling-5th-edition/346/problems/3/#

I can get the solution just fine, but I don't understand where this user got the equations to graph this (scroll down to just above the image).

>>12793156
>>12793156
[eqn]-\frac{1}{x^2} e^x + \frac{1}{x} e^x = \frac{-e^x + xe^x}{x^2} = \frac{e^x(x-1)}{x^2} = e^x\left(\frac{1}{x}-\frac{1}{x^2}\right)[/eqn]

>>12793169
How you get to the second step is a bit confusing for me, probably due to lacking fundamentals. Like the x^2 staying at the bottom while the other x goes to the top. Can you break it down how you add those two together exactly?

Also I understand that using 1/x instead of x^-1 is maybe a bit faster, but it's possible to reach the result with that way of writing as well, right? Where did I go wrong?

>>12793231
To sum fractions you need to have a common denominator, therefore you multiply the left one by [math]\frac{x}{x}[/math] to achieve that result.

> it's possible to reach the result with that way of writing as well, right?
[eqn]-x^{-2}e^x + x^{-1}e^x = x^{-2}(-e^x + xe^x) = x^{-2}e^x (x-1) = e^x(x^{-1} - x^{-2})[/eqn]

>>12793231
>>12793258
*you multiply the right one

Is there a version of the mean value theorem which holds for cauchy principal value integrals? What I mean is, can I say that
[eqn] pv \int_a^b f(x) g(x) dx = g(c) \bigg[ pv \int_a^b f(x) dx \bigg], [/eqn]
for some [math] c [/math] if the function f has only one singularity within [a, b] and otherwise satisfies the MVT assumptions on both sides of the singularity. You can assume that I'm a brainlet

>>12793258
>>12793262
Thank you. And to get to the last step? I understand -1/x^2 but why does x/x^2 result in 1/x?

>>12793296
[eqn]\frac{x}{x^2} = \frac{x}{x}\frac{1}{x} = \frac{1}{x}[/eqn]

do you normally have to be in the honors program to do undergrad research?

not sure if i should apply for honors, always thought it seemed kinda gay

Is it normal to think about dropping out of graduate school every day?
Not really sure what I'm looking for in writing this post. I'm towards the end of the second year of my Ph. D. program in a STEM field. I have felt like I'm really not doing anything productive since COVID shutdowns started. I was only out of the lab for about 2 months. Current COVID guidance makes me uneasy about going in - do I really need to be there right now? The postdoc who I work with has told me a few times when I've been in working with him that he would perform an experiment and I don't need to stick around. Due to the nature of the work we do it isn't really possible to perform more experiments even if I were there, though I could help with some cleaning or preparation. Is doing clerical work such as making equipment/reagent purchases, and working on my reading / written exam documents, good enough? I feel like I am hardly contributing anything, which is giving me doubt as to whether I should continue in this program. At our weekly meetings it seems everyone else in my research group has some update on some experiments they are doing. Between getting my B.S. degree and entering the Ph. D. program I had an industry job where I knew where I fit in to the company, and could easily see how my work was useful. I feel as though I am more or less a leech here. The department has documents saying to consult with them as soon as possible if you are considering withdrawing from the program, but I don't have an exit strategy... I'm worried that starting that discussion will remove my option to stay in the program.

Thanks for reading my blog post. Also fuck reddit r/gradschool, they autodeleted this shit probably because I don't have enough reddit circlejerk points.

>>12793623
To do undergraduate research, you just need to reach out to your professors. Honors stuff isn't necessary, but it can help if the prof doesn't already know you since it shows that you're not a complete dumbass.

Why does this summation don't show up in the final answer?

I worked through Hefferon's "Linear Algebra" a few weeks back. Would it make sense to go through Axler's "Linear Algebra Done Right" as well or does it have mostly the same material?
If it is very similar, what book would you recommend to read to get more into linear algebra?

>>12793925
im not familiar with Hefferon's book enough to be sure, but LADR is intended as a second course on linear algebra, is more conceptual and proof-based not mechanical or computational like most first courses.

>>12793811
The summations disappear because they're just considering each term of the sum individually - two of the integrals (2 & 4) go to zero for all n, while the first and third integrals goes to pi * the nth coefficient of the cosine series of f, and so their difference is 0 for all n.

>>12780620
It is a tensor, other guy is wrong. Equation is much more elegant in tensor notation but it will cost you a day to learn it

>>12794014
Thank you, anon.

how do i solve something like this?
[math]
x - 3 y + 2 x y = 0, 4 x - 6 y - x y = 0]
[/math]

>>12794317
xy = 4x-6y
x-3y+2(4x-6y)=0 --> 9x = -15y --> x = -5y/3
4(-5y/3) - 6y - (-5y/3)*y = 0 --> 5(y^2)/3 = -38y/3 --> y^2 + 38y/5 = 0 --> y(y+38/5) = 0
so either y = 0, or y = -38/5
then x = -5y/3 --> x = -5*(38/5)/3 = -38/3
possible I made an arithmetic error in there so double check it for yourself.

>>12794317
same as any system of linear equations: solve one equation for y in terms of x (or the opposite) then plug it in the other and solve it.
btw >>12794399's solution is wrong

>>12794317
x-3y+2xy=0
x(1+2y) = 3y
x = 3y/(1+2y)

4x-6y-xy = 12y/(1+2y)-6y-3y^2/(1+2y)
= 12y/(1+2y)-6y(1+2y)/(1+2y)-3y^2/(1+2y) = 0
-> 12y-6y-12y^2-3y^2 = -15y^2+6y = 0
-> y = 6/15 = 2/5

x-3*(2/5)+2x*(2/5) = x -6/5 + 4x/5 = 9x/5 - 6/5 = 0
-> x = 2/3

Check again:
2/3 - 3*(2/5) + 2*(2/5)*(2/3) = 2/3 - 6/5 + 8/15 = 10/15 -18/15 + 8/15 =0

4*2/3-6*2/5-(2/5)*(2/3) = 8/3-12/5-4/15 = 40/15 - 36/15 - 4/15 = 0

Good thing I checked, because I made a mistake earlier. Always check your solution.

>>12794430
huh, my calculator is unable to do it, so i thought somehting funny must be going on.

so i have
[math]
y = \frac{-x}{-3x+2x}
x = \frac{3y}{2y+1}
[/math]

and substituting for say x, then my original equation is
[math]
3 y / (2 y + 1) - 3 y + 6 y^2 / (2 y + 1) = 0,
12 y / (2 y + 1) - 6 y - 3 y^2 / (2 y + 1) = 0
[/math]

of which my calc still says no solution

>>12794470
>>12794430
thanks, I was trying to solve it like >>12794470. why is my calculator unable to do this?it handels normal systems fine

>>12794489
what's the difference between normal systems and the one you input in your calculator?

>>12794514
is this rhetorical, because I'm wondering the same. normal system usually don't have x*y, but that should be superficial.

even when I step through it by letting it solve for x or y (which does correctly as confirmed here: >>12794470, then I let solved variable take the new value, and try to solve for the remaining one, but it still says no solution.

this is output from my calculator: >>12794489

it normally solves these system fine, but something is diffrent here and Im not sure what

>>12794527
>is this rhetorical
yes
calculator solvers are pretty dumb, you need to input simple equations. your equation has rational fractions which can fuck with your calculator as it attempts to solve it numerically and encounters a singularity. try to get rid of the (2y+1) denominators, it should make stuff neater

>>12794556
huh, weird. usually it's pretty good. thanks for the help. unfortunately, getting rid of the denominators didn't fix it. fwiw im not worried about the algebra, just about how to get my calculator to solve these sorts of things so i can check my work and go fast on exams.

>>12793980
Thanks! I thought about it some more and might just pirate it to get a feel for how similar they are.

>>12794670
i didn't finish LADR but i really liked the parts I did, it was at just the right level of rigor for me. I like the typesetting and style in the second edition more as well.

>>12794670
oh, Axler also has lectures on YT for the book

How do you know if a quadrangle is a square in a certain perspective?

Linux Question sort command

I have to sort through lines but I want to ignore number.

My files are in this order 'numbers;numbers;Text'

I've tried with-k but without luck.

>>12795474
Does ; mean a newline or a csv-like file or something else?

Either way you can't do it with just sort.

>>12795577
Don't think It means new line bc I tried -k 3.3

They are simply separated by ;
No space or anything else.

I have 10 lines like it (#;#;text)
so it's not possible?

While we are at it, can I excuse tr replacements simultaneously?
Like tr 'a' 'b' and 'c' 'd'

I tried to put add (), {}, && between them but with no success

>>12795614
if its always in the format #;#;text then

cut -d';' -f3 file.txt | sort

>>12795624
I can only use grep and sort, I guess I'm fucked.
I don't think grep can send back while ignoring numbers.

>>12795651
why da fuck can you only use grep and sort?

>>12795667
I'm not working on my own shelf, it's my professor whos forcing us to do this shit as a test.

>>12795676
egrep -E -e'[a-zA-Z]+$' -o file.txt | sort

>>12795689
I'll try to work with this, Cheerio mate

>>12795286
Any convex quadrilateral can be represented as a projective transformation of a square. Given the four vertices of the unit square [u_i,v_i] = [±1,±1] and the four vertices of the quadrilateral [x_i,y_i], transformation by a 3x3 matrix followed by projective division gives you eight equations:
x_i = (c11.u_i+c12.v_i+c13)/(c31.u_i+c32.v_i+c33)
y_i = (c21.u_i+c22.v_i+c23)/(c31.u_i+c32.v_i+c33)
Add a ninth equation c33=1 to get nine equations in nine unknowns (c_ij) which can then be solved.

The math actually works for a non-convex quadrilateral, but you'll end up with an inversion (points inside the square map to points outside the quadrilateral, including the point at infinity).

>>12795474
Did you set the field separator with -t?

sort -t\; -k3 ... should work (you need to quote or escape the semicolon if using the shell).

>>12796209
How do you know it's not a rectangle or a rhombus?

>>12796216
Yeah it worked, thanks. What does the '\' mean? It erases the semicolon?

btw Im trying to do many letter replacement at the same time with tr. Any idea how can I do it at once instead of piping and using tr every time?