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/sqt/ - stupid questions thread
/qtddtot/ - questions that don't deserve their own thread

I have a question but don't want to start its own thread for it edition

What is the usual pKa range for hydrocarbons? Is it like 40-50? Or do you usually say about 50?
>>
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/sci/ I NEED YOUR HELP URGENTLY.
I have this unusual problem and I absolutely need to get it done until tomorrow: >>>/wsr/690202.
PLEASE HELP.
>>
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I have 42$ in amazon points what should I buy
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>>10798342
onahole
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>>10798346
BRILLIANT
>>
>>10798139
I created a polynomial time algorithm that checks whether two graphs are isomorphic.
What do I do now?
>>
How does one simplify this.
Nm^2/n^-2
Please mention rules used in each step.

In my textbook this appears in an example.
243w^10/36w^-4
=27w^(10-(-4))/4
I don't understand how the w is gone on the right side can someone explain.
Please resbond.
>>
>>10798874
(x^a)*(x^b) = x^(a+b)
=> (x^n)*(x^-n) = x^(n-n) = x^0 = 1
=> x^-n = 1/x^n
>>
I got it right hydrogen and helium produced in the Big Bang are being fused into other, heavier elements in stars and their supernovas.
Questions:
Is there a natural process that divides heavier elements into hydrogen and helium?
If it exists, does it or will it ever be done a rate even close to the rate at with hydrogen and helium are being fused?
If the answer to the previous is no, does that mean the universe will eventually run out of hydrogen and helium?
>>
How the fuck did people graph things before Descartes?
>>
Checking out a problem while studying for an exam: Let a triangle be ABC, and to find the point D where the sum of the distances from A to D, B to D, and C to D is minimum (in the XY grid). Let's name these distances a, b, c for convenience. The proposed solution uses the sum of the squares of a, b, c (which simplifies the situation massively); however I don't get how the sum of the squares of a, b and c being minimum implies that the sum of a, b and c shall necessarily be minimum.
>>
>>10798294
You need to include measurements in your question and a picture or drawing of what you are doing. We aren't psychic.
>>
>>10799532
> I don't get how the sum of the squares of a, b and c being minimum implies that the sum of a, b and c shall necessarily be minimum.
In general, it isn't.

If you can program, try some test cases. Pick three random points, find D which minimises the sum of the distances, find D which minimises the sum of the squares of the distances. They won't be the same point.
>>
>>10799645
Already did it, and it showed as you commented, thanks. Guess I'll have to write an equation that determines the third distance as a function of the other two, then run the sum as a Hessian.
>>
>>10798978
> Is there a natural process that divides heavier elements into hydrogen and helium?
No. Although radioactive decay (and fission) may emit free neutrons (which decay to protons, which are hydrogen nuclei) and and alpha particles (which are helium nuclei), these processes are far less common than fusion. In general terms, the process starts with hydrogen and produces heavier elements as it progresses.

> If it exists, does it or will it ever be done a rate even close to the rate at with hydrogen and helium are being fused?
No.

> If the answer to the previous is no, does that mean the universe will eventually run out of hydrogen and helium?
No. Eventually you end up with matter that's not hot enough or not dense enough to sustain fusion.
>>
What's the quickest way to check whether two matrices commute without going through the pain of computing the matrix products?
>>
>>10799703
Let the matrices be A and B. Treat them as operators in the canonical base. If you can find a base of eigenvectors that diagonalizes the two simultaneously, they commute.
>>
>>10798294
Can't you like, dremel it?
>>
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----------BABBY R HOMEWORK HELP-----------

d. For each dataset, create a summary table for each variable in the data set. The descriptive statistics should include the mean, standard deviation, range, and number of missing values for that given variable. *Hint:* A very simple way to do this is to create an empty matrix, fill it with the needed values, and to name the rows and columns appropriately. Print your table nicely using `kable()` or `pandoc.table()`

What's with the hint? That sounds like a really gay roundabout way of doing that. There's got to be a way I can leverage summary() somehow?
>>
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>>10798139
What does Tau mean here? The rest i get, but not that part.

https://web.stanford.edu/class/psych209a/ReadingsByDate/02_25/Williams%20Zipser95RecNets.pdf

Page 4, if you need more context.

//t.engineer trying to compsci
>>
>>10800384
Didn't read but since in your screenshot functions always have parentheses, the tau is a constant. Since it has dimensions of time, I assume it's a characteristic time scale or something like that
>>
>>10798795
ANSWER ME
>>
Retard here, can somebody explain why the gradient is the orthogonal vector to a surface in R(n-1)?
>>
>>10800708
it's orthogonal to the "surface" of constant value that goes thru the point.

you can visualize it by saying it's calculus, so near the point, it's a "plane," and the direction of greatest increase is orthogonal to the direction of constant/no change
>>
Let's say there is a function [math]f[/math] that is discontinuous at [math]a[/math], while there is a function [math]g[/math] that is continuous at [math]a[/math], is it possible to find a pair of the two functions where [math]f(x)/g(x)[/math] is continuous at [math]a[/math]?
pls help
>>
>>10800979
No, the product of two functions that are continuous at a must also be continuous at a.
f(x) = f(x)/g(x) * g(x)
>>
Hey, I want to know if my vague intuition on wavefunctions and QM makes sense or is wrong.
I know I should just take a qm course but I will only do it in a year, and my friend taking that course asked me some questions I have too so maybe an introductory qm course doesnt necessarily answer it.

I imagine particles as waveforms which can be interpreted as describing where the particle probably is.
When a photon hits an electron then the waveforms move towards eachother, interact and chance eachother and then maybe a photon bounces off again or whatever.
Analogously I imagined the collapse of a waveform simply as a waveform contracting to a very small location, so the particle is probably in a very very small area.
So in the double slit experiment I imagined the electron waveform interacting with the screen so it continuously contracts to some tiny area where the photon hit but it isnt a dirac delta.
A professor said though that waveform collapse is an absolutely discrete process.
I also dont get how obververs would work with this, can the waveform collapse for one observer but not for another?
this is probably a really stupod question because I dont even know qm
>>
>>10800989
Yes, that's a theorem, but that theorem only says that if we have two continuous functions at the same point, then the product of the functions must be continuous at that point, but this theorems doesn't mention what happens when you take a product of a function that is a discontinuous at one point and a function that is continuous at that same point, sometimes, that product, can yield a continuous function at that point, for example, let's define the function sgn(x), which gives the sign of a number, when the number is negative, the function returns -1, when 0, 0 and when 1, 1, this function is discontinuous at x = 0, but if we take the product of sgn(x) with x it gives a new function which is abs(x), and this function is continuous at x = 0.
So there may be some cases where the product of a continuous function at a point and a discontinuous function at the same point may yield a continuous function at the same point, but can this happen with division?, especially where the numerator is the discontinuous function and the denominator is the continuous function?
>>
>>10799664
Thanks.
>>
I'm having a bit of trouble discerning cause and effect regarding the second law of thermodynamics. What is the fundamental cause of heat flow? Is it perpetual increase in entropy within a system, or tendency for a system to move to a state of minimal energy? I believe it's the former, but Wikipedia explains the second law as if entropy increase is caused by heat flow.
>>
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Wtf is "right triangular region"?
>>
How did you all decide on what university to go to? Im getting out if the military at 25 and am trying to go back to school. All the reviews i look at talk about shit i dont care about like dorm life, frats, and diversity. I can reasonably any state school.
>>
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>>10798139
How do I calculate the capacity of a capacitor that is hooked on sinus voltage source
u(t)=100sqrt(2)sin(10^3t) [V]
if the value of el.current ( I ) flowing thru the capacitor is 1[A] ?
>>
Is the neurology of religion understood?
I've been an atheist for my entire life but recently started reading the bible twice a day and pray and now I'm 50/50 on if there is a god, how does belief about religion form? placebo?
>>
>>10803176
entropy doesn’t have a cause its a mathematical construct, you can’t measure entropy
>>
>>10803918
>entropy doesn’t have a cause *yet*
It's perfectly reasonable that humans could physically quantify entropy in the future, maybe the far far future, but I wouldn't discount it yet.
>>
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Since blood carries oxygen to the brain, isn't bleeding to death just another form of asphyxiation?
>>
>>10803748
If the voltage across the capacitor is a sinusoid, the current flowing through it is also a sinusoid, with a 90° phase shift. I=C*dV/dt.
> u(t)=100sqrt(2)sin(10^3t)
=> i(t)=C*10^3*100sqrt(2)cos(10^3t)
> if the value of el.current ( I ) flowing thru the capacitor is 1[A] ?
Is the 1A peak or RMS? Given that there's a √2 in there, I'll guess that it's RMS. The peak current is 10^5√2C, the RMS current is 10^5C, so if that's 1A, then C=10μF (assuming u(t) is in volts and t is in seconds).
>>
>>10804037
>Is the 1A peak or RMS? Given that there's a √2 in there, I'll guess that it's RMS. The peak current is 10^5√2C, the RMS current is 10^5C, so if that's 1A, then C=10μF (assuming u(t) is in volts and t is in seconds).
yeah that checks out with the answer sheet thanks
>>
>>10804008
No. Asphyxiation is the failure to get enough oxygen into the blood through breathing. This includes obstructed airways, paralysis of lung muscles, being in an environment with insufficient oxygen (e.g. drowning), etc.

Failure to get enough oxygen to tissue is hypoxia, which can be caused by asphyxiation, blood loss, heart attack, blocked arteries, anaemia, carbon monoxide, etc.
>>
>>10803918
Subtle point: I didn't write about the cause of entropy (which is merely a defined quantity), I wrote about the cause of heat flow (from high-temperature objects to low-temperature ones). We take it as completely that hot things give heat, and cold things draw heat, but is this as a result of the second law? Or something else?
>>
Are there natural CCR5 supressors
>>
Where x≥0 and y≥0
x/(x+y+5)>y/(x+y+5)
Why can't I simply multiply both sides by (x+y+5)?

Instead I'm told to
x(x+y+5)>y(x+y+5)
x^2+5x>y^2+5y
x(x+5)>y(y+5)
x/y>(y+5)/(x+5)
>>
>>10804519
> Where x≥0 and y≥0
Are you sure about that?

If x+y+5≥0 then multiplying by x+y+5 is safe. But in general, it isn't; if x+y+5<0 then the comparison gets flipped. Whereas multiplying by (x+y+5)^2 is always safe, as a square is always positive.
>>
>>10804862
thanks, thought as much
I can only assume it was using it as an example of squaring to ensure equality (even though it specified x and y are positive)
>>
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>Wash my clothes wrong
>Lab coat is now a mesh of purple and pink
>Friends at uni make fun of me for looking like a pimp
>>
>>10805271
Embrace it
>>
>>10803492
nvm it's a triangle
>>
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Can I do Calculus problems in LISP? I want to take down two flies with one hit, if possible.
>>
>>10801174
>photon bounces off
Think it might be absorption then ejection.
>waveform contracting to a very small location, so the particle is probably in a very very small area.
Not with conventional measurements. Usually it's a wall splat, or orientation. I'm still getting back to date. But I think a waveform in collapse has been measured over milliseconds. For a super position in a device of sorts.
>electron waveform interacting with the screen so it continuously contracts to some tiny area
Extended field interaction? Or moments before impact?
Well either way, it's not settled yet. You've got a few operations of Q waves.
Bohmian mechanics suggests quantum states observe quantum states. I read it as a wave peaks adding up but not a measurement itself. But can be measured.
Those wave peaks are correlative when a wire frame is drawn from them. Giving pathing knowlege.

then there's Copenhagen interpretation
>Despite an extensive literature which refers to, discusses, and criticizes the Copenhagen interpretation of quantum mechanics, nowhere does there seem to be any concise statement which defines the full Copenhagen interpretation
Basically you're golden whenever you reference it ;)

The statistical interpretation.
Literally it's just a statistic.

There's more. But Copenhagen kinda spams over them.

>A professor said though that waveform collapse is an absolutely discrete process.
Doesn't seem the case if a wave in collapse was measured, But I can't remember source.

>can the waveform collapse for one observer but not for another
>this is probably a really stupod question because I dont even know qm
I've been interested in this for too long. And I still have to check basics.

For observers. Seemingly yes, some sort of wave blind spot to interaction constituting a observation. If that wave collapse was true.
>>
>>10805271
you lucky mac daddy player , bet that calc hand strong....
>>
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How is the "average" range of a distribution determined? Is it defined by falling within one standard deviation, something entirely different, or arbitrary depending on what we're talking about?

For example, when do you call someone short/tall, and when can they still be considered average?
>>
If I have some group [math]G[/math] then the torsion subgroup of [math]G[/math] is defined as [math]\{g \in G | \mathrm{ord}(g) < \infty\}[/math]. If [math]G[/math] is abelian then it's easy to prove that the torsion subgroup is a subgroup but I heard that if [math]G[/math] isn't abelian then this doesn't have to be true.
So does anyone have an example of a group whose torsion subgroup is not a subgroup?
>>
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Is it worth to read Hammack's book on proofs before starting calculus? I know precalc and I already know propositional logic (mostly forgot predicate unfortunately).
>>
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At t=0 the S opens after it achieves stationary state
How do I calculate induced voltage on the inductor at t=0 and its magnetic field at t=40ms

a direction to books with similliar problems would be appreciated
>>
Test
>>
>>10806598
[math]\text{GL}_2(\mathbb{R})[/math]

[math]\begin{bmatrix} 1 & 1 \\ 0 & -1 \end{bmatrix}[/math] and [math]\begin{bmatrix} 1 & 0 \\ 0 & -1 \end{bmatrix}[/math] are both torsion, but their product is not.
>>
>>10807099
no
>>
>>10807146
Fuck.. need to brush up on EE
>>
>>10807146
With the switch closed, the equilibrium (stationary) state is when the inductor's current is constant and the voltage across it is zero (V=L*dI/dt). The inductor current will be 120V/10Ω=12A.

Opening the switch will cause that current to circulate through the 10Ω+30Ω resistors = 40Ω, so the initial voltage will be -12A*40Ω = -480V. Current and voltage decay exponentially with a time constant of L/R = 1H/40Ω = 25ms. After 40ms (= 1.6τ), both will be reduced by a factor of e^-1.6 ~= 0.202, so 96.9V and 2.42A.

The magnetic field is proportional to the current, but the actual value can't be determined from inductance alone; you need to know the geometry, dimensions and core permeability.
>>
What's the best CAS?
>>
>>10807099
I tried it. Was bad. Just find a babby combinatorics book and start proving
>>
how can I build a calculator with cellular automata
>>
Just something to keep an eye on: >>>/wsr/691714
Help if you can. It takes only 5 minutes.
>>
Why do cars accelerate in a spin so fast when rolling over? Is it conservation of angular momentum?
>>
>>10808442
if I wanted an approximate value of energy stored in inductors magnetic field at 40ms
Could I use
U=1/2*L*(I*0.202)^2
>>
>>10798139
Depends on solvent, but in DMSO reaches 57 or so. Check in Bordwell acidity tables: https://www.chem.wisc.edu/areas/reich/pkatable/ In general, acidity increases as in sp > sp2 > sp3 carbons, in sp2 and sometimes in sp3 directed metalation may happen. Think of it like an chelation of lithium by something, and if you want details read Baran's lab directed metalation survival guide
>>
>>10800468
Follow the AMS guidelines and write up a paper on it.
>>
>>10803176
>>10803918
>>10803949
>>10804206
The whole matter comes down to the nature of heat. Remember that heat is not a "primitive notion" like mass or charge is. It is the aggregate result of the nature of matter as a bunch of atoms which interact with each other. For the case of gases at common conditions, this can be approximated as hard, minuscule spheres frictionlessly bouncing off of each other. Given all their masses, positions, and velocities, you can completely describe the evolution of their states with Newtonian mechanics alone. Indeed, some simulators which exist to teach the concept of heat do exactly that.

In practice, we don't have that much information for real systems, so we describe them using aggregates that are easy to measure, like temperature, pressure, and volume. Since these don't give us the full picture, we use statistics to tell us what information these variables provide us. In particular, entropy says that, given our usual choice of macroscopic variables, how many microscopic states could produce those values, hence:
[eqn]S = -\sum_i^n \mathrm{Pr}(\Omega = i) \ln \mathrm{Pr}(\Omega = i)[/eqn]
Where [math]S[/math] is entropy and [math]\Omega[/math] is the current state of the system, expressed as an index from a list of all [math]n[/math] possible states.

Hence, since most of the states (a vast majority, in fact) only belong to the higher entropy classes, a system which does a random walk across its state space will spend most of its time in the higher-entropy states. So much so that once it reaches them, you are astronomically unlikely to ever see it go down for a system of any macroscopic size.

That is the origin of the second law of thermodynamics, and hence all heat flow. A system with heat spread out across it has a much higher entropy than one with hot spots and cold spots.
>>
>>10803561
You're much better off doing your associates at a junior college and then transferring, it's cheaper and better for older people.
>>
>>10808927
Yes.
>>
>>10809505
ok tnx
>>
>>10808877
Angular momentum isn't conserved (at the start of the roll, there isn't much angular momentum).

If the car starts to slide sideways, there's an inward force applied in the plane of the road, below the centre of gravity, resulting in torque about the forward axis. In order to cause a roll, this has to be enough to counteract the torque from the offset between the centre of gravity and the outside wheels, which normally keeps the car level. Once the car starts to roll, the horizontal distance between the centre of gravity and the outside wheels decreases and the corrective torque decreases, resulting in an increasing net torque and thus increasing angular acceleration.

It's the same as an pole which is balanced on its end. Once it starts to topple, any deviation from the vertical increases the torque which accelerates the fall. With a rolling car, the effect is magnified by the fact that the roll will tend to push the outside wheels against the road, increasing the friction.
>>
>>10798342
Nonlinear Dynamics and Chaos
>>
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I needed to calculate the current thru N and the powers, what do I use for φ?
>>
>>10809881
By KCL, In=-(V1/Z1+V2/Z2+V3/Z3). The Vi will typically each be the same nominal voltage multiplied by a distinct cube root of unity. There's no reason to use angles unless you need the resulting current in polar form (in which case, arg(In) is the phase angle relative to whichever phase uses the real cube root).
>>
>>10798139
What is my friend talking about?
>>
>>10810248
>>
>>10800979
I'm assuming you aren't trying to divide by zero, so the answer is no. Suppose you got a continuous function doing that, then multiply that new function by g so it'll be continuous again at a, but you get f.
>>
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>>10810248
>>10810251
He wants you to inject him with your DNA.
>>
>>10810248
>>10810251
That he's high as balls, and >>10810293
>>
>>10810293
baboom
>>
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I have a very stupid Algebra question.
Let [math]G = S_4 [/math] be the symmetric group and [math]H = \{(123),(132),(1)\}[/math] be a subgroup of [math]G[/math]. Further let [math] \sigma = (14)[/math].
The problem asks to calculate [math]\sigma H [/math] and [math] H \sigma [/math] and then asks whether [math]H[/math] is a normal subgroup of [math]G[/math].

What I calculated is
[eqn] \sigma H = \{(1234),(1324),(14)\} \\
H \sigma = \{(1423),(1432),(14)\} [/eqn]
since they are different, I concluded that [math]H [/math] is not a normal subgroup.

But the book claims that
[eqn] \sigma H = \{(423),(432),(14)\} \\
H \sigma = \{(423),(432),(14)\} [/eqn]
and that [math]H [/math] is a normal subgroup of [math]G[/math].

What is going on here?
>>
Any good resources on how to study in the most efficient manner possible? I've been using anki but usually fuck up by not making cards for the right stuff, and it's hard to utilize for remembering abstract principles.
>>
>>10810248
>>10810251
Explaining evolution and how we're just a canvas for the shitshow that is evolution, and our consciousness provides a viewpoint. Sounds like someone is studying intro biology and likes to write.
>>
>>10811328
It seems like you're right, though I did not write down the calculations
There must be a typo.

H is a subgroup of order 3, so it has index 8, so if it were normal it would contain all elements of S4 of odd order. This is clearly not the case, e.g. it does not contain (234)

why can't I build muscle?
>>
>>10811328
>>10811356
also, just saying, even if sigmaH = Hsigma, it wouldn't necessarily mean that H is normal since it has to hold for all elements. If the exercise is about normality and starts with "compute Hs and sH some some element s" then it's probably because it's not normal
ditch your book
>>
Anons who graduated from engineering programs:
Should I keep my lower level textbooks for future reference?
I have some physical copies and some from libgen.
>>
>>10811335
Don’t rely on notes, do exercises for technical mastery and nothing else, derive theorems yourself, go over your readings multiple times a week and ask difficult conceptual questions during office hours or when you can get a hold of another strong student. The thing with notes is that you can’t bring them with you for exams, anything you can commit to long term memory is an asset and otherwise you’re just loading your working memory and stretching your ability to respond to novel situations where that knowledge is going to be tested like on a difficult exam with questions you haven’t seen.
>>
>>10809358
I just realized that heat is not "energy" (i.e. total energy, the conserved quantity); free energy is. If we consider a closed system, free energy is constant; so when entropy increases, internal energy (heat) must decrease. This still leaves me to understand why heat flows from hot to cold, though. It's easy to understand the picture of fast-moving molecules colliding iwth slow-moving ones and transferring their kinetic energy, but I want to understand it at the abstract level, considering only the basic thermodynamic quantities.
>>
>>10812117
read a book on statistical mechanics, the wiki has quite a few recommendations
>>
Why does every time mechanical engineering gets brought up people say it's either outdated or for plebs? What do they mean by that?
>>
Couple of retarded questions, bear with me. Answer as complex or as simple as you want.

1. Can anyone become smart, like top level smart in any given topic they choose?

2. Are there learning methods, and memorization methods that help you learn and remember the stuff you have learnt and read? Is there maybe a book that can help me?

3. Are there any sites /sci likes that show recent discoveries/breakthroughs in space, science, technology, or anything else that advances the human race?
>>
>>10813474
>1. Can anyone become smart, like top level smart in any given topic they choose?
no. when I was at imatriculation I was sitting next to some dude and we started talking. he said that it was his second attempt to study pure mathematics. his exact words were something like "it's a great program but it takes a long time to finish. usually like twice as much." that scared me a bit. then the school started, and it WAS hard, but I was never on the edge of being kicked out. he made it through three semesters, because these are kind of the same for all programs (pure math,statistics, applied..). once the actual math started (complex analysis, measure theory, differential geometry) you could see that he just didn't have a clue what was the subject about. last time I saw him was at a complex analysis exam which he botched. he was kicked after 4 semesters, same as the first time. so where was his mistake? he just didn't have what it takes.
>>
>>10813474

>1. Can anyone become smart, like top level smart in any given topic they choose?
No. People have different talents and motivations.

>2. Are there learning methods, and memorization methods that help you learn and remember the stuff you have learnt and read? Is there maybe a book that can help me?
You could try:
Learn More Study Less 2008 Young

>3. Are there any sites /sci likes that show recent discoveries/breakthroughs in space, science, technology, or anything else that advances the human race?
You might like:
https://www.sciencedaily.com/
>>
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Why does the integrated bit, [ ] expression, go to 0?
>>
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I'm wondering if anyone can help me find that one study that shows whites have a higher degree of behavioral variability than other ethnic groups
cant remember the name or author and google isnt helping
>>
>>10814107
It's just algebra, when you replace the 1 the parentheses are 0 and when you replace the 0 x/3n is 0
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>>10814145
oh yeah, im an idiot - I saw the ns in the expression and got confused - I guess thats what these threads are for
thanks anon
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where the hell do the beta dist numbers come from? I'm completely lost here.
>>
>>10814296
https://en.wikipedia.org/wiki/Beta_distribution
Seriously nigga. I never heard of that before but you can read them right from the equation.
>>
>>10813474
>1. Can anyone become smart, like top level smart in any given topic they choose?
No, there are people who can't even build muscle
>>
>>10813831

Is there a way people can find out what their talents lie in? Is it just a lot of trial and error? I would love to become good at something in life.
>>
>>10814548
If you had any, it would be obvious in the first place
>>
>>10814550

Depends on how blind the individual is to see their talent. For example, in just 1 year of training and sparring other boxers, I could see I was way above people who have done it for 4 years. That doesn't necessarily mean I was talented, maybe others just went there for fun and others might have not listened to their instructors well.

It's hard to judge talent in my opinion. Have you tried everything out in life to begin with?
>>
>>10814557
>Have you tried everything out in life to begin with
Yes, I've tried lifting and I can't build muscle
>>
>>10814550
Its easy to delude oneself especially if not particularly bright and quite sheltered, I’ve seen this recently in the context of academic work. A lot of people need to be broken before they can comprehend how mediocre they are.
>>
>>10814577
By obvious, I implicitly meant that it would have been recognized by somebody else (typically a professional) rather than by oneself or mommy
>>
>>10814583
A lot of professionals are incompetent and project onto students in their grade inflated classes, and a lot of students take mild praise as an indication of talent they don’t have. Again, I’ve witnessed this very recently with multiple people. If you mean a pattern of recognition at an early age that’s different
>>
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I must reduce this term but don't know if I should see the missing Cs in those parts as zeros or leave them. Also how do I keep going and am I even doing the start right?

Also is the kv diagram correct?
>>
How do I prove this by induction:

[eqn]y \leq \left (1 + \frac{y-1}{n} \right )^{n}, y\geq 0[/eqn]
>>
How do i trick my brain into understanding maths.

I surpass all my fellow students in writing, politcs, humanities, understanding complex political patterns and history. I got the best score ever in my university in a test wich you have to analyze political and philosophical dilmmas.

Still i failed basic maths as the only one in the entire class (400 people). It seems like my brain expects everything to be more complex than it is, thus fucking it up. also details is very hard, compared to "the big picture"

Is there any science to explain this? maybe not that related to the thread, but its stil a stupid question.
>>
>>10814665
by not being a brainlet
>>10814690
don’t do what the poster above you is doing
>>
>>10814697
thank you, i hope i pass
>>
>>10814701
remember to check the sticky and the wiki page
>>
>>10812121
You're right, I just should have looked it up on the book that I used years ago (Schroeder). Apparently (and quite logically in hindsight), you need to keep in mind that energy is exchanged between systems, and that total entropy tends to maximize. If one system is cooler than the other, it will gain more entropy per unit energy than vice versa, thus energy will move from hot to cold.
>>
Guys I’m in my senior year of an applied math degree and I’ve gotten lucky to make it this far honestly.
I’m $45,000 in debt, miserable, and honestly not smart enough to finish this degree. The only thing that I enjoy doing is webdev and design but my degree has nothing to do with it and I’d need to quit school to focus on that. I don’t know what to do.
>>
>>10798978
>does that mean the universe will eventually run out of hydrogen and helium?
No. Theres so much more hydrogen than anything else. Usually in the form of diffuse warm plasma floating between galaxies that is just going to sit there for just shy of forever.
>>
>>10814995

i'll trade debts and majors with you
>>
is there some obvious way to integrate this that I'm missing?
It's from the introductory chapter on integrals in Stewart's Calculus so obviously it has to be simple, but running it through wolfram alpha yields a result that requires the integral of the cardinal cosine function and other crazy shit
>>
>>10814645
> am I even doing the start right?
No. There's no purpose to your "backwards" truth table. Write out 8 rows and put ones in the row(s) corresponding to each term.

> Also is the kv diagram correct?
No. E.g. ab = abc+abc', i.e. terms with only two variables should occupy two cells (and 2 rows in the truth table). If there was a term with one variable, that would occupy four cells (and rows).

The abc' term is subsumed by the ab term: abc'+ab=ab. You can combine the ab and a'b terms to just b: ab+a'b=(a+a')b=b. In the remaining terms, any b' is redundant: a'b'c+b=a'c+b, ab'c'+b=ac'. So you end up with b+a'c+ac'.
>>
>>10815091
The function is clearly odd (antisymmetric): f(-x)=-f(x). Thus the definite integral over [-1,1] (or [-a,a] for any a) is zero. The antiderivative isn't required.
>>
>>10815474
That makes sense. I'm not really trying to evaluate the indefinite integral so much as just get experience doing the symbolic integration
>>
>>10815091
0 by symmetry, or am I mistaken?
>>
>>10815505
yeah, the definite integral is 0 due to symmetry but I was just trying to practice finding the indefinite integral of the function
>>
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How did the Greeks manage to calculate stuff like the area of a circle, or the volume of a sphere without integrals?
Wouldn't they be making wide assumptions and approximations without the concept of infinity?
>>
>>10815543
The indefinite integral is only expressible as a power series or using special functions. The observation about symmetry is the main idea behind this problem.

>>10815760
You need ideas from calculus to provide a coherent general theory of volumes. You don't necessarily need it to examine the volume of simple shapes stemming from observations/assumptions about even simpler shapes, for example the volume of a cube or the area of a square.

I believe the greeks calculated the area of a circle by constructing a series of alternating over- and under-estimates via circumscribed and inscribed (resp.) polygons with an increasing number of sides, finding the area of such a polygon (which can be represented as a sum of squares and triangles), and observing that the two approximations converged to the same number. Within the framework of modern math, we would use the epsilon-delta definition of limit to show that the sequences actually converge to the same number, but even before such formalism it was considered obvious.
>>
>>10814665
Use the binomial theorem.
>>
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not sure if this is the right place to ask, not homework and more of a philosophical question for someone with more knowledge of physics than i.
im wondering about "true" randomness in the universe. what are the current thoughts and ideas about this? by true randomness i mean not random in the sense that its not practical to ever be able to predict events because of incredible complexity that is beyond us, but things that are truly random at the smallest and most fundamental level.
i feel like there should always be a way to work out predictions, even in something as complex as a person deciding to skip work one day. should be able to figure such a thing out if ALL of the information needed was known, no? or is that a very uneducated view?
>>
>>10815856
In classical physics, there is no randomness. In quantum mechanics, there may be. Quantum mechanics is all just theory at this point, but it is commonly believed that quantum mechanical systems can exist in a superposition of states which then collapses (wavefunction collapse) into a single state. What state it collapses into is determined by probability; it is not necessarily guaranteed that it will collapse into a given state.
What causes wavefunctions to collapse is not known. It is generally said to be 'observation', but not everyone agrees on what 'observation' is. This is part of why there are multiple interpretations of quantum mechanics.
Of course, quantum mechanics in its entirety is just theory, so even the concept of wavefunctions and superpositions is just theory.
>>
If the sun is hot how is outer space cold?
>>
>>10815867
thats interesting, thanks. theory or not, it does produce many real world results, no? people have also gotten pretty good at predicting stuff then discovering things; LHC and the higgs particle ya?
still, thats weird. i dont think i can really wrap my head around randomness like that. might this stuff about probability and disagreement on what observation means indicate a lack of understanding of some deeper part of quantum mechanics? somethin that might make this part of it somewhat "simpler" to predict? or at least remove the element of randomness/probability
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>>10815880
Vacuums don't have a temperature
>>
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I own a pair of hot gloves with what I assume is either an ablative or grip-aiding coating (nitrile maybe), but it's an unknown substance. I was doing something foolhardy inside my house just now and melted the coating pretty well, inhaling a lot of the fumes before I turned off the heat source and got out of dodge. After confirming my house is not burning down, I'm quite worried what the fuck was on those gloves that made so much smoke that I managed to melt. Any ideas?
>>
>>10815883
>might this stuff about probability and disagreement on what observation means indicate a lack of understanding of some deeper part of quantum mechanics? somethin that might make this part of it somewhat "simpler" to predict? or at least remove the element of randomness/probability
Maybe. I think it's not so much that the explanations are necessarily complicated or complex, but rather that we just don't have the information/knowledge/etc to know whether or not said explanations are correct.
For example, in the 'consciousness causes collapse' interpretation of quantum mechanics, a wavefunction (probability) collapses from a superposition into a definite/real state when a conscious being observes it. That explanation in itself is very simple, clear, and straightforward, but it would be extremely difficult if not impossible to prove, because if conscious observation causes a wavefunction to collapse, then attempting to measure something in a superposition would necessarily make it collapse, which means observing it while in a superposition would be impossible.
Of course, the 'consciousness causes collapse' is just one example of a theory/possible explanation.
>remove the element of randomness/probability
I think it's too soon to say whether or not randomness truly exists in quantum mechanics, but it certainly seems likely.
You may find this interesting: https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics
>>
>>10815880
the same reason why we use vacuum thermoses to insulate our drinks
>>
>>10815880
Because outer space is a long way from the sun (or any other star). That's what "outer space" means: space that's far from anything.

Also, "space is cold" refers to the equilibrium temperature of an inert lump of matter, the temperature at which energy lost through black-body radiation equals that which is absorbed.

Anything that actively generates heat (e.g. powered machinery or warm-blooded creatures such as humans) will get rather hot without some kind of cooling system. Spacesuits generally cool by evaporating water into space, spacecraft have heat pumps and radiators.
>>
>>10816060
>asks bad question
>receives bad answer
really makes you think
>>
what do i email a professor to get into his lab and get undergrad research as well as get credit hours for it? should i just say some shit like "hey this is John Smith from your class last semester, i thought your research was cool and could help me gain valuable experience and gain credit hours through X class." and then maybe some more fluff? should i attach my resume?
>>
Can you tell me about some of those cases where you're presented with a mathematical problem and they're putting letters in there? I only ever took basic algebra in high school and when you'd see something like "x" in an equation, you'd just be trying to solve for it (like x = 2 in "x + 2 = 4"), so I know about that. Give me some examples of something else.

This is going to sound insane, but I had a dream where I was at the first day of a new job and didn't have a clue what I was supposed to be doing there and a woman just hands me a math worksheet to test my aptitude and while I can't exactly remember the problems, I was presented with nonsense like "CCOS + 86" and "AY / 42" and got fired on the spot for not having a clue what the hell these letters in the basic math problems represented. I woke up and felt a little angry about it, honestly.
>>
>>10816578
me

basically, I wanna be a little more knowledgeable incase that dream comes up again. I want to be ready for it.
>>
What's a good formulae collection and where can I get the pdf/epub?
I'm talkin' 'bout one of them there comfy reference books with physics, math, and a bunch of tables and lookups
>>
Do you think it's possible to learn everything between algebra and calculus in a year? I want to do comp sci at university, but I never really learnt algebra, let alone anything after that because my high school just passed me anyway and because I hated math back then. I'm willing to put in the work, but I'm 24 now and don't want to waste anymore time.

I'll be using Khan Academy btw.
>>
Smoke detectors contain radioactive stuff. Is it dangerous in the smoke detector factory?
>>
>>10798795
You post on Vixra and then start a thread here as proof, duh. If you aren't bullshitting there's between 7k and 9k as various bounties by mathematicians, plus a lot of money from whatever journal you pick to publish with.
>>
>>10816774
Only old ones really use radioactive stuff nowadays.
Most of em just use a low power led.
>>
>>10814690
Have you even tried to understand math?
Have you dedicated at least 3 hours to the first thing that made you fail? Do you thing you can learn without really working through problems, if you didn't even read carefully the chapter?
>>
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Can someone give me a bijection from (0,1) to (0,1)x(0,1)? I've tried for like an hour but I'm coming up blank.
>>
are US grad school admissions first come first serve? as in they will accept people as they come in and fill up, so it is beneficial to apply early for acceptance. or would someone applying at the deadline be fine?
>>
>>10809285
>>10816781
Thanks for the answers.
What if I don't have proof that my algorithm works and I only have experimental evidence?
Does that make it basically useless?
Also, who gets the money? The one who created the algorithm or the one who proved its validity? Probably the latter right?
>>
I was using the university wifi on my phone and posted on 4chan from it to find out the ISP had been for posting for CP. It was back in February apparently, but should I be worried that, because I opened the banned message, I can be tracked, or be accused of posting fucking CP from the university's wifi?
>>
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>>10817400
Use AoC to get a well-ordering on (0,1) then use this trick
>>
>>10817474
might depend on the field, but in my experience the latter is much more common
>>
Can the L of a solenoid end up negative in an AC RLC circuit? Trying to solve a problem but it's always ending up as negative no matter how I try to solve said problem.
>>
>>10816773
yeah algebra and precalc are piss easy
>>
Got stumped by this question. I can't see why option III is false. Any explanations why?
>>
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>>10817944
D'oh, forgot to provide the actual answer. It's supposed to be C.
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>>10817681
Can't really say for your situation, but it's the fear of stuff like this that prevents me from posting on 4chan while on university WiFi.
>>
>>10817971
It's the last fucking time I try to help an anon out on campus, that's for sure.
>>
where do i find internships thats no from indeed or through my school?
>>
>>10817961
Consider a small neighborhood of 1 in (0,1] and think about its preimage under f
>>
>>10817994
why not those places? and you could always just email a random company nearby even if they dont have an opening online or walk in and ask
>>
>>10818148
It's hard looking for internships as an electrical engineer. Most stuff I see is related to energy, but I'm not going down that route. I'm looking into laboratory work in communications, not energy.
>>
Any actuaries or someone with an applied math degree here? Do you like your life?
>>
ME/EE brainlet here I took an intro to numerical analysis course and fucking loved it. I want to go further but don't have the availability to take any upper level math courses again for a while. What do? I thought of reading the textbook since I have my own but idk if that's a good idea.

I've also taken intro to linear and ODEs formally, I personally enjoyed linear more but diff eqs has been more useful in other classes.
>>
>>10817817
No. You can't have negative inductance.
>>
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I'm a brainlet trying to wrap my head around optics.
I need to get an image from the circle to the square. My dumb caveman first solution was to just put a screen at A and use two 45° mirrors to reflect it to the square. However, I need the circle assembly to be as small as possible, and the smallest non-garbage-quality screen I could find was 32mm, which is still larger than I'd like.
Then I thought I could mount the screen to B instead and reflect it to the first mirror with another mirror, but now I have little idea how wide the assembly including the new diagonal mirror would have to be, and what angle it would have to be at, in order to get the whole image. Dumb caveman brain says it'd have to be at 45°, same as the others, making it just as wide as the screen itself would be and defeating the purpose. But that'd only be if the light came straight out of the screen and didn't go in any other directions, right? I don't need the screen to be perfectly straight, only entirely visible. I tried modeling the reflections in some free online tools, but they all seem to be built for things like lightbulbs, lasers, and flashlights rather than "Is this legible from this angle?"

Tl;dr is there a way to use lenses and mirrors to reflect an image X mm wide around a corner while keeping the assembly to do so less than X mm wide?
>>
>>10818374
>I thought of reading the textbook since I have my own but idk if that's a good idea.
Why not?
There is a lot of fun to be had with numerics, and there is a huge variety of topics.
The study of solving linear systems, on its own, is already pretty interesting, but you might also want to look into the numerics of ODEs.
>>
>>10817681
Almost certainly not.
The University most likely has logs of what each user did, thus they can easily see that you are innocent.
Also, firstly some three letter agency has to get involved in order for any relevant action to be taken and they have to gather some evidence that it was actually you.
>>
>>10818815
If you're trying to view the screen using only one eye, then the mirrors can be smaller than the screen. Draw an isosceles triangle where the apex is the viewpoint and the base is the screen. Adding a mirror lets you flip everything on one side about the plane of the mirror.

If you're trying to view the screen using two eyes, the size of the mirrors would need to be based upon the distance between the eyes.

Using lenses or curved mirrors would allow you to condense the path, but then any errors in positioning become magnified. E.g. even small vibrations can result in the image shaking.
>>
Does current science say that time has to move linearly? Would shit be fucked if it didn't?
>>
>>10798139
Hows the exchange of Heat between a beach rock and the sea?
Lets say 32°C vs 20°C
>>
>>10817944
Enjoy mein neigar.
>>
Show that the series converges
[math] \sum\limits_{n=1}^{\infty} \frac{1}{F_n} [/math]

Where [math] F_n [/math] is the n-th Fibonacci number.
>>
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>>10818883
I'm only viewing it using one eye.
>the apex is the viewpoint and the base is the screen
I see, so if the base is X mm wide, then depending on the distance the mirror is from the screen, the mirror can be less wide than that since it only needs to be wide enough to touch both sides of the cone, right?
Thinking of vision as an isosceles triangle really helped me in modeling it.
It looks like using an additional 45° mirror wouldn't really help make the assembly thinner at all, and may even make it larger. But it appears that using a mirror at a smaller angle followed by multiple mirrors to bounce it down a corridor would make it a bit thinner and quite a lot shorter as well.
>>
>>10819390
First use induction to prove that
[eqn]\frac{1}{F_n} < \left( \frac{2}{3} \right)^n [/eqn]
for large enough [math]n[/math] then use the comparison test.
>>
What would happen if a very small black hole appeared and dissapeared a second later on the surface of Earth?
>>
>>10819633
1 < 2/3
>>
>>10819814
>for large enough [math]n[/math]
>>
>>10819045
You could start with something simple like [math]Q=mc\Delta T[/math].
>>
>>10819805
How small are we talking?
>>
>>10817565
>experimental evidence
>No actual proof
Might as well say you took it on faith.
>>
>>10819951
Small as a particle of sand
>>
>>10816256
help
>>
>>10817565
>I only have experimental evidence?
You are trying to do mathematics, not physics.
There is no value in an "empirically validated" mathematical result.
>>
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>>10819281
Thanks, desu. I was stuck on this for a whole day. I tried to follow the hint given by >>10818002, but couldn't get past the suggestion.
I was able to show that if [math]f[/math] is a bijection then [math]f^{-1}[/math] can't be continuous by connectedness arguments, but couldn't say anything about the continuity of [math]f[/math] itself.
I figured IVT would show up somewhere.
>>
>>10819970
Looks like everything the proximity of that black whole is gone.
>>
>>10819390
We know that
[math] \lim_{n\to\infty}\frac{F_{n+1}}{F_n}=\phi [/math]

Which means for sufficiently large [math] n [/math]
[math] F_{n+1} \approx \phi F_n [/math].

So for some large number N,
[math] \sum\limits_{n=1}^{\infty} \frac{1}{F_n} = \sum\limits_{n=1}^{N} \frac{1}{F_n} + \frac{1}{F_{M+1}} + \frac{1}{F_{N+2}} + \cdots [/math]

But
[math] \frac{1}{F_{N+1}} = \frac{1}{\phi F_N} [/math]
and....
[math] \frac{1}{F_{N+2}} = \frac{1}{\phi^2 F_N} [/math]
and...
[math] \frac{1}{F_{N+m}} = \frac{1}{\phi^m F_N} [/math].

So....
[math] \sum\limits_{n=1}^{\infty} \frac{1}{F_n} \approx \sum\limits_{n=1}^{N} \frac{1}{F_n} + \sum\limits_{m = N+1}^{\infty} \frac{1}{\phi^m F_N} = \sum\limits_{n=1}^{N} \frac{1}{F_n} + \frac{1}{F_N} \sum\limits_{m = N+1}^{\infty} \frac{1}{\phi^m} > \infty [/math]
>>
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>>10819390
>Show that the series converges
i show look flat
>>
>>10819961
>>10820047
alright, guess I'll try to find a proof for my algorithm
probably won't be able to
>>
>>10820048
You’re welcome my dude. That’s some slightly deeper stuff you’re using, let me see if such a method could work.
>>
>>10820085
1/(phi^m) = (1/phi)^m is a convergent geometric series.
>>
>>10819390
https://en.wikipedia.org/wiki/Reciprocal_Fibonacci_constant

It's asymptotically a geometric series with ratio = 1/phi < 1, so it converges.
>>
>>10820684
Oh, I meant "<", not ">"
>>
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goals for this board?

influence mainstream and directed science
i think it's fair to say, we are around the 13-17% of influence

i think we've settled on ai as possible. we saw that quantum clone thing but i think theyre just confusing semantics (that gram of information was interesting, wasn't it?)

climate change, when ice melts in a cup, does the cup overflow? i feel like we are are missing some fundamentals. if its all displacement when its solid then its just fear mongering

greenhouse, i think there's some sort of geopolitical war in regards to...whos gonna survive.

we've got the rat universe problem chasing up on us and ai doesnt look like its going to provide the virtual world to dump 30+ billion souls yet

the understanding is that energy is, manageable. its like last generation gaming on a console, all the quirks are solved. ..

its just people farting and collecting money..

JFK. how about another JFK or directed attack on communism/capitalism / this retarded anarcho mass stagnation ideology.

UPON THE FOUNDATIONS OF SCIENCE AND MANIA
>>
I’m the guy who made this response >>10819281
I believe climate change is a hoax
>>10821588
>>
>>10821588
cultural pipelines

better psychological health and education

bioinformatics, data in general.
inb4 the machine learned world order
>>
How much more weight per cubic meter of air displaced would an atmosphere where we replace all the nitrogen for sulfur hexaflouride? Is there a formula for how much more weight it would increase by for every 1% change from nitrogen to sulfure hexaflouride?

What would I even need to study to not be stupid and work out problems like this on my own?
>>
>>10822617
Basic arithmetic and chemistry?
Just read off molar masses and atmospheric compositions.
>>
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Why are NAND-NAND implementations of a Sum-of-product expression preferred over an AND-OR implementation?

I'm not trying to be a theoretical faggot, my professor is asking this on his study guide and I legitimately have no idea why it would be preferred.
>>
>>10805966
kek.
>>
>>10799532
because iff you find the minimum of a monotonic (non-decreasing) function, you know that it is also the minimum of that function squared or cubed or etc.

so if you find out when X^2 is minimal, you also know when X is minimal. You just have to prove X increases monotonically over its domain.

or something like this, I'm an EE/CS major, not an actual mathematician.
>>
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>>10823002
>inb4 t.butthurt mathfag
>>
How much smarter is the average person today than the ancient Greeks?
>>
>>10823024
maybe like 2 or 3.
>>
I wanna learn about path integral formulation for QM. I'm told Feynman's original book is a good place to start. I've gone through most of the required undergrad physics curriculum. Am I ready for this book?
>>
>>10799703
spectral theorem
>>
>>10822978
In TTL, NAND gates require fewer transistors to realise than any other type of gate. In CMOS, NAND and NOR are equal but both are simpler than non-inverting gates (AND/OR gates are just NAND/NOR followed by an inverter).
>>
What the fuck are radio waves made of? Are they created out of nothing? Do we "lose" matter when creating radio waves (does the antenna lose matter or electrons or something)?
>>
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pic related seems to be what happens when i drink too much coffee and stay up for too long with no sleep. what do?
>>
>>10824377
Stop getting high with caffeine and sitting all day and sleep more, I believe you already told the issues in your own post. Cardio regularly wouldn't be bad as well
>>
>>10824393
Don't go too hard on the cardio on the beginning though, I don't want you to have a heart attack
>>
is there any scientific way to stop being in love with someone? I thought about snapping a rubber band on my wrist everytime I think of them but then I worry it'll ruin the friendship since I'll associate them with pain, what about only doing it with feelings of love?
>>
>>10824700
That rubber band thing might just make you a sub one day
>>
I need to draw a few simple image for my thesis, basically I just need them to illustrate a few of my points, so nothing complicated. What is a good free editor that's gonna do the job?
>>
I'm going to upload an article to a preprint archive.
This is the first time I'm ever making something available to the public and I was wondering if I should set up a separate email account to avoid spam.
>>
>>10824700
Yeah - tell them how you feel, let them laugh in your face, and you'll no longer feel that way.
>>
>>10825540
it's a close friendship I don't want to ruin personally I doubt she'd even reject me since she's showed interest in the past but realistically she deserves better
>>
Brainlet here, what are antigens and antibodies? (learning about blood shit)

If i understood correctly, lets say i have blood A type and i got blood B type injection, my blood A cells have antigens that makes antibodies to fight the blood B type?
>>
>>10798139
Does the prostate start producing semen while simply watching porn with an erection, or does the prostate only produce semen when masturbating?
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>>10825691
An antibody is a thing your body produces to attack foreign (usually) materials. They're part of the adaptive immune system. An antigen is the thing that gets attacked by an antibody. Think of it as antigen = thing that generates an antibody response. Antigens can be pretty much anything, but some common ones include surface proteins on bacterial cells and allergens.

In ABO blood grouping, antigens are surface proteins on red blood cells. They happen to be similar to the surface proteins on common gut bacteria. Your immune system is used to the ones on your RBCs, so it doesn't produce antibodies. But it does produce antibodies to the gut bacteria with surface antigens similar to the ones on other blood types. That's why you get an immediate reaction if you give blood of the incorrect type to someone -- your body essentially mistakes them for bacteria. There are two types of antigens in the ABO system, A and B. A person with type A blood has A antigens on their red blood cells and makes antibodies for B antigens, a person with type B blood has type B antigens and makes antibodies for A antigens, and a person with AB blood has both antigens on their RBCs and makes no AB antibodies. A person with type O blood has no AB antigens on their RBCs and makes antibodies for A and B antigens. That's why O is the universal donor (no AB antigens) and AB is the universal recipient (no AB antibodies).
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>>10825691
>>10826057
To clarify, the antigens don't make antibodies. The immune system (specifically B-cells) make antibodies in response to antigens. Macrophages and dendritic cells wander around the body picking up antigens and taking them back to the lymph nodes, where they present them to B-cells. If the antigen activates the B-cell, it will produce antibodies against that antigen.
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>>10826057
>>10826070
Thanks bro! this is very clear to me now!
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>>10824159
Radio waves are electromagnetic radiation, just like visible light. They're made of alternating electric and magnetic fields. They have no mass, but they do have energy. On a fundamental level mass and energy are equivalent, but antennas do not lose mass. The oscillating electrons in the antenna induce electric and magnetic fields, which takes energy.

>>10824846
What kind of illustrations? Krita is good for raster, Inkscape is marginally usable for vector, draw.io is good for quick vector diagrams and flow charts.
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207 KB JPG
What's the best precalculus textbook?
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what is the best way to study for the gre quant section? in all the practice tests i usually either run out of time with 1 question left or misread some question, maybe every other test i miss one i genuinely dont know.
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What is the most efficient way to check whether two grids are a 2D permutation of eachother?
By 2D permutation I mean you can swap rows with eachother and you can also swap columns with eachother.

This is an example of two grids that are a 2D permutation of eachother:

1 2 3
4 5 6
7 8 9

2 3 1
8 9 7
5 6 4

I do have something in mind, but I'm not sure if it's the most efficient way.
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>>10827128
Locate the elements from a specific row to determine the permutation of columns, locate the elements from a specific column to determine the permutation of rows. Then check that all remaining elements are in the correct position.
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>>10827128
do all the column permutations of the first grid and sort by row
>
123
456
789
>
231
564
897
>
312
645
978

sort the second grid by row
>
231
564
897

start comparing them:
>123 =/= 231
move on to the next
>231 = 231
>564 = 564
>897 = 897
it's a match

skipping the sort might help the worst case time complexity (not sure), but it will probably help the average case on large grids. also you could do a binary search on the first row so you don't waste your time comparing 12345678, 12345679 etc if you're looking for 91234567
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>>10827213
sounds good but could get messy if you have a lot of duplicate elements
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>>10827214
another idea is that you could find the row with the lowest value in the second grid (231), and then in the first grid you go through each column and find the lowest starting digit e.g. 123 doesn't match so you can skip that permutation, then the next column has the lowest digit 2 which gives the rows 213 and 231, 213 doesn't match but 231 matches, then you can proceed to check the remaining rows of the 231 permutation
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>>10827261
but then again this could also get messy if you have duplicate elements
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>>10827214
Sorry but I'm not sure what you mean by "do all the column permutations".
Do you mean literally list every column permutation?

Then why didn't you list
>
321
654
987
for example?

I think this is a pretty inefficient method btw.

>>10827218
guy who asked the question here
I can confirm that duplicate items can occur
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>>10827278
>Do you mean literally list every column permutation?
yes but this could be optimized to eliminate dead-end permutations like do just one row and check if that row is present in the second grid
>Then why didn't you list
my bad
>I think this is a pretty inefficient method btw.
the solution will be have poor efficiency regardless of what you do since it's a 2d grid and it's almost completely shuffled in 2d
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>>10827296
>yes but this could be optimized to eliminate dead-end permutations like do just one row and check if that row is present in the second grid
also, if the second grid rows are sorted (you only have to do the sort once) it allows you to do a binary search to find the row faster if the grid is large
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>>10824377
your penis vanishes?
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>>10827296
It still sounds like you want to generate all permutations first. That is in O(n!) and is not even feasible for say a 15x15 grid.

I think the method I had in mind is more efficient.
I'll explain my method with a different example:

8 8 3 4
7 2 2 5
3 6 3 5
9 8 1 6

List each sorted row:
[3488, 2257, 3356, 1689]
Now sort this list:
[1689, 2257, 3356, 3488]

List each sorted column:
[3789, 2688, 1233, 4556]
Now sort this list:
[1233, 2688, 3789, 4556]

So now we have A = ([1689, 2257, 3356, 3488], [1233, 2688, 3789, 4556])
Do the exact same thing to the other grid to calculate B.
And then just check if A == B.
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Do I need to know the details of any parts of calculus for a computer science phd? I did well in calculus but barely remember most of it.

I know that I need to know combinatorics, graph theory, proofs and linear algebra. But do I need to review things like L
Hopital's rule and methods of solving intergrals?
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>>10827415
no. and even if you do, if you're at a phd level and you did well back in the day, you can pick it up again in a week
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>>10827379
these would be a false positive though:
8 8 3 4
8 2 2 5
2 8 3 5
9 8 1 6

8 8 3 4
2 8 2 5
8 2 3 5
9 8 1 6
and you'd still be doing an assload of sorting and comparing
>>
but i guess you could use >>10827379 initially to see if it's a potential match but then you still need to check it more thoroughly
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>>10827428
>these would be a false positive though
ah, my bad, you're totally right
>and you'd still be doing an assload of sorting and comparing
bro O(n^2 log(n)) is nothing compared to O(n!)
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>>10827443
>bro O(n^2 log(n)) is nothing compared to O(n!)
ok but how about you sort the second grid by rows so in the first example you know 231 is the lowest
then you check each column in the first grid and look for a 2
in the first column you find a 1 which is lower than 2 so you skip to the next column
in the second column you find a 2, now you're looking for a 3 in the same row and then a 1
then check the remaining rows using the same column permutation
>>
>>10827461
How does that work when there are duplicate elements though?
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>>10827484
if you didn't find a match you continue looking for the next 231 in the same row and then the next 2 in the same/next column
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>>10827495
the problem isn't really not finding a match, it's finding many matches and not knowing which one to choose

another issue, you said:
>using the same column permutation
but let's say the row you just checked is 555155 for example
then you still know very little about the column permutation
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>>10827524
as long as what you found so far is a match you just gotta check the rest of it. there's no way of getting around the complexity of the problem. if you're looking for 12345 and have found 1234, you gotta look for the 5, if there is not a 5 you go back to 123 and check the next 4, and so on

>but let's say the row you just checked is 555155 for example
then if the next row is 123456, the permutation could be 123465, or 126453, and so on, you gotta check them until you run out of options
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>>10827558
>if you're looking for 12345 and have found 1234, you gotta look for the 5, if there is not a 5 you go back to 123 and check the next 4, and so on
or in this case you would abandon the row and look for the next 1 to start the next row with
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What are good degrees that have lots of emerging fields/products that would make it easy to be self employed in the next 10 years or so? The degrees i was looking at were EE/CS/SE but i also really like math. Can someone help point me in the right direction? Thank you for your time in advance.
>>
>>10827568
>emerging fields/products
this is generally cutting edge stuff that only the big companies can do. like OLED display technology for example. starting a business and becoming self employed is never easy.
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>>10827591
I understand that it wont be easy anon. Is best path to work for one of these corps for a few years to get inspiration for my own products? Also which degree has the most well rounded education for someone? is EE/CS/SE the best path to go or should i follow my passion as a math major? Although, I dont see the transition between that major and being self employed.
>>
I need to know the plasma frequency of a Lead-indium alloy. Anyone know of some repository for this kind of information? Google gives me absolutely nothing.



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