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user228700
10:01
Wow :-)
user228700
I need some homework-tsy help in Math for now...
user228700
I've been asked to check if the function $f(x) = x^2 + \ln x$ is injective and surjective or not.
user228700
To check if it's injective, I found $f^{'}(x)$, which came out to be $(2x^2+1)/x$
injective is easy, just assume there is $a, b$ such that $f(a)=f(b)$, plug them into the function, and see if you get $a=b$ in the end. Alternately, as you said, check that the inverse function $f^{-1}$ is such that $(f^{-1} \circ f) (x) =x=(f^{-1}\circ f) (x)$.
user228700
For $ x \epsilon (-\infty, 0]$, $f^{'}(x) \le 0$
10:06
actually, is $f'(x)$ the inverse function or its derivative?
user228700
@Secret No, no, I'm checking the monotonicity of the function. I typed inverse by mistake because I'm not very fluent in LaTeX.
Ah I see
Indeed, if a function is monotonic, then it must be injective (i.e. have a left inverse)
user228700
Yeah, so I see that $x \epsilon (-\infty, 0] , f^{'}(x) < 0$.
user228700
And for $x \epsilon [0,\infty), f^{'}(x) > 0$
user228700
Dammit, I didn't mean to include 0. At 0, I guess the function has a vertical asymptote.
10:11
that's ok, it just means the domain does not include 0
user228700
Okay, so there seems to be no turning point for the graph...which is weird, because for awhile, it decreases, and then it increases. Huh :-|
user228700
Oh, wait.
user228700
Nope, nothing.
user228700
@Sec: Uh, u there..?
I am thinking a bit, because when I graph it, it seemed to be increasing without turning points
and also, thinking being complicated by wacky cases like as follows:
0
Q: Can the product of an increasing monotonic function and a strictly increasing monotonic function have turning points

MichaelSuppose $f:\Re \rightarrow [0,1]$ is an increasing monotonic function and $g:\Re \rightarrow [0,1]$ is a strictly increasing monotonic function. For the product function $h(x) = f(x)g(x)$, will there exists a local maximum? If the gradient of $f$ and $g$ are different, I think there exists a tur...

user228700
10:19
@Secret It first decreases and then increases. I mean, the derivative always increases, but not the graph itself...
Ok, I guess the better way to say is that, a monotonic function does not have minima nor maxima except for the boundaries
user228700
Wait, what does "monotonic function" really even mean?
It means given any $x \leq y$, $f(x) \leq f(y)$ or $f(x) \geq f(y)$.
in calculus terms, the function only increases or decreases throughout its domain
user228700
> "A monotonic function is a function which is either entirely non-increasing or non-decreasing. A function is monotonic if its first derivative (which need not be continuous) does not change sign."
Yes, that's true, if the first derivative change sign at some interval, it means it is decreasing for that interval
user228700
10:23
What? That means that the function $y=x^2$ is not a monotonic function.
user228700
Oh, wait.
yeah, $x^2$ is not monotonic, because it decreases for negative x and then increases again for positive x, with a stationary point at x=0
user228700
Nvm me, I'm just confused. All cleared up now.
user228700
Ow, I fell off my chair trying to grab a pen that fell down without getting up. And now my back hurts :-(
user228700
Alright, so functions functions, um...
10:27
@Kaumudi.H Ah now I see why it does not initially seemd to fit the definition of monotonic function: ln (x) is complex for all x<0. Since you are dealing with real functions here, the domain is $x\geq 0$ hence f'(x) > 0 for all x, and hence it is monotonic
user228700
Wait, so since the derivative first decreases and then increases, the function itself is clearly not monotonic so it's not one-one, correct?
user228700
@Secret Oh, yes! You're really smart, I missed that completely! Thanks so much! :-D
user228700
11:00
@Secret: I tried to read that paper and I also sort of read your question and truth be told, I don't really understand what's happening.
@Kaumudi.H Deletion means an account is anonymized and completely removed from the system permanently (except for some forensic information kept by SE). Suspension (which is what people probably mean when they say "ban") is just temporary, and has no permanent effect on the account.
While suspensions are imposed by mods as a disciplinary measure (more or less), deletion usually happens because the user requested it, because they are a spammer, or because the account is a sockpuppet being used fraudulently.
user228700
Oh, I see. Thanks :-)
No problem
user228700
If ANYBODY is familiar with the Baker-Nathan effect, I request you to PLEASE help me out _/\ _
user228700
@SirC: I'm assuming you didn't get any sleep last night, as usual?
11:26
Hello,

I am having trouble calculating the heat capacity in the Debye model because I am confused about how to take the derivative of

$$U = U_{zero} + 9NK_BT (\frac{T}{\Theta_D})^3 \int^{\frac{\Theta_D}{T}}_0 \frac{x^3}{e^x - 1} dx$$ where I have made the substitution $$x = \frac{\hbar \omega}{K_B T}$$

If anybody could give me a hint, or point me to something that might help that would be great

Thanks!
Is there a rule I need to consider that will help me solve this?
Basically, if we temporary set aside on why only C-H can hypderconjugate for that reaction (that is something that confuses me and no journal articles nor website sources covers that and hence that ChemSE question).

Then if we use the inductive effect, the halide X will draw electrons from the C that is attached to the benzene ring, which in turn will draw electrons from the para group, thus when you drew all 3 resonance structures, you will get for each structure a formal positive charge at one of the 4 locations where I labelled as $\delta^+$ in black. Meanwhile the carbon at the C-X bon
So as the article said, both hyperconjugation and inductive effect facilitate the reaction. The trend reversal is due to Me being the best hyperconjugating group and tert is the best electron donating group
Wow.
Will I even understand chemistry? I doubt.
Ah, the leibniz integral rule is what I want
11:41
@Kaumudi.H Who is that?
that's Sir cumference
On hyperconjugation. Actually I think it is not very straightforward especially when you realise that a sigma orbital formed by C-C is expected to have a better overlap than its hydrogen counterpart (being larger), yet we observed the opposite trend
user228700
12:09
Wow. Wokay, I don't think that I need to go into such depths in this topic. I'm going to move on. Thanks very much :-)
user228700
@G.Bergeron Yep, that's Sir Circumference.
Well for your level, I think you only need to remember that adjacent C-H does hyperconjugation, and groups with a lot of carbon are more electorn donating than those with less carbon, plus both hyperconjugation and inductive effect facilitate the baker nathan reaction. The trend reversal is due to Me being the best hyperconjugating group and tert is the best electron donating group
should be more than enough to answer any question about Baker Nathan
I have a tendency to massively overanalyse a question because of my perfectionism to find the most perfect answer. This explains why I often cannot respond to people in time because I am thinking way too hard
user228700
I see. Thanks :-)
user228700
I come with a homework question again. I'm trying to find the range of the function given by $y = (x^2-2x+5)/(x^2+2x+5)$
user228700
12:24
Writing this equation as a quadratic in terms of $x$, I get $x^2(y-1) + 2x(y+1) + 5(y-1)=0$
user228700
To find the range of the function, I am trying to find a condition on $y$ such that the above quadratic equation has real roots. For this, we know that $D \ge 0$, where $D$ is the discriminant of the quadratic equation given.
user228700
After simplification and everything, this gives me $y^2-3y+1 \le 0$
user228700
I dunno if I'm correct so far...in terms of what to do, I think I am but I'm not sure about what I've done, you know, steps-wise...if it's all free from errors, because I've tried this question bout 5 times already and I'm still not getting the correct answer.
I think you want instead to find condition for x such that y is real. Also checking the denominator with the discriminant, $D < 0$ thus the denominator has no real roots hence the domain will be all of $\mathbb{R}$
The function $y$ can also be simplified using long division to become as follows:

$$y=1-\frac{4x}{x^2+2x+5}$$
user228700
Huh..? No, to find the range, I'm pretty sure I want a condition on $y$, not $x$. $y$ is the output and $x$ is the input, remember..?
user228700
12:34
And what is it that you're saying about checking the denominator with the discriminant..? I'm unable to fully get what you're trying to say...
user228700
@Secret What to do with this..? To find a condition on $y$, I don't think writing it like this would be very helpful...
Check the discriminant of the denominator $k=x^2+2x+5$ gives $D=2^2-4*5<0$. Therefore the denominator has no real roots thus there are no vertical asymptotes. Now check whether $k$ is positive or negative (above or below the axis).
Completing the square gives $(x+1)^2+4$, which is $\geq 4$. Therefore the denominator will be any number $\geq 4$. Now for the numerator. $4x$ has the range of $\mathbb{R}$, so the fraction has the range of the reals. Finally 1- the fraction also have the range of all reals (Since 1- any number in reals is going to lie in the reals). Hence y has the range of all
user228700
That is...almost convincing but the answer is unfortunately wrong :-(
user228700
> "The denominator has no real roots thus there are no vertical asymptotes."
user228700
Um, why would any polynomial have vertical asymptotes? Surely, u are talking about the whole function itself..?
12:45
yup, I am actually talking about how the demominator affect the whole function y (as for the answer, I am still figuring why somehow I missed out the max and min that bound the function between two numbers)
@Kaumudi.H The best way is to draw a graph. The maxima of the function occurs at -sqrt(5) if you differentiate the function y. And the minima of the function occurs at +sqrt(5). BTW there will be no vertical asymptotes.
At either infinites the value will tend to 1
Hmm, I guess that's a lesson for me to always check for extremas (whether cusps or stationary points or boundary points)
user228700
Well, yes, but the answer still doesn't quite match. The range is given as $[(3- \sqrt{5})/2 , (3+ \sqrt{5})/2]$
user228700
Remember that equation I got..?
user228700
$y^2-3y+1 \le0$
user228700
12:53
The real roots of the quadratic on the right are exactly $(3 \pm \sqrt{5})/2$
Hmm, I think I can see how that real root argument works above. Because your function is real, for every real y there must be a corresponding real x, thus rearranging into the quadratic involving y like you did above allow you to use the discriminant to pick out the ys that guareentee the x are real hence giving the range of y in the process from the bound set by the discriminant
How does that ensures it gives you the upper and lower bound of the range, I am not very certain
user228700
You put it exactly the way I didn't know how to :-P So, yeah, it works, but that inequality..? It doesn't give the range. I can't factorize that into anything.
You can find the y s where the discriminant is =0, (i.e. double roots), this is where you can use the quadratic formula to solve for y, which gives you the roots that forms the range
This is because the discriminant is at least = 0 in order for x to lie in the reals
user228700
Yes, it gives me the roots and all, but it doesn't give me the range. Dyou know what I'm saying? I need to get the range using that inequality.
user228700
Ohh, hang on.
user228700
13:00
DANG IT, I knew the answer all along. I just. Ugh. Okay, I just didn't know how to arrive at the last effing step.
x is real when $D \geq 0$. Therefore the minimum requirement that x is real is when that discriminant $D = 0$. You have found that $y^2-3y+1 \geq 0$. Therefore the largest possible interval of y you can get x to be real (the range) will be given by the interval formed by the solution of y in $y^2-3y+1 = 0$, that is the two roots together form the bound that is your range
user228700
Yeah, yeah, I was just...being dumb (Sigh, @DanielS: I can't help it. I kinda am, okay? Or maybe I'm just tired.)
user228700
Alright, thanks a lot for your help :-) Dyou want to discuss another similar question? :-P
Don't worry , I intiially forgot about this method despite I learnt that back in my high school grade 5 because it is too hard for me to visualise.

Well, ask
user228700
Grade 5? Man, what kind of a high school did you attend? O_o
13:03
sorry mistake Grade 10
user228700
Ah, okay :-P
can't count because in HK we use the Form somethnig system and form x = Grade 6 + x
user228700
Well, if u're still interested to learn to visualize this method, then I suggest that you read yesterday's transcript. Daniel Sank and I had a brief discussion about this, which cleared it up for me...
user228700
@Secret Oh, okay...
user228700
Anyhoo, this is the question:
13:08
hello
user228700
$y = 1/\sqrt{x^2-x}$
ooh, bijective and surjective
cool
user228700
@heather Hey :-) Yeah. How's it going?
user228700
What I did was, I squared both sides, after noting that the domain absolutely cannot include 1 and 0.
the domain does not include anything that is $x^2-x \leq 0$
user228700
13:10
Oh, hang on...
user228700
@Secret Dang it, yeah.
@Kaumudi.H good, how about for you?
user228700
@heather I wanna say "good", but in all honesty, kinda miserable :-P (Probably should've just said "good" :-P)
@Kaumudi.H oh, I'm sorry! What's making life miserable?
fwiw, thanks for your honesty =P
I am kinda multitasking: helping kaumudi on the maths, investigating convolutions in the maths chat and still trying to divide by zero
2
user228700
13:13
@heather Nothing that won't go away in awhile :-) Thanks.
user228700
@Secret Lol. I'm sorry I'm taking so long to type...
wait, Doraemon was banned?
why?
user228700
Deleted.
er, deleted.
but why?
user228700
(Just to be clear, this is not me "speculating", because I know (and can prove, if needed) this for a fact) Sockpuppet account. He was S007.
13:19
ah.
is, um, S007 banned?
@heather Like suspensions, reasons for deletion are not public unless the user in question chooses to make them public or they are otherwise in plain view. (Some deletions are actually requested by the users themselves)
user228700
@heather Probably.
@ACuriousMind, ah, sorry.
just woke up, not exactly thinking.
@Kaumudi.H, nice hat, btw.
bacon is a good scarf.
user228700
Thanku thanku :-P
@ACuriousMind, oh, while you're around, I think I got serially downvoted.
13:21
@heather Oh, I see another mod misunderstood your flag
@ACuriousMind oh, really?
I almost made the same mistake; you shouldn't have chosen that exact flag text on a post where there were three other answers which had a single downvote :P
oh, i didn't notice that!
sorry =P
i don't know if mods can see this, but I ended up deleting one of the answers anyway because it was bad, too
hmm, i wonder if the other posts got downvoted across the answer board
@Kaumudi.H XD
@DanielSank, hello
user228700
13:23
@DanielSank :-P Hi.
@heather yo
mmm, bacon
this is a great hat
@heather It read as if you were saying someone had "serially downvoted" all the answers to a particular question, which is what the text for declining your flag is about; I left the flag untouched because I could not uncover evidence for serial voting beyond what ticked you off. I'm afraid that unless the perpetrator returns there's little we can currently do about it.
user228700
@Sec: God, this is horrible. Which method do you usually choose to find range?
@ACuriousMind oh, okay
sorry about that (that was really stupid on my part)
it's not that big a deal, i can live with a couple downvotes =P
13:27
@heather Just a simple misunderstanding - it's hard to predict how others will interpret what you write because you know what to wanted to say.
Well, I tend to figure out what the domain is, and then worked from there, as well finding extremas and such. But really after year 2 in uni, I rarely need to worry about functions for some time as I shift towards more abstract algebra pursuits
and not to mention chemistry made up most of my year 2 to honours
so it is not suprising I am kinda rusty on that aspect
user228700
Oh, I see. Good for you :-) I think I'll chuck this and move to other stuff for now. Thanks ever so much for helping me out so much _/\ _ :-)
But I am pretty sure I will get better once I finish reading munkres, which deal with even more general mappings
user228700
@Secret Say, before I go, dyou ever like, do stuff other than studying and trying to divide by 0? :-P
Well I do other things in the past, such as analysing time travel stories and games, some quantum mechanics calculations, reading some physics jourhal articles for fun, browsing facebook and so on
user228700
13:31
In the past..?
not to distant past, like 3 months ago. Currently I am mostly hanging out in the SE chats while continue my divison by zeor pursuit in the backgorund
Aftr all, my phD starts next year, I can do nothign until at least january arrives
user228700
I see. So the short answer would be: No? :-P
mostly no, other than asking and answering question . Pretty much my role is kinda in parallel to johnrennie nowadays, except I don't drink coffee and I am not actually working on a formal project
user228700
:-P Wow, I see. And u're not even 20 yet, right..?
I am 21
user228700
13:34
Oh. Sorry :-P I thought you were like, 19.
user228700
Anyhoo, thanks for all your help! :-D
well, my mental age is actually around 13, and my parents have noticed I have a quantised trend in my maturity
lol
that's fabulous
that is, I become mature at some intervals
like a quantum jump
user228700
x'D I see. That's...interesting.
13:35
But, whatever the next jump will be, hopefully it is not one that turn me into a souless working machine, that will be TOO SAD
user228700
:-) Indeed. Good luck! I'll see u tomorrow.
user228700
Bye, everyone.
@Kaumudi.H, hope your day gets better =)
have a good one.
user228700
(I might come back with questions, who knows :-P)
user228700
@heather Thanks! :-) Have a nice day.
13:45
ooh, got the trendsetter secret hat
hats are great
14:19
Sup
nothing much
how are you?
oh, general hbar question: thoughts on having a tag ?
@heather I couldn't even tell you what that is beyond mumbling something about GR ;P
Have you seen a sizeable number of questions who would benefit from such a tag?
I'm fantastic ty
If there are enough questions that need the tag then why not
@heather that seems a rather specific tag. I'd be surprised if there were that many questions on it ...
How much rep is required to make a tag
14:33
@Kenshin 1k.
27 questions in fact. And given how easy a term it is to search for I'm not sure a tag would be that useful.
@JohnRennie Well, don't forget that at least some people filter the site by tags
I'm not saying that Shapiro delay specifically is a tag we need, but there are several other tags (especially the large ones, like quantum-mechanics) who are actually not useful to search for anything specific but nevertheless are useful as an organizing tool
But weren't we discussing whether Shapiro delay specifically is a tag we need?
Yeah, just saying that tags don't need to be difficult to search for to be useful
I think with 27 questions a tag can be created
14:38
And actually, there are only 5 questions about "Shapiro delay", your other results are answers.
oh
Well that changes my view
(add is:question to a search to view only questions)
hmm, true.
thanks for the advice
np but feel free to still create the tag as you have earned the rights to do so
@Kenshin Technically true, but it is considered good style to ask for the creation of new tags first, especially on a site as old as ours.
Basically, it's very unlikely that there's a really good tag we haven't yet created
14:43
Yes but I think a 1k rep user would have good enough judgement
not really..i'm 4.5k and i don't; that's why i asked.
@heather According to the canonical meta.SE post, it's for changing your hat three days in a row. I just got it, too
Sounds like an interesting thing. I have two secret hats.
hi wendy
Heather u have sufficient judgement. We wouldn't create that tag but creating it wouldn't cause harm its a borderline choice
hi kenshin
14:55
Wendy r from Brisbane?
yes, þe dark side of þe planet.
Where are you from
Same
What is ur job
retired invalid, mainly.
14:57
Sounds good
but i do weights and measures, number bases and physics as a hobby
base 120, that sort of stuff
Do u lift
Wats ur favourite physics topic
I cover much of it. At the moment, i'm trying to simplify relativity. I have done a really good job on easter.
Yeah my favourite topic is GR
Some of special relativity was pretty easy, the curvature thing.
15:02
Do u mean Christmas or easter
GRT
a formula to find easter. It's down to about four lines of code.
That would be handy for planning
I solved the schwarzchild metric in my head. You can pull GMm/R^2 as a tension in the fabric.
Wow u must be a genius
But generally, i don't believe GRT is the right answer, so i read up on this stuff.
15:06
Na Einstein wouldn't be wrong
hmm, i just earned a bunch of hats
@Kenshin eh, he's been wrong before.
@Kenshin yes he was
Anyone who says Einstein is wrong is wrong
Ah, you just pop a bit of arithmetic in base 120 at them, and see what they say.
15:08
U need base 121 tho
Oh good lord, no. Base 120 is straight-forward.
How come
Cause i've been using it since the seventies.
Do u have 120 fingers and toes?
You know, six by twenty is one hundred.
eight fifteens is a hundred
I sometimes translate decimal numbers to it to calculate.
15:11
No six by twenty is ten
I'm pretty sure my calculator says six by twenty is a hundred. Twenty is two by ten.
Hmm it depends on how you define hundred
No one ever said the base you use has to be called 'ten', or you need digits from the unit to the base. I use just twelve digits.
A hundred is 100. You know 6*20. As it was at the time of christ, and at the time of the crusades.
6*20=120 which in base 120 is 1,0 where a comma separates each order
Y at the time of crusades?
You really did not learn arithmetic at school, i take it.
15:15
No
Y r u awake so late
Prolly born after 1960 even. Never used real money £sd.
Correct
it's only 0116 here
But it's 2016.1230.0116 here, or 16.96.99F.06 using twelfty time
Wow twelfth time dats cool
It's not so late when u put it Lyk dat
Most parts of the world, the days start with Y. 'yesterday'. :)
so i must to bed go.
15:21
Ok u gtg sleep now
I gtg*
Didn't mean to type u lol
Laterz
15:50
What is going on in here?
I can barely read the chat log.
Hi, @Stoby.
Hi @DanielSank I'm not sure I just got here
@Stoby I don't recognize your username. New around here?
Yes
Welcome.
Thank you I'm really enjoying this stack exchange, it's a great reasource
15:52
Yes, it is amazing.
This chat room is a lot of tun too.
We have a good rapport.
@Stoby Hi and welcome :-)
Yo, @AccidentalFourierTransform. What's shakin'?
@Stoby, hello, welcome!
'sup @heather?
@AccidentalFourierTransform, hello
@DanielSank, nothing much, how are you?
15:54
Doing well. Getting ready to release a LaTeX package on CTAN.
The system is arcane.
nice!
AFAICT there is no authentication needed to upload a package, so there's no real way to make sure that other people can't modify it.
as for the transcript, it started off with Kenshin and someone else talking and I ignored them both as i didn't really want to read the conversation.
@DanielSank that's...strange
15:55
Yeah it's pretty weird.
It's not like PyPI where you need your account credentials to upload a package.
@DanielSank the first bad sign was Kenshin saying Einstein was never wrong and anyone who said he was wrong is wrong (which means I am, therefore, wrong).
o_O
Kenshin = John Duffield
?
@DanielSank seems like there's a not insignificant possibility for spamming then
@heather Yeah I guess we'll see.
::shrugs::
@DanielSank some recent activities have made me unsure in my position as moderator on the new site...on the one hand, I do want to moderate, on the other, Kenshin sometimes takes....difficult positions.
15:57
Mostly I just want TeXLive to pick up my package so I can \usepackage it. Right now I'm using a git submodule which has its own problems.
@DanielSank what's your package?
@heather You could consult the mods here for general mod advice ;)
=)
yeah maybe
Really. They have more experience than you.
They know things.
that does make sense the more i think about it =)
15:57
::spooky music::
rob
rob
I KNOW NOTHING
3
LOLOLOL
Man, what is it with the mods lurking like that?
Y'all just sit silently in front of your keyboards waiting for someone to mention "mods" and then BAM you pounce.
rob
rob
@DanielSank I was writing something in another chat room, and happened to notice your comment on the sidebar.
Wait a minute... do you guys have a chatbot waiting for the word "mod"?
Ah, so you are the chatbot.
you guys really do just pop out of nowhere sometimes
rob
rob
15:59
In fact I was writing in the secret mods-only chatroom where we all discuss our ignorance.
3
ah
your secrets have been revealed!

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