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12:00 AM
Also, as an aside you do a far better job of teaching than what I recieve in my professional development courses!
 
@AndrewMicallef Thanks. I hope that I am helping.
 
(These days if I ask a question at school my instructor is of the opinion that delivering the answer is the best course of action, as opposed to gentle nudges in the right direction)
In deed, a great deal
 
Andrew has stumbled on @robjohn :)
 
stumbled?
nay been elevated by
 
@TedShifrin I hope I gave bites small enough to digest, but too big to be spoon-fed.
 
12:07 AM
Indeed. Andrew stumbled on me first. Robjohn is nicer.
Andrew is trying, but very rusty!
 
Very Very rusty
In a side note, we had some friends over the other day, they saw my maths books out on the coffe table and we had a conversation about nevber having used algebra since school
and while I have done plenty of algebra since school, it's been aeons since I have actually had to apply any of my calculus :(
 
Learning stat makes that no good.
 
@AndrewMicallef It's good that you are dusting off the books. I hope it is a rewarding experience.
 
Yeah, pre pandemic, I thought, hey how hard can learning stats proper be, I dug up an MIT OCW for stats, looked at the first few problesm and realised I was way outta my depth
 
I have to go walk my dogs before they stare me to death.
 
12:11 AM
Go wild :P
I have to start cleaning my apartment anyways
And yeah, it sure is rewarding :P
tough tough in a good way
 
my cat stares at me any time. she's just like that.
 
actually before I do bug out, I kinda wanted to talk a bit about use and abuse of this site from the teaching and learning pers-pective
because I saw Ted, you were bemoaning that recently, and by the sounds of it, perrenially
 
"bemoaning" is a funny phrase. i think we all go through it.
 
lol, like I see the issue with getting your homework answers spoon fed to you
that prevents you doing the work and develoiong the understandings, but if you arent beeing spoon fed, and talking around a problem, that winds up being the sort of behaviour you want students to engage in, right?
 
i would defer to people with more experience in professoring. i had some unfortunate experiences, it was always a race to the answer. my colleagues told me that it used to be different. but it wasn't different for me
 
12:25 AM
@leslie a race to the answer?
what does that mean?
 
many of my students were uninterested in anything other than the numerical answer to the question.
not all. i am still in correspondence with some of them.
one of them babysat my daughter about a year and a half ago. our parents had known each other prior to the whole academia thing. we knew each other without knowing it.
then coronavirus and all the nonsense.
 
I mean, there is a larger issue around the problem of education system being largly a certification excercise and seen by the majority of students to be a means of improving their job prospects, hence understanding being less important than passing exams
but that is a BIG issue, that really is a doozy to try and resolve
 
i think you have adequately identified the problem. and also why many feel left out of this system (= people who do not have four-year degrees, including several members of my family who do not have time for other debates).
it is a divisive issue.
my mother did not go to college. she has more sense than a lot of people. but in the US at least going to college involves a significant investment of money. if this money is not available, people become upset. they rightfully recognize that they are being shut out of something.
i have no answers.
her sisters went to college. she was very conscious of the fact that the money was there for her sisters but not for herself. i really don't understand why education is expensive. the providers of education are not overpaid.
there's all of this infrastructure on top of university instruction. it seems to become more and more expensive, without limit.
 
Yeah, but now we are straying a little too far from the scope I had intended (though into a larger and really interesting issue). I was more thinking about how my uni experience might have panned out if I had of been an avid user of this site. On the one hand I probably would have improved my grades, handing in fewer assignments with silly errors.
education isn't unrealisticly priced globally though
 
i am generally in favor of students using all resources. i copied a lot from textbooks. but i understood what i was turning in. that's the hard part to test.
 
12:34 AM
In Austrlalia it is really the opportunity cost you need to be concerned with, 4 years is a long time not to be earning full time wages
 
my wife is an academic and sometimes in the position of encouraging people to go to graduate school, when it is a near certainty that they will not financially benefit. she is conflicted about this role. i suggested that she abandon it.
 
And at least in Germany ( the one other example I know about because my partner finds the Australian system so backwards) university is more or less free (there are trivial admin costs)
 
one of my best friends is german and she had a very different experience.
 
Yeah, I had a friend who used to work in uni admin, who was in the position of councelling students not to abandon their PhD's despite any evidence it was in the students interest not to
she hated that job
didn't last long
 
she is now being shunted out of the workplace due to childcare protections that we would love to have in the united states. i want the option. i don't want women to be shunted out of the workplace. it's goofy.
policy is hard.
 
12:38 AM
shunted out of work?
The maternity leave policies are another point that my partner finds outragous in Australia by comparision to Germany :P
Although no one does it better than the Scandies :P
 
maybe that is too strong a phrase. i have first-hand experienced the incentive to not work after having a child. germany has quite a bit of systems in place to compensate people, give them child care, etc. on the surface it is a positive thing. underneath the surface, maybe it's a tool for pushing women out of the workplace. i don't pretend to know.
i was very annoyed with my wife when she did not get a job at a swedish university where she had studied as an undergrad. "everything could have been fine!" i said to her.
 
hahah, everything would be fine living in Sweden, except for the Loong Darks
 
i love horrible weather. my wife hates it, but there's a lamp for that.
 
Perhaps, though as a former student of psychology, I think there is a difference in the mood altering powers of artifical vs natural light
But then we really are miles from where we started, and maybe that should be my cue to get some cleaning started :P
 
my daughter doesn't know what snow is. she reads books about it. has never seen it. will never see it.
 
12:47 AM
I live in Australia, never saw snow until I was 19. Man did I act like a small child that day (incidently in a country where snow was pretty normal, I musta looked like such a tourist in hindsight)
Snow angels are so much more fun than sand angels (which no one does for pretty good reason)
 
i threw a lot of snowballs.
that's all i remember.
i also went down a hill on a piece of cardboard. my hosts had to explain to the local children that i had never seen snow before (which is why i was pelting them with snowballs). and that i didn't speak swedish. they pitied me.
 
Just for the sake of drawing my mental image, how old were you?
 
My son saw his first snow on Mt Wilson (3 or 4 years old). Then he went to college in Chicago, so saw more than enough there.
 
So I think I did make another mistake on my integral, but I think I have to leave it for today. For the integral of the $y^2$ term, I followed the advice to to integration by parts, setting $dV = y^2$ and $u = e^{y^2}$ which leaves me with a more complicated integral to solve $\int y^3 2e^{y^2}dy$
(I left out all the constants in that expression because I am lazy)
will have to pick this up tomorrow
thanks for the mental image / chat @leslietownes
 
1:03 AM
i was 21 when my hosts had to explain my lack of facility with snow. good day.
 
@AndrewMicallef No, if $u=e^{-y^2}$, you'll want $v=-\frac12y$
Then $v\,\mathrm{d}u=y^2e^{-y^2}$
 
1:35 AM
I made a weird torus
$E^1:= \big\{(x,y)\in\Bbb R^{2+}:\log^2(x)+\log^2(y)=1\big\}.$
 
is that a torus?
 
with metric on $\Bbb R^{2+}=ds^2=\frac{du^2}{u^2}+\frac{dv^2}{v^2}$
$E^1 \times E^1=O^2$
where $O^2$ is the weird torus
 
what's weird about it. is it not topologically a torus?
no mind games here, just wondering
 
It's topologically a torus yes
but can you visualize it?
how do you calculate the product metric btw?
would it just be $2ds^2?$
 
1:55 AM
robjohn if you have time could you plot this?
 
2:25 AM
44 mins ago, by geocalc33
$E^1:= \big\{(x,y)\in\Bbb R^{2+}:\log^2(x)+\log^2(y)=1\big\}.$
 
2:56 AM
@robjohn I mean the torus like object $E^1 \times E^1$
 
3:50 AM
 
Not the right geometric picture at all.
 
@TedShifrin then what?
That is the torus using the curve I plotted before
ratio of sizes 1:4
 
4:25 AM
The actual torus is a flat torus in 4-space .
 
how do you even look at a torus in 4 space then?.....
 
@TedShifrin I'll have to buy a 3D printer for that...
 
I know.
I used to tell my classes that my 3D-blackboard was on back order.
 
so then we would only be seeing the surface of the torus is it was "printed" out?
 
4:34 AM
yes, but I can only reasonably print a 3D shape on my 2D printer
 
Flat perspective is tricky.
 
Nothing to do with 4D
 
Huh? Even with geocalc's weird hyperbolic metric the extrinsic picture in 3D is never flat.
He's trying to see geometry, which he can't, not topology.
 
4:49 AM
awesome gif
 
@copper.hat just rotating the second copy of $E^1$
 
But, yes, the undulating figure is nice !
 
@robjohn how do you create the gif?
 
My comments are more to warn geocalc than for you, @robjohn.
 
@copper.hat with Mathematica
 
4:51 AM
ahhh
i'm too cheap to buy it
 
I guess I haven’t tried uploading a movie to here.
 
wow, its $$$
 
uploading a movie to here? i'll do it, half price.
 
They told me I had to upgrade for OS Big Sur. They lied. The older version still ran, but I'd already paid for the upgrade
 
i hate the effective rental model
 
4:56 AM
i'm still using the cell phone i bought in 2015. that's where i win.
 
@copper.hat The home version is $250 I think
@leslietownes Mine is from 2015, too
 
I used to get it free from UGA’s site license. I figured I could buy it now. No, It cost more, robjohn.
I upgraded cell phone a few months ago from 2015.
 
i dropped mine and cracked the screen, so am 2021. i prefer my nokia e51
i don't get the whole giant black slippery phone thing
 
i have the galaxy s5.
 
i put electrical tape on the back to reduce the slidiness
 
4:58 AM
I'm an Apple person through & through.
 
my bank kicked me off of its app recently. it told me that it needed to upgrade the app.
 
we had a lisa a long time ago. actually it belonged to apple.
it nearly killed apple, those nice rounded window edges.
 
@TedShifrin ack! it's up to $350
I have a higher version than I can use on my current computer until I upgrade my OS
so I use 11.3 for now
 
i just use linux stuff.
 
@TedShifrin I upgraded my son's phone last year. I may do the same for my wife and me this summer.
@copper.hat Wow... I saw one of those when I was interviewing at Apple, but I have never used one.
 
5:05 AM
my dad did the finances for the apple factory in cork long time ago.
the guy who ran the place was a family friend.
 
hello, foreign tax credits
 
apparently when jobs visited he wanted the entire assembly line shut down because the slight noise bothered him when he was presenting.
tourism & tax. what else do we have?
 
that's it
 
nice cheese. dairy in general :-).
 
a number of good beers, too.
 
5:08 AM
broom bridge. but that's not a big draw.
big in aviation as well.
kcs chipper in douglas.
 
we used to have aviation in long beach. all of the big companies were here. then they shut down.
i think they mostly do NFTs now. you can buy an image of a plane. if you want
 
i presume that is a passing fad
that giant order of palak paneer was not enough for me.
 
you can never have enough palak paneer.
 
i'm a fan :-).
just occurred to me i have a thing for green veg food, like pesto. far from a vegetarian.
 
i could live on palak paneer and aloo gobi.
 
5:14 AM
i like the enormous range of Indian subcontinent food.
 
people think that being vegetarian is a chore. it's not. just eat indian food.
i'm still not vegetarian. i eat meat maybe once a week.
 
i agree entirely.
my daughter & myself discovered some incredible Bangladeshi snack food in London. have been looking for it since...
reaching for the frozen jalapeno poppers
 
when my wife and i were in edinburgh we did a lot of eating from this restaurant downstairs from our hotel room. it was indian in name only. everything came in about an inch of oil. it was the true scottish experience.
 
:-). the hottest food i ever had was in an Indian restaurant in Scotland.
 
we are growing jalapenos on our porch.
 
5:17 AM
and that was on the back of a few months in India, Nepal, Indonesia, Thailand...
I thought, how hot can it be...
i like jalapenos in my scrambled eggs
 
yes jalapenos are definitely breakfast food.
a tortilla, some eggs, some jalapenos, grill it. it's amazing. you can put some spiced potato in there but that is optional.
 
ahh, potatoes. stereotype applies here.
can only find them in gardens nowadays, but new potatoes are to die for.
 
the simpsons episode where homer sneaks into a u2 concert. "potato man." "where the hell have you been?"
 
Groening is a genius
 
hey robjohn, I just got told off for asking about mathjax in tex.se, is there a good place to go for specific help re mathjax?
I ask because you authored the chatjax bookmark
 
5:23 AM
folks are very territorial on se/so
 
mathjax is mostly tex, innit?
 
yeah, I noticed
I thought so too
 
so much better than troff
 
i have no idea what boundaries people are patrolling. i will offer unsolicited advice on your tex. unsolicited is the key. don't ask me for anything, i only chime in when i'm not wanted.
 
I asked a question frammed in $$ and got told off :P
haha, I was actually doing something kinda unsightly in retrospect
trying to fit three line of equations in a single integral using align
 
5:24 AM
i once graded homework for a professor where literally all of his documents were in custom troff. this was, like, 2000ish. no excuse for it.
 
but in retrospect that was just a nasty sight
 
@AndrewMicallef You got told off in the chatroom?
 
troff what was that?
 
my advisor would not let be use latex
 
more like a poltite repromand
 
5:25 AM
a predecessor to tex.
 
@copper.hat why would he care?
 
because he wanted to use my papers for other stuff
 
in TeX, LaTeX and Friends, 59 mins ago, by Andrew Micallef
sorry, I'm actaully using mathjax in markdown, I just thought this was the appropriate place to ask :P
 
@robjohn hi, I wanted to talk to you in my room with you, but it was so long since we last talked, you have become un-pingable there.
 
@copper.hat Ah, I forgot about academic reuse...
 
5:26 AM
well, that's you told.
 
i think he just kept me around for entertainment
 
@satan29 Hmm... I know someone else who pings me from his room after long absences. ok
 
I just wanted to not make this look awfull, but I thin kthat is just unpossible ![i.imgur.com/1fRSZWd.png]
 
sorry, that's gross
 
yeah I know right :P
 
5:28 AM
yeah, i can't get on board with that.
 
i can't unsee it
 
I just thought it could work
 
@AndrewMicallef It should be, but they only want to deal with $\LaTeX$
 
are you a physicist? do you want your notation to suck?
 
5:28 AM
No I'm a mechanic
 
there is hope for us.
 
I think I should stick to writting by hand
 
so much faster to do stuff by hand
 
@AndrewMicallef I can help make it better. Later
 
yeah, I feel like I have seen multiline sums that didnt look awful beforew
no need to trouble yourself
it actually is more readable as a one liner I discovered
@leslietownes So it isn't just me, physics notation is weird huh
 
5:31 AM
suppose we have $$(I+A)^{-1}$$
 
@AndrewMicallef Take a look at this answer. The formatting there might help.
 
where $I$ and $A$ are matrices
 
just spit it out
 
can we say that this expression is = $I-A+A^{2}.....$
 
physics notation is what it is. i do not think it is designed for people who are not members of the club.
 
5:32 AM
?
 
if $\rho(A)<1$.
bra, ket, really?
 
the neumann series for (I+A)^(-1) is always a good idea, even if it does not make sense.
if it does make sense, so much the better.
 
@copper.hat whats $\rho(A)$?
 
spectral radius
 
@robjohn whats the [6pt] at the end of one of the lines about?
 
5:34 AM
spectral radius! we were talking about matrix norms recently. the unprecedented gelfand theorem.
 
@AndrewMicallef that adds extra space so the lines are evenly spaced.
 
still find it curious
 
what perturbs me about that theorem is how easy it is to prove once you have the audacity to think of it.
 
ahh, cool, thanks
 
15 mins to jalapeno poppers.
 
5:37 AM
poppers or peppers? we do both at my house, but at different times.
 
some costco stuffed jalapenos wrapped in bacon. what were they thinking
not a huge fan of costco food offerings, but i am desperate
i like their meat & fish.
 
we sometimes purchase fish from them. it is ridiculously cheap.
 
i am a salmon fan. something about all those stories of the fianna growing up
the salmon of knowledge
 
@copper.hat, speaking of truley awful formatting, see exhibit b: ![i.imgur.com/Og8rJ6k.png]
 
aaarrrrghhhh
 
5:42 AM
i also enjoy salmon. i'm for it. my cat is, too.
 
a friend in hercules smokes steelhead from time to time. needless to say i am his slave.
 
there are even some weird fonts in there. andrew, what are you doing. why have you chosen chaos.
 
I'm using ipython notebooks to keep a log of my studies,
 
Andrew, for gods sake, where are you from, what happened?
 
so I'm using markdown + mathjax, which is a mess
um, Australia?
 
5:44 AM
there's a 10 step program for that
i mean your formatting, not Austrailia
 
step away from the abyss.
 
I tried pure latex before, but it's a pain to keep re rendering
 
a few of my brothers lived in Sydney for a while
 
a lot of convicts and criminals do go there.
 
so I use ipython, and then I can throw in code for numerical work too
Ouch
 
5:46 AM
the word in ireland was transportation.
 
A ten step program for formatting?
 
yes, you need help :-)
 
I know
thats why I appeal to good natured folks on the internet so often
 
it's sometimes twelve but they've shortened it to ten just for you. to get you out of this nightmarish latex.
 
America has n step programs for recovery from x-aholism
i am in a 20 step program to learn how to step away from bad investments
 
5:47 AM
I guess real mathmaticians do everything in tex documents huh
 
i am real, but not a mathematician
 
@copper.hat GME, BTC, DODGE?
 
i wouldn't discourage anyone from the twelve-step stuff. most of it is BS but not all of it. it will not reformat your TeX.
 
maybe I should go back to .docx
 
ms word is not an option.
 
5:49 AM
i did try to buy $10k of BTC many many years ago, but the exchange wanted 5% in either direction so i passed on it.
 
nah, I'm not paying for licensces, screw that, Libre office and saving as .docx
 
i still do not understand how ms was/is so successful
 
i approve.
 
god the smell of the jalapenos is driving me insane
 
I do. work in the real world,. WHAT you see is all there is to most people, so WYSIWYG is reallythe only viable option
 
5:51 AM
i still push people. are you sure you want it in word? really? i make them ask for it
 
also most people aren't writting maths manuscripts in word
 
i knew a lot of ms researchers, all of them used linux.
i have to pay for a windoze 10 license, my ubuntu is free.
the more i pay for windoze, the fewer features it has
 
I feel like that is the opposite of what I imagine feature creep to represent
 
everything is a service now. i now expect to be charged for the privilege of logging into my desktop
 
a friend of mine works with gates from time to time, speaking of feature creep
something to do with cows in Africa.
i don't mean gates that you open & close :-)
 
5:57 AM
yeah, good that you clarified, becuae that was an ambiguous statement, but then after seeing you claimed Obama number I got the drift
 
my friend got to meet him in the white house
 
:) neat
 
bacon wrapped jalapenos are gooood
 
I dunno, that sounds kinda gross, but I'm happy for you :P
ciao
 
:-)
 
 
2 hours later…
8:04 AM
I really really should just stick to pen and paper and stop f$cking about with tex. @robjohn I've just been working through excercises in my calc book and I get it now. I was wondering how you determined to use $u=\frac{y} {-2\lambda}$ but now I realise you just went backwards. Rather than decide what substitution to make, you differentiate the exponential term, see what is left over and then collect the remaining terms into the $u$ function!
That feeling of realising you understand something that you know you didn't five minutes ago is great
 
 
2 hours later…
10:26 AM
@AndrewMicallef exactly!
 
jay
If I have a continuous function $f$ over an interval $(0,T)$ and I consider any partition $P_h$ of $[0,T]$ where each interval $[t_i,t_{i+1}]$ is length $h$ is it true that $\lim_{h\to 0}\sup_{P_h} \sup_{s\in [t_{i},t_{i+1}]} | f(s)-f(t_{i}) | =0 $ ?
seems like this should definitely be true but im having trouble with details
 
11:02 AM
I don't know what "any partition where each interval is length $h$" is supposed to mean. there is at most one such partition. In any case, this follows from uniform continuity.
 
jay
Yeah I meant max length $h$ sorry.
So I need uniform continuity :/
 
continuous on compacta automatically implies uniformly continuous
 
jay
im not on compacts :(
wait
 
A simple counterexample if $1/x$ over $(0,T)$
 
[0,T] is compact
 
jay
11:09 AM
around the interval [0,t_i] @Astyx
 
$1/h - 1/(2h) = 1/2h\to \infty$
 
jay
yh
thanks for the example
 
I mean, you need a function defined on [0,T] for the question to even make sense
cause you're evaluating at endpoints
 
jay
how about continuous with compact support
implies uniform continuous?
 
6 mins ago, by Thorgott
continuous on compacta automatically implies uniformly continuous
 
jay
11:13 AM
and outside its constant 0 so ocf
ofc*
 
 
1 hour later…
12:25 PM
I noticed some really weird things regarding identities that if I say

$1+ tan^2 x $= $sec^2 x$is an identity but$ tan^2 x $- $sec^2 x $= 1 is not. Why is that ?

Now , ofc examples are endless. But why does this happen ?
 
Because one of those is true and the other isn't. I'm not sure what exactly you are looking for, this seems like a really arbitrary question.
 
Second equation should be a -1.
 
12:57 PM
well, then it would be correct
 
1:18 PM
@Thorgott Like , why isn’t any value of x possible for both the equations?
Because I just shifted one value from LHS TO RHS.
It shouldn’t change out result
 
@jay You need uniform continuity in as much as $\lim_{h\to 0}\sup_{P_h} \sup_{s\in [t_{i},t_{i+1}]} | f(s)-f(t_{i}) | =0$ is essentially a restatement of uniform continuity. So, yes.
 
because it always takes that one value and not the other
this feels like you're asking something akin to "why is the sky blue and not green". there's an answer for why it's blue, but there's no good reason for why it is not green other than it being blue and blue not being green.
 
@SrijanM.T Indeed, $1+\tan^2(x)=\sec^2(x)$ is an identity, and so is $\sec^2(x)-\tan^2(x)=1$. Why should $\tan^2(x)-\sec^2(x)=1$ be an identity? It's always false.
 
@robjohn My teacher told me that sec^x - tan^2x = 1 is not an identity
 
@SrijanM.T get a new teacher, or question what your teacher says from now on.
or at least ask your teacher what it means to be an identity.
 
1:31 PM
either you or your teacher are making a sign error
you started by talking about tan^2-sec^2 and now it's sec^2-tan^2, those differ by a sign
 
@Thorgott I think they made a sign error before.
 
Yes. I made mistake there. It had to be sec^ x - tan^2 x.
This is one. I am sorry for the mistake in typo earlier
 
then of course it is correct, you just put the tan^2 term from 1+tan^2=sec^2 on the other side
 
So , here.
X means it is not an identity and tick means it is
 
Wow...
 
1:34 PM
That’s why I asked like why should it be. There is one more example which is true.
The second one I checked is correct what my sir says
 
Perhaps he means: "These are identities listed in your book, the others are derived from those identities."
 
@SrijanM.T hey ,what's difference in first and second equation?
 
@robjohn No.
 
@SrijanM.T then, do ask him why those are not identities.
 
@robjohn He said like it is valid for some x values but not all
 
1:37 PM
then your teacher is wrong
 
they are both false for $x=\frac\pi2$
 
@robjohn Ohk.
 
of course, the identities only make sense for those values of $x$ for which sec and tan are defined, but that applies to both of the mequally
 
Ohk. I’m getting your point better now.
 
0
Q: $\text{exp}(\frac{x}{x^2+y^2})$ is bounded on $\sqrt{x^2+y^2}\geq\epsilon>0$

love_sodamI want to check if my proof is valid. $\text{exp}(\frac{x}{x^2+y^2})$ is bounded on $\sqrt{x^2+y^2}\geq\epsilon>0$ where $x,y\in\Bbb R$ and $x>0$. Proof. Since $\frac{1}{x^2+y^2}\leq\frac{1}{\epsilon^2}$, for each $x\in\Bbb R$, $\text{exp}(\frac{x}{x^2+y^2})\leq\text{exp}(\frac{x}{\epsilon^2})$...

 
1:38 PM
So they are both false for the same values of $x$
 
I wonder if my proof is correct
 
@robjohn Ok. I got it
Thanks a lot everyone
 
@robjohn @Thorgott any hints?
 
@Rover can you write what $f'(x)$ means?
and possibly use the relationship on the page you posted to compute it?
Think about that. I have to go for a while
 
@robjohn ok
 
Wherefore didst i posteth this junk
Any nice book for cryptography?
 
"Serious Cryptography" by Aumasson
 
Wow t shouldst beest a s'rious booketh
 
what
 
2:06 PM
I bethink thee knoweth english?
Seemeth liketh thee aren't und'rstanding mine own w'rds
 
your words make no sense
 
Those w'rds maketh p'rfect senseth
I preview'd the booketh thee recommendeth and t seemeth nice
I thank ye
Farewell, i has't to leaveth
 
2:36 PM
@Rover any thoughts?
 
@robjohn I found f'(x) and tried by substituting f(x) by g(x) and x, but it's not making any sense .
 
2:52 PM
@robjohn thanks for making the torus!
 
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