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9:00 PM
@TedShifrin G'night for now! Nice to chat up again :)
 
@Studentmath Explain Ramsey theory like I'm 5 :D
 
oh. yes, (-1)^(n^2) is certainly periodic
 
Night, @Studentmath. Be safe!
 
so then why does my textbook not say it is lol
 
What does your text say exactly?
 
9:00 PM
@TedShifrin Good point
 
it just says that its not nonperiodic
im doing homework problems
 
NOT NONperiodic?
 
um. not nonperiodic would just be periodic
 
oh wait
i typed that wrong
 
@Danu I'll work on that :P
 
9:01 PM
it is "non periodic"
 
@Danu: It's not much of a stretch. You're sorta like 6 or 7. :P
 
@pingOfDoom Apart from what the others said, that is surely not all it says. For example it probably defines the function more precisely than you have so far
 
i mean, (n+2)^2 = n^2+2n+4. so (-1)^((n+2)^2) = (-1)^(n^2+2n+4) = (-1)^(n^2).
 
how do we submit pictures?
 
so if you're working with discrete values of n, then (-1)^(n^2) is period-2
 
9:02 PM
"Upload" button
Somehow I thought this was supposed to be in signal space, so this should be Fourier coefficients.
But I'm giving up :)
 
@TedShifrin :'( Crying in bed tonight
But really though, Ramsey theory sounds interesting.
 
if it was, I think it'd end up being a Jacobi theta function
 
One of the few discrete-type things that I like
(at first sight!)
 
@Danu Then don't start your sentences with "Now, now."
You pat your children on the head. Not most others.
 
@MikeMiller Oh, you meant that? It was meant as a joke, because I didn't assume you were actually offended.
 
9:04 PM
 
Tone on the internet... Always a tricky issue. Sorry for the misunderstanding @MikeMiller.
 
1.42(e)
btw this is from haykin and vanveen's signals and systems
 
Oh, I see, they mean literally the function on the integers.
 
its a signal processing text but it might as well but a math textbook
might as well be'
 
Do you understand (d)?
 
9:05 PM
yeah of course
 
(e) is identical.
 
^ :)
 
So the answers at the back of the book are wrong.
 
This fact is so useful, so many times :D
 
ughhh lol
 
9:06 PM
@Danu I wouldn't find it particularly funny if you'd put on the same tone in person. Anyway.
 
@MikeMiller Let's forget about it.
 
@MikeM: I did not find the use condescending. You must have a particular trigger for that phrase that I don't.
But, yes, let's move on.
 
even the profs solutions have it as nonperiodic
 
Also @TedShifrin you're exactly the type of person to use that kind of phrasing yourself :D
 
which is weird
 
9:07 PM
Does the prof explain, doomed Ping?
 
Perhaps the square brackets mean something?
 
Good point. Why not parentheses?
 
nope lol
the square brackets is discrete time
 
OK. Then we stay where we were.
 
thats all. isnt that a math thing or is that specifically engineering?
 
9:09 PM
Hey again :)
 
$(-1)^n = (-1)^{n^2}$ for all integers $n$.
 
@pingofDoom you can easily prove yourself that $n^2$ is odd iff $n$ is. That's equivalent to the statement above this one, by Ted.
 
Hi @Mahmoud
 
@pingOfDoom Definitely not a math thing :)
 
hold on so if im understanding correctly
do you guys do any z transform stuff?
 
9:11 PM
@TedShifrin The problem had to do with logic at the end, where I'm asked to simplify a dis-junction of two conjunctions.
 
OK, @Mahmoud ... But there's an awful lot of algebra to get to that. Remember that showing $x+y\ge 0$ dismissed half of the disjunction.
I taught the z-transform 30 years ago, doomedPing, but I'm rusty.
 
ohh k
 
Yikes, time doth flee.
 
Yes, I thank you very much and appreciate what do you do.
 
@TedShifrin Does that make me older than him?
 
9:14 PM
Thanks for letting me know, @Mahmoud.
@BalarkaSen Some days.
 
By the way, what is it like to be a mathematician/professor/researcher ? Are you happy with your choice @TedShifrin ? I'm just wondering please excuse my curiosity.
 
I'm now retired, @Mahmoud. I very much enjoyed my career, and, as you can tell, I miss teaching students.
Well, some students :)
 
@Mahmoud If you like doing math research and teaching math, then being a mathematician is great
 
Most of us, I think, find it a rewarding career. Some are in it just for research, some just for teaching, and a lot for a blend. But mostly we're doing something we enjoy doing and find fulfilling. Not sure how many people in other careers feel that way.
 
What does it require to go deep in the world of mathematical research... ?
 
9:17 PM
Danu's becoming the resident complex geometry expert next to you, @Ted, so probably not.
OK, my job's done. I have to sleep.
 
Night!
 
G'night.
 
@BalarkaSen LOL
 
@Ted "Most of us", of course, only applies to those who get jobs.
 
@Mahmoud require in what sense?
 
9:18 PM
can someone explain to me how to find if cos[2n] is periodic?
 
@MikeMiller This is the main message to keep in mind :(
 
A PhD would be an obvious answer, but not necessarily the sort you are looking for
 
note that its discrete ttime
 
Yeah, @MikeM ... I'm not going to say the state of academe in this country is much better than the state of the populace or politicians.
 
@TobiasKildetoft Cleverness, ability to appreciate math ... ?
 
9:19 PM
@Mahmoud now that is a tough one
 
doomedPing, if only there were a $\pi$ in there ...
I think most people who study math seriously and do research love to puzzle over problems and don't get too easily discouraged.
doomedPing, you're not expected to give a "proof," I'm sure.
 
oh dear no
i just need to determine algebraically
if it is or not
 
Do multiples of $\pi$ ever differ by an integer?
 
im sorry?
 
what does cos[2n] with square brackets mean?
 
9:22 PM
You know the cosine function is periodic with period $2\pi$.
 
yes
 
We recently got into the basics of Set Theory (The course wasn't really named that), and after a few minutes I realized that they were absolutely no numbers on the board, can we consider in the case Sets being the mathematical object of the study ?
 
Make the brackets parentheses, @Alessandro.
 
@Alessandro it means its discrete time, so the function isnt continous
 
@Mahmoud: Most math is a blend of abstraction (theory) and concrete calculations/applications. But not all.
 
9:23 PM
(almost) every mathematical object is a set when you're doing set theory @Mahmoud
 
@Alessandro the input samples cos(2n) at a particular frequency
 
I see
 
doomedPing: If you had $\cos(2n) = \cos(2n+2k)$ for some $k$, what would that tell you about $k$?
 
that k was some multiple of pi?
 
Right. Precisely. Is that possible? What famous fact do you know about pi?
 
9:25 PM
its irrational
 
Bingo.
 
I like it ! @TedShifrin It makes the ability to generalize facts and apply math pretty much everywhere, I don't consider abstraction bad, it makes you feel the purity of math.
 
i still dont follow, im sorry
 
If $k$ is an integer and $k=m\pi$ for some integer $m$, then what is $\pi$?
 
k/m
 
9:26 PM
uh huh ... which makes it what sort of number?
 
ohh rational
which cant happen
 
Bingo. :P
 
ohh but thats assuming that theyre both integers
 
How old are you @TedShifrin ?
 
@Mahmoud Indeed, abstraction is one of the great strengths of math, in that it can sometimes remove unnecessary details that were not needed for the desired conclusions
 
9:27 PM
But they are, doomedPing.
@Mahmoud: Ancient. :) 63.
 
@TobiasKildetoft Agreed ! It's the absolute opposite of literature(No hating) :)
 
There's a lot of artistry to mathematics. It takes a while to appreciate that.
 
Grothendieck's memoirs merge the two nicely.
(also random hate spewing, as an aside :P)
 
I recall laughing when I saw the first few proofs, I think in calculus where they did few algebraic tricks. It was how it transformed such complex things into such simple 'magic tricks'...
 
What still fascinates me is the axioms, we don't have to check every single one of the infinite cases, after setting the corresponding rules of the game.
 
9:34 PM
yay k that makes snese now
thanks @TedShifrin
 
aren't we assuming that $\cos$ has no rational period somewhere? I mean it could in principles have period $2\pi$ and also period $2$, right?
 
@TedShifrin It's is a great honor for us humans, as finite creatures, to put our hands on those incredible facts and grasp them, arriving at Infinite results, in a finite world.
 
How are you guys doing in here?
Btw, @Mahmoud, chill with the commas ;) But I do agree with your sentence there
 
Do I use too many commas @dsillman2000 ?
 
I don't mean to be a stickler, I'm sorry if it came out that way, it's just I'm ocd sometimes
*It's a great honor for us as humans, as finite creatures, to put our hands on those incredible facts and grasp them, arriving at infinite results in a finite world
But yeah I really do agree it is very divine for us to be able to use infinity - something that can never exist - to arrive at conclusions that very much exist.
 
9:46 PM
No it's not like that @dsillman2000 just tell me if I use many commas and it'll be good.
 
Long message incoming...
You know, I was thinking about division by 0 as it relates to Peano arithmetic and first-order logic (I'm not a logician, so bear with me):
When we make a statement about $\frac ab=c$ (in the natural numbers), we formalize it similarly to For all $c$ such that $bc=a$...
If we set $b=0$, then we get For all $c$ such that $0c=a$... which is vacuously true for $a\neq 0$.
However, if we also set $a=0$, then it becomes For all $c$ such that $0c=0$...
 
if i have a piecewise function, and then i square it, do i square what the ranges are too?
 
This means that we can only make a statement about $\frac00$ if it is true for all numbers (e.g., $\frac00+1=\frac00+1$.)
 
or only what the actual function values are?
 
As another example, the proposition "$\frac00$ is either even or odd" would be true, but "$\frac00$ is even" and "$\frac00$ is odd" are both false.
Any comments?
@pingOfDoom I believe you'd only square the values
 
9:50 PM
are we in consensus?
 
@ΛεγίωνΜάμμαλϠΟΗʹ most divisions are not defined in peano arithmetic, so why try to define one that cannot be defined even in more general structures?
 
Test
 
just based on the logic of the thing.
 
@TobiasKildetoft I know it's undefined and anything using it becomes unintelligible gibberish, I was just thinking about it in the realm of first-order logic.
 
the values @ping, think about what squaring a function means
 
9:53 PM
Is it possible to change my user profile image ?
 
ur only ever operating on the output values. the inputs stay untouched
k thanks
 
@ΛεγίωνΜάμμαλϠΟΗʹ first order logic itself does not have any multiplication, let alone division
 
@TobiasKildetoft (with Peano arithmetic on top of it)
 
@Mahmoud go on math.stackexchange.com, click on your reputation and medals at the top of the screen and go to edit profile and settings
 
I did @Alessandro But it didn't change here in chat.stackexchage
 
9:55 PM
@ΛεγίωνΜάμμαλϠΟΗʹ Peano does not have most divisions as I said. I have no idea where you are going
 
you might have to either leave and rejoin the chat or restart your browser @Mahmoud
 
Ok @Alessandro
 
@TobiasKildetoft IDK, my sum knowledge of this stuff is GEB + a whole lot of reading Wikipedia
 
@ΛεγίωνΜάμμαλϠΟΗʹ There is not much to have knowledge about (no idea what GEB is). You cannot divide by zero in any meaningful way and retain a useful structure
 
Test ?
 
9:58 PM
hm, it didn't get updated
 
Meh.
 
it's supposed to automatically use the same image on the main site and in the chatrooms, I suppose it'll get updated eventually?
 
I see that it's updated here math.stackexchange.com/users/376394/mahmoud
WB @TedShifrin
 
@Alessandro: If a function has period $2$ and also period $2\pi$, then all the periods will be dense in $\Bbb R$. What will that mean?
Thanks, @Mahmoud. May have to clear cache of the webpage.
 
that the function is either very discontinuous or constant I believe
 
10:01 PM
Right. If it's continuous, it'll be constant.
Same fact as noting $e^{in\alpha}$ is dense in the circle whenever $\alpha$ is an irrational multiple of $\pi$.
 
Back.
 
makes sense
 
Heya @PVAL
 
Hiya @ted
 
www.youtube.com/watch?v=Kc0Kthyo0hU Impressive.
 
10:07 PM
hi
 
Hello
Oh it's updated now :D
Aleph-Null the first smallest infinity.
 
@PVAL is there anything i should be applying for this year?
 
?
 
@MikeMiller Probably. I don't know what I should be applying for.
 
There's a program at the Newton center about low-dimensional topology homology theories. I emailed one of the organizers to ask if they have funding for grad students to attend.
 
10:15 PM
Glad to see a proliferation of non-generic avatars :)
 
But that's not really your style of thing.
 
@TedShifrin :)
 
I have trouble following mainly algebraic talks about any kind of Floer theory.
I'm wondering if I should give a 5 min talk in december at georgia tech.
 
@PVAL-inactive only 5 minutes?
 
Ya
That's the main reason I'm wary.
 
10:19 PM
@PVAL A lot of the algebra is beyond me. I can read it but I don't understand it.
 
The few times I had to do 10 minutes, I found the experience excruciating.
 
@TedShifrin I think I'd just say heres this person's theorem, and here's my theorem see how its an improvement of this person's theorem. Are there more thereoms like these two?
 
LOL ... But can you make the notation and terminology understandable in the remaining 4 minutes?
 
I wish i had known about the conference in Bonn right now before two minutes ago.
 
@Ted Well for 5 minutes, I'm really only catering to people who might already care about the result I have.
I wouldn't be trying to convince anyone to care.
 
10:22 PM
Fair enough, @PVAL. Go for it.
Hopefully you can put in an example or an illustration of what your theorem can do that the original couldn't.
 
Maybe i should go to the Tech thing.
 
im still confused about periods :(
 
Ask your mother.
 
Is there a way to have a question moved to a different stack exchange?
 
ahaha lol
 
10:36 PM
Oh, yikes, the application cycle for next year's PhD's starts in a about a month. I'm so screwed!
 
Deadlines in the US start early- to mid-December ... yup.
 
Is anyone here more than an acquaintance of Fractional Calculus and can recommend a good text for it?
 
@TedShifrin Bonn is also mid-December.
 
Oh, hey @TedShifrin
 
They have an option to apply for PhD after one qualifying year. I should do that.
 
10:38 PM
hey @Axoren
 
One more year of courses yay :P
 
I remember I had a question for you, but it slipped my mind. If I remember, I'll ask you later.
 
how do you find the period of a continuous sum of sinusoids?
 
Reciprocal of their frequency, isn't it?
 
does it add tho
sin(2t) + sin(3t)

first has period pi, the other 2pi/3
so then am i allowed to just add them up?
 
10:41 PM
Draw the graph.
 
algebraically
 
Draw the graph. You'll see what it should be.
Then you can work algebraically, once you know what you're supposed to find.
 
@Danu What sort of sorcery lets you draw the sum of sinusoids like it's no big deal?
I can't even draw a parabola half the time.
 
It might take 5 minutes.
 
Hi @Ted. How goes it?
 
10:42 PM
Or one second using graphing software.
 
Heya @Fargle ... :)
 
"You're not going to have graphing software everywhere you go."
 
@Axoren Then take 5 minutes :P
 
Definitely not adding them up, doomedPing.
Think least common multiple.
 
I remember learning how to do least squares on paper. Now it's just a function on my smartphone.
 
10:44 PM
Least squares is about solving the normal equations: projection on the image of a linear map :D
 
Now it's me taking a picture of a list of points, using an OCR app to copy and paste them into a math app
 
Back
 
I no longer need to know math.
 
wait what?
 
I've transcended.
 
10:47 PM
@TedShifrin how?
 
@Axoren Impossible
 
Try it out, doomed ping.
 
Draw the graph :'(
Or solve something like $f(x)=0$
 
@Mahmoud Don't you see? I'm not algebraic in the least!
 
Thinking is easier, @Danu.
 
10:49 PM
@TedShifrin What did you think of mathematics when you were younger ?
 
oh so its just 2pi?
 
@TedShifrin SAYS YOU
 
It kind of bothers me that the people who will get my pun won't find it funny.
 
Yup, doomedPing. Check it out.
Sorry, @Mahmoud. I'm busy on an MSE question.
 
Wait which one? I thought I remembered seeing 2/3 x as the argument
 
10:50 PM
Ok @TedShifrin :)
 
whats the logic behind that @TedShifrin
 
Oh, I saw 2pi/3 and then my memory messed it up :D
 
like i can graph it and its fine but why does that work like that?
of the lcm being the period of the combination?
 
Because you need to have the smallest number that's a multiple of $\pi$ and also a multiple of $2\pi/3$. Periodic with period $a$ also means periodic with period $ka$ for any integer $k$.
 
ohh
 
10:52 PM
Alternatively, if you're more of the programmatic school of thought, think of it like a game of Fizz Buzz and the period is how often you say both Fizz and Buzz.
 
welll im an electrical engg student so i should be thinking like that @Axoren lol
 
So, if the first sinusoid is Fizz and you say it every $2$ times, the second sinusoid is Buzz and you say it every $3$ times, then you say FizzBuzz every $\gcd(2, 3) = 6$ times.
Happens to work out just like Ted said
 
Yeah, I think the $\pi$s confuzled doomedPing. :)
 
Definitely scary for an engineer.
 
Nah, he's smart.
 
10:56 PM
we dont like floating point values as engineers
 
I had a physics teacher who told us to assume $\pi = 3$ and $g = 10$.
 
they're hard to deal with :P
yeah in AP they make you assume that
 
Ugh.
 
so that you can do all the math without a calculator
 
Leave the numbers $g$ and $\pi$ in the answer, dammit.
 
10:57 PM
Alternatively, represent things symbolically.
^
Also, doing Physics in English units.
 
i had a mech prof who had a hard-on for unit conversions like that
it was so annoying :(
 
I went to the hardware store today looking for an Alan Wrench. Oh my goodness, every product had a metric counterpart, except that the set I was looking for only had a Metric set.
I needed an eighth-inch Alan Wrench
 
its like here solve this system that takes you 15 minutes, now convert that torque to foot pounds or fuck you and deduct half the marks for the q
 
I don't know what that is in metric.
 

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