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00:03
Bonne nuit.
@DanielFischer De même
00:41
I am having some trouble with this problem: Let $\omega$ be a complex number such that $\omega^5 = 1$ and $\omega \neq 1$. Find
$\frac{\omega}{1 + \omega^2} + \frac{\omega^2}{1 + \omega^4} + \frac{\omega^3}{1 + \omega} + \frac{\omega^4}{1 + \omega^3}$, (precalculus, roots of unity). Does anyone have any hints? I tried rearranging it to cancel some terms out in the fractions but it ended up becoming a large, messy fraction.
not that I've tried working anything out on paper but it might get nicer if you multiplied the first term by $\frac{\omega^4}{\omega^4}$, the second by $\frac{\omega^3}{\omega^3}$, etc.
okay I worked it out and it simplifies really nicely
That worked, @SamuelYusim? I had conjugates in mind, maybe; $(1 - \omega^2)/(1 - \omega^2)$ for the first, for example
But that's probably not a great idea the more I think about it
I don't really see that working out @pjs36
I can confirm, @Samuel has a good method there. I got a really clean answer after not much manipulation.
Yeah, I agree.
00:53
+1
I did it!
Thanks, I'll try it! @SamuelYusim
What I have so far is $\frac{2}{\omega^4+\omega} + \frac{2}{\omega^3+\omega^2}$.
Yep!
yep, now just get it over a common denominator and you'll be on your way
I got the answer (2). Thank you! :)
01:03
Where are you taking pre-calc? At least where I'm from, we hardly touched complex numbers, much less roots of unity, but then again, I didn't exactly have a stellar math education.
Art of Problem Solving! @Fargle
Ahh! That explains it.
It wasn't used in solving this particular problem, but as a side-note, in future problems it might be useful to remember $1+\omega+\omega^2+\omega^3+\omega^4=0$, so the final fraction (although it simplified without realizing it) could have been seen as $\frac{-1}{-1}2$
Yep! :) @Fargle, and thanks for the tip @JMoravitz!
(where $\omega$ is of course a primitive fifth root of unity. similar results hold for primitive $n^{th}$ roots of unity)
01:08
Great, thanks! @JMoravitz
r9m
r9m
@DanielFischer I was dragged afk (sorry for late reply) /// Nice one!!!! :D (+1)
I'm working as a grader for a linear algebra course this semester, and have found that they were taught the definition of a determinant of a matrix as $det(A)=\sum\limits_{P\in \text{patterns}(A)} sgn(P)prod(P)$. I'm curious if anyone else here learned this definition before the recursive definition.
@JMoravitz Not before, no, but this was taught to me in my abstract algebra class last year.
Definitely not before, and not until doing some pretty serious group theory
It only managed not to be awkward because we had established definitions for working with permutations long prior to ever seeing a matrix.
(A matrix as formally defined in ring theory, that is. Not just the typical $GL_2(\Bbb R)$ example from elementary group theory.)
01:17
So it's raining so hard here I can't go to school. I'll get flayed alive on Monday. :(
D:
Spend the time well! Study!
(He said as he updates TF2 and fiddles with his guitar)
At least tf2 is somewhat social. Better than wasting more time on touhou like I have.
Hey, everyone has their vice. Some have more than one. ;)
I'm doing nothing instead of writing a final exam! I did put Ted's Multivariable Mathematics book on hold at the library; I guess that's something.
@BalarkaSen no
@Fargle oh, you play too?
01:28
Yeah, since mid-2010 or so. (Right after the WAR! update.)
I'm going to $\LaTeX$ up more solutions to Hatcher's notes.
@Fargle no, guitar
...oh.
Yeah, I dabble. I've played piano much longer. The trouble I'm having right now is tuning for Sonic Youth songs.
01:30
Standard tuning is, of course, EADGBE. Examples of their tunings include: F#F#F#F#EB, DDDDAA, GABDEG, CCEBGD, and my personal favorite, ACCGG#C.
You can have an all-D tuning.
But really, all of these are what I like to call "diabolical".
Yeah, it's just hard on the guitar after a while to be tuning it up and down.
I can tune to drop C and D and DADGAD by ear, but that's it. I prefer standard, although I toyed with perfect fourths tuning for a while.
I can tune by ear to most things, but that's the ten or so years of piano at work, giving me a damn decent sense of pitch (I tend to be only a few cents flat).
(How do you post videos like that? Just put the link in?)
yes. my ear sucks, though. not even kidding.
classical piano?
01:33
At first, yeah. I took lessons starting at the age of nine, ending when I was about 17.
Now I mainly play rock and jazz stuff.
Anyway. I can't really argue with Sonic Youth's method when they get results like this:
oops.
That was in my clipboard for a reeeeeeeeally long time.
This is really nice. Melancholic, introspective.
Herp.
This is in quite a different direction from what I've been listening to lately (alt and punk rock that's older than I am). I really like it.
That was what pushed me to learn. I've been playing for a year now and can barely manage that. :)
Want to look at an unashamed show-off? (turn your speakers down)
This is, like, the limit of technical skill.
Look at him sweep with his fingers. It's almost noise, but it's impressive noise.
01:41
Dude's got chops.
I don't really listen to much metal--I don't know why, just something about it turns me off--but there's no denying that it takes an immense level of technical skill to do it well.
And she, well, looks like Jennifer Lawrence, which is a plus in my book.
That piece is what I would nominate for "greatest single-human achievement" in music.
"Hey, uh, Bach, you have fifteen minutes. What can you get me?"
"Oh, nothing much, just a guided tour of every human emotion." :P
I'll see your classical and raise you more Sonic Youth--I've been obsessed lately.
They were quite technically skilled, but didn't concern themselves with typical notions of consonance and dissonance--they cared more about the "wall of sound", so to speak, the texture and timbre of the guitars as they weave between one another.
oh, but listen to that piece first.
01:45
I am--I just linked that one to push my music taste on this chat room. >_>
@Fargle well, do you know whom "wall of sound" was originally meant to refer to?
Phil Spector.
It's not from the same historical tradition--I just really like the term.
Oh, man, Coltrane. I love this song.
eh, Coltrane, not Bach. :P
Give this a shot, then. Pretty interesting things he does.
And now I should get to work.
01:47
I'm somewhat a fan of Abasi, but I haven't seen this.
Well, don't let me keep you, haha.
$\hskip -1.2in\longrightarrow$ the fan
ah, perfect
so I was on the radio yesterday for a quiz, and the RJ asked me what genres of music I liked. I said "upto 19th century classical, inclusive, some metal and jazz, and . . . dramatic pause Pink Floyd".
Pink Floyd is a genre.
Because "progressive rock" isn't a thing. ;)
02:13
@SohamChowdhury May I ask why you have an arrow next to your pic?
@Khallil I believe it was done with $\LaTeX$, by offsetting the longrightarrow $\longrightarrow$ some amount to the left. That is, Soham put it there (to emphasize that in fact, he is the fan) :)
02:46
@Fargle oh, they both are. :)
03:21
lol'd @SohamChowdhury
r9m
r9m
03:54
can anyone upvote just one of my questions so that I may get the reputation score $8888$? :D
Aah, I finally understand what a $\rm{coker}$ is. :)
@r9m there you go
r9m
r9m
@SohamChowdhury Thanks !! took a screenshot :D
when you're on 9995, someone will go and give you an upvote. you'll be mad. :P
r9m
r9m
:P lol
04:22
Hi.
"Does $G_1/H\cong G_2/H\implies G_1\cong G_2$?"
Is $V_4/C_2\cong C_4/C_2$ a counterexample?
How's things, @Kaj?
04:42
Not bad @MikeMiller. I've almost gotten myself to the categories section in Hungerford, which is exciting since I've never studied them before.
You've touched most of the things in that, haven't you?
Most things before that I have. But I'm seeing everything presented differently from when I was introduced to it, and his writing is so lucid. It's great.
Ah, I was confused, it was a typo.
I agree that you should get exercises elsewhere since his are rather tepid, but I agree with your taste in the book. I think I said this earlier but it's where I learned my algebra.
But the categories section will be 100% new stuff for me.
Yeah. I guess I'm just really excited because I like algebra quite a bit, and I've been away from it for a while now.
Hi pal :-)
04:48
@KajHansen should be fun!
Hey @skillpatrol
Can I say that an action of a group $G$ on a set $A$ is a map $G\to S_A$?
Nice to see you back.
@SohamChowdhury: You should be able to answer both of the questions you've asked in the past twenty minutes. :)
@SohamChowdhury, in fact, that's how group actions are defined in many texts, as a homomorphism $G \rightarrow \text{Symmetric Group on X}$.
04:50
@MikeMiller It's a very small sanity check, is all.
Some places define it as $G\times A\to A$ instead.
I just wanted to check that these were equivalent.
(Of course, I can construct a bijection really easily.)
Wiki has a good discussion in the article on Group Actions @SohamChowdhury
It feels good @skillpatrol. I was out for a while with my REU taking up a lot of time.
I'll give that a look when I'm done studying, sure. :)
We lost a legend :'(
of the 70's
@Kaj, @Mike, are any of you familiar with some functional programming language?
That was a valid counterexample from before @SohamChowdhury. Both of those quotient groups have order $2$, so there's only one thing they could be. And obviously $C_4 \ncong V_4$.
05:00
There's so much partial application/currying going on in algebra.
I'm either unknowledgeable about or incompetent with any programming language you can name.
Nope, I don't know anything about programming beyond a little Sage that comes in handy from time to time.
@KajHansen I know, I discussed that with Balarka a while back in the context of short exact sequences.
@MikeMiller Oh.
It'd be useful to get better sometime but not today.
Even BASIC? @MikeMiller
05:01
$S_3/C_3$ and $C_6/C_3$ works too, haha
@Mike, I was essentially talking about this idea: when you have $f:A\times B\to C$ (say $f(g,h)=gh$), you can "partially apply" the first argument.
So you can define $g=f(a)$.
And now $g:B\to C$.
This is very, very common in Haskell and Scala and all those languages. It's also sort-of all over the place in the little algebra I've seen.
@KajHansen yep, I hadn't thought of that!
Sure. If I learn how to program properly it'll probably be Python, which to my understanding isn't that.
Python does support functional-ish ideas, so it's a good gateway drug.
Say you have some list of numbers $l$ and a function $f:\Bbb R^2\to\Bbb R$ defined by $x\mapsto 2x$. You can get a list $l_2 = \rm{map}(f,l)$, which does exactly what you think it does: it doubles everything.
If you'd like to type this, that's ok, but I should warn you that I'm not really paying attention. Sorry. I might be more interested another day.
Haha, I'm done there. That's all right. :)
Just tell me when you take the plunge. (interesting choice of words there)
05:09
You'll be waiting a while.
He indeed does choose his words interestingly. @SohamChowdhury
Just like robjohn
Okay, a proper question. Does "free" mean anything special in "free action"?
As in, does it have anything to do with free groups etc.?
@Balarka, for when you come back: wow, group actions do trivialize Cayley's theorem. I mean, I blinked, something suddenly happened, and it was over.
@SohamChowdhury: No, they don't. It's just terminology overload. Free as in 'does not fix anything', which one might say as 'nothing is restricted', or 'free'.
Ah, right, thanks.
Sup chat?
05:21
Group actions.
LOL those come up in music theory
Really? That's interesting.
Yeah, I guess with 12-tone stuff?
PLR group? @StanShunpike
05:25
this one is pretty good
Ah, thanks for the link
Yeah, Parallel, Relative, Leading-tone exchange
I haven't really managed to implement it in composing but it's still fun to think about.
There's also the behemoth of a book Topos of Music by Mazzola
it's like 1400 pages or something
Bib
Bib
05:53
Can someone point me to a reference to the fact any two semisimple categories are equivalent if and only if the numbers of their isomorphism classes of simple objects are the same?
You'd probably have more luck in the MO homotopy theory room.
Bib
Bib
Thank you for the pointer.
off topic. Suppose $N$ is a normal subgroup of $G$. I know that $(gn)N=gN$, but I'm not sure whether $(ngN)=gN$. Is the latter always true? It's not obvious to me since $ng$ need not be in $N$.
$ngN = nNg = Ng = gN$, the first and last equality by definition of normal subgroup.
sweet, thanks Mike!
06:03
Sure.
 
1 hour later…
07:08
Do I see a functor here?
Anonymous
07:24
@KajHansen Hey there!
07:41
Note the last sentence :-) @Chris'ssistheartist
Ehhh, now there'll be a fight.
Hey @Ashwin. MSE just let me know I got pinged, lol
@Mike, is there any "deeper" analogy between a ring element not being a left/right zero-divisor and the idea of mono/epimorphisms?
08:01
Hey @KajHansen which math topic should I read to understand about the n sphere. I seem unable to understand the wikipedia page
In mathematics, the n-sphere is the generalization of the ordinary sphere to spaces of arbitrary dimension. For any natural number n, an n-sphere of radius r is defined as the set of points in (n + 1)-dimensional Euclidean space which are at distance r from a central point, where the radius r may be any positive real number. Thus, the n-sphere centred at the origin is defined by: It is an n-dimensional manifold in Euclidean (n + 1)-space. In particular: a 0-sphere is the pair of points at the ends of a (one-dimensional) line segment, a 1-sphere is the circle, which is the one-dimensional c...
Well, you should understand what $\mathbb{R}^n$ is first and the so-called "metric" associated with it.
Metric spaces?
Yeah, but no need to be so general.
So $\mathbb{R}$ is just the real line, $\mathbb{R}^2$ the plane, and $\mathbb{R}^3$ is $3$ dimensional space as we think about it.
@skillpatrol oooooooo, "In the study, mathematicians rated Srinivasa Ramanujan's infinite series and Riemann's functional equation as the ugliest of the formulae." :-(
I know what $\BbbR^n$ is
08:03
Ok perfect
@skillpatrol For me they are the most beautiful of all. :-)
I don't know words like manifolds are
And to find the distance between a point and the origin in $\mathbb{R}^n$, we just take the square root of the sum of the squares, right?
"Brain scans show a complex string of numbers and letters in mathematical formulae can evoke the same sense of beauty as artistic masterpieces and music from the greatest composers."
@Chris'ssistheartist from this study
08:05
yes I know that
@skillpatrol In the end it wasn't nonsense at all when I was talking about beauty in mathematics (as some suggested here - maybe indirectly).
So in $\mathbb{R}^2$, the distance between $(x, y)$ and the origin is $\sqrt{x^2 + y^2}$. So all the $N$-sphere is is the collection of points in $\mathbb{R}^{n+1}$ that are all the same distance from a given point.
@skillpatrol Yes, thanks.
np pal :-)
08:07
That's pretty much all there is to it. Now, it's true that the $N$-sphere is a manifold, and there are tons of equivalent ways of defining a manifold. One way of thinking about a manifold of dimension $n$ is that a small neighborhood around any given point is "homeomorphic" to $\mathbb{R}^n$.
A homeomorphism is a function from one space to another that is bijective, continuous, and has a continuous inverse.
Well, it's hard to explain what a manifold is just right off the bat.
Well, an $n$-dimensional manifold "looks like" $\mathbb{R}^n$ locally. So if the Earth was a perfect sphere, it would be a $2$-dimensional manifold because you can't really tell the difference between the ground you're standing on and a plane ( $\mathbb{R}^2$ ) @zed111
And likewise the $1$ sphere, what we call a circle, is a manifold because it "looks like" a straight line ( $\mathbb{R}$ ) if you're standing at a point and looking at points very close to you.
And there are ways of making these statements mathematically rigorous.
Mew
Mew
08:23
Hi I need some stats help
can u help?
@skillpatrol I wonder if there can be great mathematicians that don't perceive mathematical things as beautiful. I have doubts about that.
(I can't even imagine such a case)
I don't know anything about statistics
Mew
Mew
how come
I just haven't taken any statistics courses. It's never really interested me.
Mew
Mew
stat's is great
it's one of the oldest branches of mathematics too
The universe is ultimately a statistical one at the quantum level
It is the most profound and practical field of mathematics we know
08:31
I make a distinction between probability and statistics
Then again, I've never been that interested in probability either :P
Mew
Mew
It's an acquired taste
@skillpatrol The hugely influential theoretical physicist Paul Dirac said: "What makes the theory of relativity so acceptable to physicists in spite of its going against the principle of simplicity is its great mathematical beauty. This is a quality which cannot be defined, any more than beauty in art can be defined, but which people who study mathematics usually have no difficulty in appreciating."
I just read that quote somewhere else
I don't agree with the last part though, I saw a lot with a lack of appreciation toward its beauty.
Anyway, I have to finish some solution.
BBL
Later my friend :)
Mew
Mew
08:51
Skill patrol pls help
with my stats question
09:08
what's your stats question @Mew?
Mew
Mew
Hi Karl
I have some paired data
e.g. weight before and after some treatment
If I want to find confidence interval of the difference I can use a simple paired t-test,
but
what If I want to find a confidence interval for the % increase in weight
Is it correct to simply calculate the % difference for each set of paired data, and apply same methodology as paired t test from there?
hello @KarlKronenfeld
Ah, I am not strong enough in stats to give you a certain answer, unfortunately @Mew.
Hi @Balarka
Mew
Mew
aw k
@SohamChowdhury you're seeing functors everywhere.
not everything can be translated into category theory and/or is worthed translating
09:14
how can you say that as a person in the 21st century? :P
@SohamChowdhury right.
@Karl is it a modern trend to translate everything in category theory? if so, I haven't been following.
better than translating everything into sets, amirite?
2
@Soham another tautological corollary of group actions : let $G$ act on the underlying subset of $G$ by conjugation : $gx \mapsto gxg^{-1}$. partition $G$ into the orbits of this action. $|G| = \sum_i |O_i|$. for each elt in $Z(G)$, the orbit is of card. 1. for the other $|O_i|$s, apply orbit stabilizer to conclude $|O_i| = |G : C_G(x_i)|$ where $x_i$ is some representative of that class. now sum up.
the resulting thing is called the class equation of $G$.
@KarlKronenfeld :P
@Karl I guess I am just ignorant towards category theory (probably because I don't know much mathematics)
09:37
nah
And with that extremely clear and precise remark, I shall go.
lol
you call that extremely clear.
but bye, anyway.
09:52
@r9m did you finalize my integral? :-)
@BalarkaSen that is a functor, I think. whether or not it's worth studying or talking about is another question. :P
@BalarkaSen this:
Sarcasm is "a sharp, bitter, or cutting expression or remark; a bitter gibe or taunt." Sarcasm may employ ambivalence, although sarcasm is not necessarily ironic. "The distinctive quality of sarcasm is present in the spoken word and manifested chiefly by vocal inflections". The sarcastic content of a statement will be dependent upon the context in which it appears. == Origin of the term == The word comes from the Greek σαρκασμός (sarkasmos) which is taken from σαρκάζειν meaning "to tear flesh, bite the lip in rage, sneer". It is first recorded in English in 1579, in an annotation to The Shepheardes...
don't mind, haha.
did you go to school?
@SohamChowdhury ok.
no, I didn't. water, water everywhere.
same here.
@SohamChowdhury why are you linking that to me :P
we had some sort of "career guidance" crap today, thank goodness I couldn't go.
10:02
cool, I'd give anything for missing such a thing.
@BalarkaSen because you apparently didn't understand Karl's "precise" remark.
@BalarkaSen and it's based on pseudoscientific 70s psych tests.
aka "CONGRATS YOUR BREN IZ JUST LIEK EINSTEINS WOW RITE????"
yes, I guess he was being sarcastic.
he was, indeed.
what are you doing?
still writing that up?
@SohamChowdhury I? trying to do some math.
wait a sec
@SohamChowdhury no, have finished doing that.
what about you?
going to work out some groupy stuff, briefly look at rings and then do a few Hatcher/Armstrong exercises.
10:11
did you see the class equation message i pinged you above?
verify it with small groups like $S_n$, etc.
I've seen that before, when I skimmed Aluffi's second groups chapter.
@BalarkaSen sure
oh, ok.
it's mainly used to prove that p-groups have nontrivial center
that in turn is used in Sylow theory.
woo. but I'm very excited about starting rings. because 1) I learn what homology is, properly; 1b) diagram chases; and, the most important of all: 2) number-theoretic applications like the Christmas theorem
you won't find homology in rings.
modules as well are in that chapter.
10:15
and diagram chases are part of modules
eh, sorry.
you're doing this from Artin right? then there's no mention of homology
do you think? ;)
I read Artin afterwards, always.
10:16
@Soham no point in having a prof if you're not going by his suggestions, as already told.
I am doing Artin. That's all he asked me to do.
Doesn't everyone use a bunch of books?
:p but you're doing Aluffi, mostly (don't get me wrong : it's a good book)
nah, I use a single book most of the time
I did things from Artin, and had D-F as an encyclopedia.
yes, I am. for some reason, the explanations "click" so well.
Artin does everything so slowly and in such small steps that you never realise when he managed to silently prove something right under your nose. :P
but be warned : homological algebra differs wildly from homology in algebraic topology. I'd say you'd'nt have fun with the former.
Artin gives geometric intuition
and it's your loss : symmetry is cool
Thanks @KajHansen
10:20
(Artin also does a bit of covering spaces theory and Riemann surfaces alongside Galois theory :P)
listen. I read both concurrently, more or less, so I think I don't end up losing anything.
Aluffi doesn't have symmetry
@BalarkaSen Aluffi does some algebraic geometry in the second modules chapter. More than what Artin does.
@BalarkaSen eh, I read both.
@BalarkaSen cool.
@SohamChowdhury I bet he won't teach you covering space and riemann surfaces.
just plain old algebraic curves
31 secs ago, by Soham Chowdhury
@BalarkaSen eh, I read both.
10:22
that's ok. I'm just telling you what Artin has and Aluffi doesn't :P
sure, thanks.
Artin was Grothendieck's colleague, remember that. his algebraic geometry is of a different style.
which Artin? the dad or the son?
Mike Artin
Emil Artin was a number theorist.
hmm, the son.
3 hours ago, by Soham Chowdhury
@Mike, is there any "deeper" analogy between a ring element not being a left/right zero-divisor and the idea of mono/epimorphisms?
I shouldn't have used "analogy", you'll flay me alive.
:P
10:24
I don't even know what's the analogy between the two.
left-/right-cancellative
oh. nah, I don't think so.
shucks. it was a long shot, anyhoo.
are the only cyclic integral domains the $\Bbb{Z}/p\Bbb Z$s?
(sanity check)
You've had your sanity check for today.
what's a cyclic integral domain?
10:29
One per day please :P
if you mean finite charecteristic domain, then sure, that's the only thing.
@BalarkaSen sorry. I meant $\Bbb Z/n\Bbb Z$ when I said "cyclic", for obvious reasons.
@skillpatrol I'm not very sane; I need a bunch of those to get through the day.
@SohamChowdhury then that's obvious
Just kidding pal ;-)
@skillpatrol I might be too.
10:35
True, some days are better than others...
that thing is a domain iff n is prime
there seems to be some sort of duality between zero-nondivisors and units
wanna see why?
10:39
left-zero-nondivisor $\implies x\mapsto ax$ is injective
left-unit $\implies x\mapsto ax$ is surjective
cool, imo.
ok, sure
is there a notation for the group of units of a ring?
$U(R)$
the name's just "group of units"
:P
10:42
i was lucky to edit that quick.
i realised that would happen :P
r9m
r9m
@Chris'ssistheartist sorry ,, I keep forgetting about it .. I'll try it sometime later .. there are a bunch of trouble at hand I have to deal with first ..
@r9m OK
@r9m Does it seem to me or you're too serious compared to the previous days? :-)))
r9m
r9m
@Chris'ssistheartist perhaps .. I'm relatively troubled :P

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