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12:00 AM
STOP.
STOP. STOP.
Symbols are useless now.
 
What are you talking about?
 
If $R$ is a relation on a set $S$, then $RR$ is defined as the set of pairs $(x,y)$ for which, for some $z\in S$; $(x,z),(z,y)\in R$.
The question is, does $(x,y)\in RR$ imply $(x,y)\in R$.
This holds when $R$ is transitive. For $(x,y)\in RR$ means exactly $(x,z),(z,y)\in R$ for some $z$.
 
my book doesn't write it as $RR$ it wrote it as a composite relation
 
Same thing.
 
ok so how do I prove the transitive part
 
12:03 AM
$(x,y)\in R\circ R$. Then by definition $(x,v)\in R$ and $(v,y)\in R$ for some $v\in S$...
 
Since $R$ is transitive, $(x,z),(z,y)\in R$ implies $(x,y)\in R$.
 
and we know R is transitive..
 
This concludes the proof: $(x,y)\in RR\implies (x,y)\in R$ when $R$ is transitive.
 
so I didn't need to expand to go all the way to $(x,z)$?
 
I don't know what you mean by that.
 
12:05 AM
Definition 6.2.3 states that $R$ is transitive if $( \forall x, y,z \in S)((x,y) \in R \land (y,z) \in R) \rightarrow (x,z) \in R)$
my previous homework was prove that R is transitive and the end result became (x,z) which I had no idea how to get there
ugh I'm making this hard for myself... should've subsituted v = z for def 6.3.9 and I could see it right there
$R_2 \circ R_1 =[(x,y) \in S \times S :(\exists z \in S)((x,z) \in R_1 \land (z,y) \in R_2$.
$R \circ R =[(x,y) \in S \times S :(\exists z \in S)((x,z) \in R \land (z,y) \in R$.
it is transitive... seen x y z on here and in the transitive def
 
@seaturtles I guess I have an alternative to Rotman's solution.
 
@usukidoll What exactly are you trying to prove?
and what is the problem you're facing?
 
this
http://math.stackexchange.com/questions/711529/r-is-transitive-if-and-only-if-r-circ-r-subseteq-r
 
Okay. Can you prove $R$ is transitive $\implies R\circ R\subseteq R$?
 
I could try
Definition 6.2.3 states that $R$ is transitive if $( \forall x, y,z \in S)((x,y) \in R \land (y,z) \in R) \rightarrow (x,z) \in R)$
SO for $R$ being transitive implies that $ R \circ R \subseteq R$
 
12:14 AM
Usually to prove $A\subseteq B$, you would assume $x\in A$ then show $x\in B$.
 
Definition 6.3.9 states that we let $R_1$ and $R_2$ be relations on a set $S$. The composition of $R_2$ with $R_1$ is the relation $R_2 \circ R_1 =[(x,y) \in S \times S :(\exists v \in S)((x,v) \in R_1 \land (v,y) \in R_2$.

For this problem :$R \circ R =[(x,y) \in S \times S :(\exists v \in S)((x,v) \in R \land (v,y) \in R$.
Definition 6.2.9 states that we let $R$ be an equivalence relation on a set $S$. For each element $x \in S $ the set $[x]=[y \in S: (x,y) \in R$ is the equivalence class with respect to $R$.
 
In your case, let $(x,y)\in R\circ R$ and then try to prove $(x,y)\in R$.
 
x belongs to $ R \circ R$ and x belongs to $R$ by def 3.1.2
$[x:x \in R \circ R \rightarrow x \in R$
 
Well, if it is 'obvious' from the definition then sure.
 
yesss
now how to put transitivity into the picture because that's just without transitivity
maybe there exists some z for $ R \circ R$!?
$R_2 \circ R_1 =[(x,y) \in S \times S :(\exists z \in S)((x,z) \in R_1 \land (z,y) \in R_2$
 
12:17 AM
But usually, you would go like this: Let $(x,y)\in R\circ R$. Then $(x,z)\in R$ and $(z,y)\in R$ for some $z$. Since $R$ is transitive, $(x,y)\in R$.
And since $(x,y)$ was an arbitrary element of $R\circ R$, $R\circ R \subseteq R$.
 
sigh if only transitivity wasn't involved, I would've gotten it already. you made it look easy
oh now I'm seeing it... but that's for if we're starting at $R \circ R \subseteq R$ which means that there are x elements that belong to $R \circ R$ and $R$. Those sets must have some elements in common...
 
Hello:)
 
Hi!
 
can we simplify this even further such that n is outside? puu.sh/7upAp.png
can we factor the exponent n and make it outside
 
@Prototank What is $\lvert z^6\rvert$ for $\lvert z\rvert = 2$, and what is a bound for $\lvert z^6 - p(z)\rvert$ there?
 
12:32 AM
@DanielFischer Hilfe!
 
@PedroTamaroff Who's bullying you?
 
@DanielFischer Rotman.
He's saying crazy things.
 
@PedroTamaroff Didn't I tell you not to listen to algebraists?
 
@DanielFischer I forgot.
Damn it.
It's Little Red Riding Hood and the Big Bad Wolf all over again.
Consider the following.
 
So, @Pedro, I saw above that he's saying even is odd. Anything else?
 
12:34 AM
Nope.
But I need to find a proof of his claim.
 
Vot Klaim?
 
So, consider the case $t=-1+2^{m-1}$
Oh.
Let $G$ be a group. Let $x,y$ be elements of $G$, with $x$ of order $2^m$; $y^2=x^{2^r}$, $r<m$ (of course?) and $x^y=x^t$.
Then of course $x=x^{y^2}=x^{t^2}$.
So $t^2=1\mod 2^m$ and $t=\pm 1,\pm 1+2^{m-1}$.
Rotman claims that if $t=\pm 1+2^{m-1}$ then $G$ contains at least two involutions.
$x^{2^{m-1}}$ of course is one.
 
@PedroTamaroff $x^y$ was $y^{-1}xy$?
 
@DanielFischer Ja.
No.
$yxy^{-1}$.
Sorry.
$(x^y)^z=x^{zy}$. Tough luck. =P
Yeah.
 
Hey when we say that things like the fourier basis are complete, is that the same kind of completeness as when we talk about a metric space?
 
12:38 AM
@Anthony No.
 
@PedroTamaroff Urk. I'm not particularly fond of the $x^g$ notation anyway, but $x^{yz} = (x^z)^y$ is YUCK!!!
 
@DanielFischer Hehehe, I know.
Will not do it again in your presence.
I swear.
@Anthony An orthogonal system is complete if $\langle x,0\rangle=0$ for every $x$ in the system implies $x=0$.
For example, for continuous functions (or pushing it a little further, Lebesgue integrable functions) over $[0,2\pi]$, $\langle e^{int}:n\in\Bbb Z\rangle$ is complete,.
@DanielFischer So, the point is to look at $x^ky$ for some $k$.
And show that is an involution for an appropriate $k$.
When $t=2^{m-1}+1$, things work out nicely.
But when $t=2^{m-1}-1$; we get wrong claims by Mr. Rotman.
 
@PedroTamaroff Okay, slowly. So $x^kyx^ky = x^kx^{tk}y^2$.
 
$\langle x,0\rangle=0$ for every $x$?
 
@DanielFischer Indeedz.
 
12:42 AM
Shouldn't that always be $0$?
 
@Anthony Sorry, I meant $\langle x,s\rangle =0$ for every $s$ in the system. =)
My bad.
 
phew
 
If that implies $x=0$; then $S$ is complete.
@DanielFischer That gives, $k+tk+2^r=0\mod 2^m$.
 
So I mean, how do we go about making things like the fourier basis?
 
And that's $x^{k(1+t)+ 2^{r}}$.
 
12:43 AM
That's what we want to happen.
@DanielFischer Yiss.
When $t+1=2^{m-1}$, we want $k2^{m-1}+2^r=0\mod 2^m$.
 
It seems like a large, perfect, package, I don't even know how to go about understanding how it came to be, or how we know that it's complete while something like the polynomials (I think?) aren't.
 
@PedroTamaroff Which is fine if $r = m-1$.
If not, not.
 
@DanielFischer Sure.
But if $r$ is smaller we done goofed.
 
I guess you just take it, show they're orthogonal, then show that condition you gave... Just such a large thing.
 
@Anthony Polynomials are not orthogonal.
Well, you need another weight.
 
12:46 AM
I see.
 
But according to Weiertrass, $\langle x^n\rangle$ is complete.
 
:o
 
For continuous functions.
And then thus for every Lebesgue integrable function by density. Over compacts subsets of $\Bbb R$.
 
@PedroTamaroff So let's try $x^ky^{-1}$.
 
@DanielFischer Let's a-see.
 
12:47 AM
Will projecting onto that give you something different than a Taylor expansion?
 
@Anthony Come again?
Oh.
You need different weights, Anthony.
To make the inner product orthogonal.
Else there is no such thing as projections and whatnot.
@DanielFischer That squared looks promising, I guess.
 
$x^ky^{-1}x^ky^{-1} = x^k y^{-2}yx^ky^{-1} = x^k x^{-2^r}x^{tk}= x^{(1+t)k-2^r}$
Nope. Not that.
 
@DanielFischer Doesn't work, right?
Still $2^r$ ruining things.
 
I understand that I need different weights-but in the end, you'll have some expansion onto the basic of $\langle x^n\rangle$ right? Do those coefficients agree with Taylor expansions?
 
You see, $t-1=2^{m-1}$ works because we can divide by $2^r$ and get an odd leading term in $ak+b=0\mod 2^{m-r}$.
 
12:56 AM
finally done with this *** ***
 
@PedroTamaroff?
 
@Anthony Recall that if $f(x)=\sum a_nx^n$ and if $x$ converges in some disk $|x|<r$ then you can show $a_n=f^{(n)}(0)/n!$.
 
Hmm, $y^ax^k = x^{kt^a}y^a$, so $x^ky^ax^ky^a = x^{k(1+t^a)}y^{2a} = x^{k(1+t^a)+ 2^ra}$.
 
@DanielFischer Right, but then $a$ cycles $y$.
So maybe we're just ending up with $x^{2^{m-1}}$ again-.
 
@PedroTamaroff Yes, maybe. the question is whether we can avoid that.
If $a$ is even, then $t^a \equiv 1 \pmod{2^m}$, so we have $2k + 2^ra \equiv 1 \pmod{2^m}$ to get.
 
1:01 AM
$x$ converges, or $f(x)$?
 
Which looks a tad impossible.
So $a$ would have to be odd, and we're back to square one because $t^a \equiv t$ then.
 
And yes! So Taylor series do represent a projection onto the polynomials then? Just maybe not for divergent functions?
 
@DanielFischer This is doomed.
I wonder if there is an errata by Rotman.
@KarlKronenfeld
 
@PedroTamaroff ey
 
What @DanielFischer and I are discussing.
 
1:05 AM
lol, a bunch of symbols?
 
@PedroTamaroff I'm almost sure the singular is erratum.
 
@DanielFischer Well, I just mean a list of corrections, which include this one. =P
 
Ah, a list of errata.
 
@DanielFischer I had to look this one up once. Erratum is the singular, yes, referring to a single error.
How do you count the number of monomials of degree $d$ in $k[x_1,\dots,x_n]$?
 
@KarlKronenfeld By degree you mean...?
 
1:11 AM
Usual definition, sum of powers.
 
So $x_1^2x_2+x_1$ has degree...? =)
 
monomials, bro
 
I hate algebra
:)
 
@KarlKronenfeld Oh.
Derpacola.
 
It's just a combinatorics problem hidden in this setting. I don't know how to translate it though.
 
1:13 AM
@KarlKronenfeld I don't get it. You just mean elements of the form $a x_i^j$ for $a\in k$ and $j>0$?
 
@PedroTamaroff Monomials are typically assumed to be monic.
 
@PedroTamaroff $x_1^3x_2^5x_3x_4^{12}$
 
Oh.
Sorry.
 
I'm thinking we are counting functions from the partitions of $d$ to $[n]$.
 
My bad.
@KarlKronenfeld I agree.
Assing to each $i=1,\ldots,n$ a number $d_i$ such that $\sum d=n$.
 
1:14 AM
But we have to account for some symmetries
 
Then form $x^d=\prod x_i^{d_i}$.
@KarlKronenfeld Example?
We just need to find out what to divide by.
 
It's just that I know the answer, which isn't a power of n
 
Oh, well.
@KarlKronenfeld What's the answer?
You could've started with that! =D
 
@PedroTamaroff What are you saying here?
 
@KarlKronenfeld $d_1+d_2+\cdots +d_n=d$ I meant.
 
1:16 AM
@PedroTamaroff $\binom{n-1+d}{n-1}$
 
That gives a monomial $x_1^{d_1}\cdots x_n^{d_n}$ of degree $d$.
@KarlKronenfeld Aha. Stars and bars! =D
@KarlKronenfeld I learnt it has balls and bins. Your bins are $x_1,\ldots,x_n$. You have $d$ balls.
When you finish, you get a monomial.
Of course the balls are indistinguishable, but the bins are.
 
Yes, right.
Cool thanks
 
1:31 AM
@DanielFischer If you want something a bit more itneresting than the above, that is used to show that if a $p$-group as a unique subgroup of order $p$, it is either cyclic or a generalized quaternion.
Which is quite cool.
 
Aha, @Pedro, why is it cool?
 
@DanielFischer Well, it shows say the importance of involutions? (2-groups).
The case $p$ odd is quite easy though.
The tough work is for $p=2$.
 
As usual. $2$ is an odd prime.
 
Indeed. =)
Rotman says "It is not unusual that the prime $2$ behaves differently than odd primes."
 
Tru dat.
2
 
1:35 AM
Hi, someone has a reference for the foliowing Theorem of Chudnovsky : let $f : I \to \mathbb{R}$ a continuous function defined over a segment $I = [a;b]$ wich does not containing integers. Then there exists a sequence $(P_n)_{n \in \mathbb{N}}$ of polynomials with integer coefficients converge uniformly to $f$ over $I$.
I searched but I did not find demo on the internet
 
1:45 AM
@PedroTamaroff Fancy idea. $G$ has at least two involutions if $t \equiv 1 \pmod{2^{m-1}}$, not if $t \equiv 2^{m-1}\pm 1\pmod{2^m}$.
 
@DanielFischer i use that a lottttt
@PedroTamaroff my advisor in texas told me that group theory is the study of what makes the number 2 special.
@DanielFischer for example the Sylow $p$-subgroups of a Frobenius group have unique elements of order $p$
 
@AlexanderGruber Good. What was a Frobenius group, again?
 
@DanielFischer they're a type of finite group that come up a lot, definitions here
basically you've got a nilpotent group $K$ being acted on by a group $H$ which fixes only the identity, so that $G=K\rtimes H$
(what i just said uses a lot of results and is by no means a definition, but it's the best way to think about them)
so for example you could have $C_q\rtimes C_p$ with $p\mid q-1$, with an appropriate action that doesn't fix anything nontrivial
 
Ignoring the fact that I know basically nothing about group theory, that sounds interesting. How well are Frobenius groups understood, cool open problems?
 
@DanielFischer a lot is known but they come up everywhere so they're part of many open problems
 
1:59 AM
If group theory is the study of what makes the number 2 special,
then calculus is the study of what makes the number 0 special :D
 
my work involves lattices of Frobenius groups (and $2$-Frobenius groups, which are a slight variation)
 
Lattices as in meet and join, or which kind of lattices?
 
@DanielFischer more like formations of them
there's something called a prime graph of a finite group, you take the primes dividing its order as vertices, and put an edge between $p$ and $q$ if there's an element of order $pq$
it turns out that in solvable groups, the nonedges correspond to Frobenius (or $2$-Frobenius) Hall subgroups, so i started orienting the edges of the complement according to who's acting on what
anyway, essentially you end up with a directed graph showing how a bunch of Frobenius groups interact, that is what I mean by lattices/formations
 
Interesting. Thanks. I won't pretend I understand what you do, though.
(I wish I did)
 
@DanielFischer it's a little involved... the idea is actually pretty simple but there are a lot of definitions so it's kind of a long story
 
2:07 AM
@AlexanderGruber And I have barely read the first lines of the introduction.
 
@DanielFischer usually i can explain it pretty well with a chalkboard, like most things involving graphs it's a little obtuse if you can't draw it.
 
@AlexanderGruber Everything can be explained better with chalk and blackboard.
 
2:26 AM
hello folks
 
hello @Mike
 
2:49 AM
@Alexander damn it
Im going to sleep now
 
@PedroTamaroff i will not be for a long time.
@Mike yo
 
yo @alexander
 
@Alex what side of the us are you Near? Ill be visiting NJ and NY on august
 
He's in FL
bottom right
It's here
 
@PedroTamaroff i'm in the south.
i'm from Ohio, which is about 8ish hours driving from NY.
but now I live in FL, which is a long way.
 
2:56 AM
would you be offended if someone sawed off florida @alexander
 
@Mike i would not
we could succeed as an island nation
 
I wonder where florida is headed in the gif.
 
@KarlKronenfeld we're going to war with venezuela
@Mike someone should put the locations of all the chat regulars on a map.
 
@AlexanderGruber 21st century war: the whole population comes to you at a glacial pace. I like it.
@AlexanderGruber Mike's assassins would like that.
 
@AlexanderGruber I'd put mine on there if you made it interactive
I wonder if you could set it up so you could sign in with your SE account
and only you could put it in?
 
3:06 AM
@Mike like on google maps or something?
 
yeah
@PedroTamaroff
 
Howdy all ... I see @Mike made it back safely and is assiduously avoiding his grading
 
I'm tutoring @TedShifrin
 
Chat-tutoring, eh? Talented :)
Hi @Alex
 
3:15 AM
@TedShifrin or is he tutoring on chat?
 
@TedShifrin hi there
 
@robjohn, he purports to be making money whilst so engaged.
 
@TedShifrin I think I sort of agree with the school about the rejected candidate. Her requests showed she does not want to be a teacher. But the school she wants to work at is a teaching school.
 
@TedShifrin oh, then I was wrong
 
I am certainly not chatting when I'm tutoring :)
 
3:17 AM
It's a tough call, @Mike. Even at small teaching-oriented schools there is research pressure. I think t
 
i wonder how hard it'd be to start a private college
 
I thought an offer was an offer, so that bothers me. She should have done negotiating on the phone, would be general consensus. Email is not for negotiating.
Be very, very rich, @Alex
 
@TedShifrin maybe i can meet a few rich people
 
@Ted I also feel uncomfortable about rescinding an offer, but I understand why the school did so - she was showing that she wanted to spend less time teaching and more time researching. This is a noble goal at a school that values research more than teaching, but that school is focused on teaching.
(Also, I only talk here when people aren't asking questions - when they are, they are my focus.)
@AlexanderGruber how do I add my name?
 
@Mike i guess search for your location, add a marker, and change it to your name
i just mussed with it till it worked
 
3:27 AM
I guess I'm bothered that the candidate didn't bring this up during her campus visit, @Mike. On the other hand, I'm skeptical about reading too much in: When I interviewed at universities, faculty were prone to take my expressed devotion to teaching as anti-research, which I certainly was not.
 
@TedShifrin I see what you mean, but at the same time, those are very much that (barring the first two): fewer classes, a pre-tenure sabbatical...
Those are very directly "I want to teach less and research more".
@alexander i'm in
 
@Mike all riiiight, nice
 
Ram
Hi All. How to write a 2 complex in form of vertices. I mean like if X is a 2 complex with ordering on vertices as in page 6 of the link math.unl.edu/~mbrittenham2/classwk/872s07/lecnotes/…
then why A_i = [z, w_i, v_i] not [z, v_i, w_i]
 
we got Karl in there too
 
he dares show his face?
i bet he's trying to trick us. i bet he's really on the opposite side of the world.
so reflect through the center of the earth...
 
3:34 AM
I added a note.
Uncomfortable with giving exact location, so I aimed at a diner in Hell, MI. '
 
yeah, I put mine a bit away from where I actually am
 
@Mike mine's just a nearby intersection
 
i was trying to find the lat/long so i could find the opposite point
 
should we sidebar the map for others?
 
Map of locations of chat regulars.
2
 
3:38 AM
let's include instructions also:
MSE chat dwellers: pin your location (just for fun) [instructions]
12
 
Ram
Hi, could some body please answer my question :(
 
@Ram sorry man, I don't know.
 
Ram
@AlexanderGruber Thanks
 
@Pedro "Punto 3", ya weenie?
 
Yes
Im on my phone it is jot easy
 
3:50 AM
I'm putting your name in
 
Im the south star.
Tehehehe
Im off to sleep now.
 
4:14 AM
@r9m I wont talk to you ever. :Plum
 
@Karl we've both got cool icons.
 
@Mike it's more customizable than you'd think huh
 
yeah
 
@AlexanderGruber How do I add myself in the map?
 
Hit the little pin button below the search bar
To the right of the hand
 
4:26 AM
@Mike Sure not a star, but the next best thing.
 
@Sawarnik add a pin with your location and name it after yourself
 
@Sawarnik You're logged in (with a google account) there, right?
 
Yes, ok.
Look where I am now?
 
i'd prefer that you actually put roughly where you are
why did we lose all our icons? :(
 
what happened to my skull and cross bones?
 
4:30 AM
@Mike idk weird
 
i'm putting mine back in
 
somebody must have been messing with the styles
 
@Mike happy now
 
mine's gone again.
i'm annoyed.
 
4:54 AM
Map instructions: click the pin icon under the search bar, click on your location, then name it after yourself. Alternatively, search for your location, click the pin, click add to map, then rename it.

- it doesn't have to be your exact address, just approximate location
- to change the style of your pin, hover your mouse over your name and click the paint bucket. (Do not change the layer style away from "individual styles"!)
- Please don't mess with other people's pins or add random crap to the map.
 
5:06 AM
$$[h$]$
wow annoying
 
5:25 AM
i want this book so bad
 
@AlexanderGruber Yay, hardcover. I wish all the books I've bought were hardcover.
 
@ccorn i wish it wasn't so expensive
 
Jeez, down from 190
 
one of my friends is falling for an MLM. :/
 
@AlexanderGruber: Is there also a map for chat-nonregulars? :-)
 
5:39 AM
@ccorn haha, go for it
 
@AlexanderGruber That seems to need a Google account. Bummer.
 
@KarlKronenfeld idk, i think i see me on there.
 
The next book I'll buy for casual reading will be amazon.com/Perfect-Rigor-Mathematical-Breakthrough-Century/dp/…
I'm quite fascinated by Grigori Perelman.
 
5:44 AM
Btw, a map of extraterrestrial chat-nonregulars: boallen.com/assets/images/randbitmap_true.png
 
 
I never read fictions anymore. Always non-fiction. I leave "fiction" to my maths :) .
 
then read borges's selected non-fiction
 
Fantastic. I'll look into it.
 
reading math all the time really does deplete one's ability to read fiction
 
5:48 AM
i'm going to start reading more nonfiction this summer.
one book i've been meaning to read is "wittgenstein's mistress".
 
@Mike Way too inexpensive. Can you suggest reasonably pricy alternatives?
 
@Karl What's your minimum?
 
it's hard to move from super dense math texts to reading linearly through a story
 
the solution is to read highly nonlinear postmodern books
 
Fair point.
 
5:52 AM
@Mike That kind of thing would actually be enjoyable to read.
 
@Karl I'm super into that kind of literature, I can recommend some of the best if you want.
 
@Mike Sure
 
do you like poetry?
 
Not really, I have also not given it much of a chance though.
 
@karl thomas pynchon is one of the original big names. "V." is one of his best, though not his easiest, books. "the crying of lot 49" is a bit easier and gives you a taste of his style so you don't read V. and not enjoy it
one of the lighter, less difficult, but still very good and very interesting is david foster wallace's "infinite jest". this book was big for a while.
my favorite postmodern author is william gaddis. "J.R." is an awesome story told in roughly 99% dialogue
 
5:56 AM
Added myself to the google "pin your location".
 
@Mike Cool thanks
 
So far away from you all. ^_^
 
@karl of those i suggest checking out lot 49 or infinte jest first.
 
borges is also good. here's a short story of his that's nice.
 

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