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09:00
s'ok. i think i'm getting there.
i just wish it weren't 5am and the birds weren't chirping
@Jeff ...complex integration at five in the morning? Finals?
Damn them to hell!
no. homework - not mine, mind you, someone else's (who's paying me to tutor them at 10 this morning on this homework).
Haha, excellent!
if it was my homework i would've given up already. but i feel an obligation to someone i committed to help
09:02
:P
This is at least 3rd year stuff, right?
@DavidWallace I do want to see that irl!
@DavidWallace yes
4th year, actually
If you're tutoring 4th years, then I'm not qualified to help you. I don't have a Masters.
i don't have a masters either. but this isn't a very elite school
09:04
...way I see it, anything past $\int \frac{\mathrm dx}{x^4+a^4}$ is messy to do properly, for that family of integrals.
Doable, but messy.
I think there must be a transform involving an inverse tangent. That's the only way I can think of to turn the integration into a closed contour.
Found a superb photo of Piri Weepu.
The polynomial in the denominator has complex roots, so an arctangent will certainly pop up... if you restrict yourself to manipulating reals. If you don't mind complexes, you can use logarithms, but that requires finesse.
@JonasTeuwen We used to have to do them at the local secondary school athletics competition.
@JM Who cares about the value? It is clearly convergent! 8-).
@DavidWallace Excellent!
@JonasTeuwen That's the pure/applied divide, I'm told. ;)
09:15
@JM I have given the students complex analysis a nice exercise on Bessel functions. Sweat they will do!
Afk :-).
dave....dave's not here....
@AlexanderAmenta
Hey
this localisation stuff is so confusing
@BenjaminLim hi
@AlexanderAmenta I have not got anywhere with that contraction map thingy too
it's not so bad
don't stress too much about the contraction map, if you can't get it work on something else
09:22
I'm stressing on localisation and absolutely flat rings
you should stress about some other things too
hahaha
algebra 2
no, more like, what IS localisation
@AlexanderAmenta I have been trying to figure that out myself too
whyy is localisation
09:24
my latest confusion
why would you care about a module over a localisation
Well if you localise at a prime ideal
you get a local ring
why would you do such a thing
and there is this nice result that absolutely flat rings that are local are fields
that's nice
I was just talking to someone today
we used this result to prove a problem in AM
that's all well and good if you want to make a living solving problems from AM
09:26
By the way do you know what is the canonical isomorphism that they are talking about
in the link up there?
i can't remember, i'd have to think about it
it would come from the map A -> S^{-1}A
but it doesn't shed very much light on why you bother localising
hmmm
i am trying to figure that out too
this is why you do comm alg alongside other things ;)
09:30
Yes, like algebraic geometry. :p
hey @ZhenLin
Do you know what is the canonical morphism that georges is talking about here: math.stackexchange.com/a/139815/5783
@ZhenLin there's always complex geometry too
@ZhenLin and number theory
@ZhenLin and topology...
Those rings are not so nicely behaved. And algebraic number theory is basically algebraic geometry.
'algebraic number theory is basically algebraic geometry' - not quite but there are many similar ideas and overlaps
(speaking as neither a number theorist nor a geometer. nor an algebraist)
I'm submitting an essay later today, "étale cohomology and Galois representations".
To a number theorist.
09:32
because you're dealing with the overlaps =)
Algebraic number theory is basically algebraic geometry.
why not 'algebraic geometry is basically algebriac number theory'
Because algebraic geometry is strictly more general. :p
how are you defining algebraic geometry?
I don't; that's a futile thing to do.
09:34
then don't speak of set containment
3
Then don't speak of overlap!
haha good point
then either both our arguments work, or both fail
@ZhenLin I don't get the so called canonical morphism that georges was talking about
given that we're contradicting each other, we're both failing...
@BenjaminLim: Recall the universal property of tensor products.
09:36
he's claiming that they are isomorphic as $A$ - modules no?
Yes. And as $S^{-1} A$ modules.
Ok so let's tackle them first as $A$ - modules
wait would canonical morphism be just the identity on elementary tensors?
Yes.
Canonical things are always trivial things like that.
then the isomorphism....is just like obvious no??
@ZhenLin I don't get the difference then between the $A$ - modules $M' \otimes_A M$ and $M' \otimes_{S^{-1}A} M$
Well, if it's obvious to you, good!
09:41
Really I don't see the difference between the two then!
But just to check that you understand correctly: are $\mathbb{C} \otimes_\mathbb{Q} \mathbb{C}$ and $\mathbb{C} \otimes_\mathbb{R} \mathbb{C}$ isomorphic as $\mathbb{C}$-modules?
hmmmm
I don't know the difference between the two
Well for the second I think we have that as a $\Bbb{C}$ - module is isomorphic to $\Bbb{C}^2$
Correct.
For the first one.....
For the first one though.....
@ZhenLin I am not so sure...
It's a $\mathbb{C}$-vector space of uncountable dimension.
09:45
hmmmm
how?
It's a property of tensor products of vector spaces. $M \otimes_K N$ has the dimension $(\dim_K M)(\dim_K N)$ over $K$.
Ah ok
so $\Bbb{C}$ as a vector space over $\Bbb{Q}$ is infinite dimensional
yes
And for change-of-base, $L \otimes_K N$ has dimension $\dim_K N$ over $L$
countably infinite though
wait, is it?
09:47
@AlexanderAmenta: No. A vector space of countable dimension over $\mathbb{Q}$ is countable (in cardinality).
ah yep, got it
my bad
@ZhenLin This base change thingy is quite confusing
Well, base change by a localisation (i.e. localisation of modules) is particularly confusing because it has some weird properties not shared by general base change.
But $\mathbb{R}$ is not a localisation of $\mathbb{Q}$.
The first property to take note of is that $R[S^{-1}] \otimes_R (-)$ is an idempotent operation (up to isomorphism).
Can this be simplified more: $$\displaystyle 2 \pi i \frac{1}{(\omega^1-1)(\omega^2-1)(\omega^3-1)(\omega^4-1)}$$, where $\displaystyle\omega=e^{\frac{2 \pi i}{5}}$
@ZhenLin I guess with experience I'll learn from it then.....
09:53
@Brian May I ask you for a reference help?
So back to your original question: $M \otimes_R N$ and $M \otimes_{R[S^-1]} N$ are isomorphic if $M$ and $N$ are $R[S^{-1}]$ modules, but this is something special to localisation.
I may not be able to help, but you can ask.
Michael Aschbacher wrote his thesis at Madison, Winconsin on the Collineation groups of Symmetric designs under the supervision of Bruck.
@BrianMScott You can ask your questions, which I may choose not to answer.
09:55
@Gigili If that was aimed at Kannappan, enough of this squabbling please.
@ZhenLin what?????????????????????
@BenjaminLim I have him ignored and never talked to him. Please you stop.
A soft copy of this is available at the Library of Univ. of Madison, Winconsin. I remember that you also have been at Madison. So, I thought you might be able to help me. @Brian
I see you feel like you are a moderator already. Very nice.
@ZhenLin So this is not always true?
09:57
Well, yeah, wasn't that the point of my example with $\mathbb{C} \otimes_\mathbb{Q} \mathbb{C}$?
@Gigili I'm sorry. I did not mean to offend anyone, just trying to be a peacemaker.
@ZhenLin Yeah so it is not so trivial to see that isomorphism that Georges pointed out anyway.
@KannappanSampath If the soft copy isn't generally available, I've no special access, I'm afraid.
@Ben AFAIK, I consider it worthless to be here when the user (abuser goes here) Gigili is here. But, I wanted to ask for some help from Brian. Thank you for trying to put things together. I am afraid, they won't go that well with the abusers.
@BrianMScott :( OK. Anyway, Thank you Brian. The copy is not available. It is available only to people at Madison.
Sorry for popping in, but does anyone know a good source for reading about how to bound series by using integrals? There is only a short section in my book about it, and alas I can not quite grasp it. It is titled "Using Integral Bounds to Estimate the Sum of Series"
10:00
@ZhenLin How does your example fit in with mine?
@BenjaminLim It's okay. Thank you. I'd leave it completely since he insulted me in every possible way.
@BenjaminLim: Here's one proof. I leave the details to you. $$M \otimes_R N \cong M \otimes_R (R[S^{-1}] \otimes_{R[S^{-1}]} N) \cong (M \otimes_R (R[S^{-1}]) \otimes_{R[S^{-1}]} N \cong M \otimes_{R[S^{-1}]} N $$
@ZhenLin Ok let me try to give an example of why this fails in general
... I already gave you an example.
If I ask whether we have an $\Bbb{R}$ - module isomorphism
$\Bbb{C} \otimes_{\Bbb{Q}} \Bbb{C} \cong \Bbb{C} \otimes_{\Bbb{R}} \Bbb{C}$
then the right hand side has dimension $4$
as a vector space over $\Bbb{R}$
But then the left is infinite dimensional because we have vector spaces over $\Bbb{Q}$. Correct?
10:03
Yes... but you really should be comparing dimensions over $\mathbb{C}$.
ok.
By the way all your isomorphisms above were $S^{-1}R$ isomorphisms ?
Yes.
Thanks Zhen for your hint, I think I can do it.
The hard part is establishing the isomorphism in the middle
the one on the left and right I already know
@Ben I have one thing to point out: You may want to add your time zone to the post (it is now a comment.); JM had posted that as a comment in the meta thread. Good Luck. :)
Actually, this whole thing is more-or-less answered here
10:10
@ZhenLin Ah that's right
I had a similar confusion as to why the tensor products should associate over different rings
Anyway, I should leave guys. Bye @Ben and @Brian.
bye kannappan
@KannappanSampath Take it easy!
 
1 hour later…
11:22
@tb: has anybody seen you here?
LaTeX question: Anyone know how to wrap \left( and \right) around lines with newlines (\) in align statements?
11:38
can you tell me (if, by 'yes', you meant that you know :D )?
11:54
@Jeff you make judicious insertions of \right. and \left. within your expression.
@JM but does that associate the sizes of the \left( and \right)?
aah. those don't appear, but are placeholders. thx
Looks more like the time I'd like to hang out: Hi @robjohn and others.
@robjohn well, if the first line of the align environment is shorter in height than the last line... things look unbalanced.
Otherwise I don't know of a better way to split expressions with \left and \right.
@JM yes, that is what I would expect, but not what I would want. Judicious use of \vphantom can fix that :-)
11:58
Ah... right, that's what it's for. :D
@robjohn since you helped so much, would you to see how i completed that problem? (i ended up not using the partial fractions - and I don't think i have the right answer)
@Jeff where is it?
i would have to send it to you (pdf file).
@robjohn or i could post it to a file server (but you will have to put up with tons of popups to get it).
@Jeff That would be preferable, but I am on Skype
12:07
which one is preferable, and how would i find you on skype without posting your address publicly somewhere (also, i'm a skype neophyte)
Highlight: Assuming you accept this paper, we would also like to add a footnote acknowledging your help with this manuscript and to point out that we liked the paper much better the way we originally submitted it, but you held the editorial shotgun to our heads and forced us to chop, reshu‚e, hedge, expand, shorten, and in general convert a meaty paper into stir-fried vegetables. We could not ± or would not ± have done it without your input
If @robjohn (or anyone) is interested (and can put up with scads of popups), the question is posted here: mediafire.com/file/j4b88d5fsar2vra/HW04_q1.pdf and my hastily typed attempt at answering is here: mediafire.com/file/j4b88d5fsar2vra/HW04_q1.pdf.
@Ilya wow...
@Jeff Some brackets are missing in the second of your links
(I have gone crazy, I am sorry.)
this link should work, too (for my answer): mediafire.com/view/?j4b88d5fsar2vra
12:17
@Jeff The question link seems to be the same as the answer link.
At New York’s Kennedy Airport today, an individual was arrested trying to board a flight while in possession of a ruler, a protractor, a setsquare, a slide rule, and a calculator. At a morning press conference, Attorney General John Ashcroft said he believes the man is a member of the notorious al-gebra movement. He is being charged by the FBI with carrying weapons of math instruction.
“Al-gebra is a fearsome cult,” Ashcroft said. “They desire average solutions by means and extremes, and sometimes go off on tangents in a search of absolute value. They use secret code names like ‘x’ and ‘y’
@rob Question (correct one): mediafire.com/file/3ty0m9ttt4uqo9z/HW04.pdf
sorry 'bout that (i think i paraphrased the qeustion in the answer, anyway)
@Jeff now that I look at it as a Complex Variable class, I would suggest contour integration...
is that different from what i did?
i didn't end up using that partial fraction decomp
12:28
@rob can you tutor or teach and so forth over skype? if yes, how do you get paid?
@Jeff Well. I'd note that $z^5+a^5$ along the path from $0$ to $\infty$ equals $z^5+a^5$ along the path from $0$ to $e^{i2\pi/5}\infty$, if you see what I mean.
@Jeff I am sure that people do, but I don't know how they arrange the payments. However, Skype doesn't have MathJax as far as I know.
@rob whant you say "path along 0 to $\infty$", you're talking about the real axis
@Jeff yes
and you say that equals the integral along the path from 0 to 2/10s of halfway around the circle (i assume you mean along the circle).
i have a so-so understanding of integrating along a path (i never took vector calc). can you tell me how that works out?
...fairly quickly (i have to jump in the shower)
<-- afk. actually, gonna jump in shower now and see any answers when i get out
12:59
Are people seeing a different Gravatar?
now yes
Thank you for confirming @Ilya.
Hello!

What do 2 linear transformations have to have identical to be equal?
Kernel, image and the same formula?

thank you!
same formula is sufficient
13:02
@robjohn Yes, precisely that @robjohn. :)
@Chris just kernel and image is not sufficient: $A$ and $\lambda A$ have the same kernels and images
@Chris i think the same eigenvalues/vectors (normalized eigenvectors, that is).
@Ilya not based on eigenvectors?
@Chris Two linear transformations are the same if they agree with how they deal with the basis of the domain space.
@Jeff I don't know what is meant by the formula in OP, but certainly in most cases the same formulas define the same objects
Let us consider transformation $T: V \to W$ where $V$ and $W$ are vector spaces over field $Bbb F$.
13:05
@Ilya do you (and @chris) mean the characteristic polynomial when you say formula?
@Jeff I meant different. I thought in OP it was about a matrix representation in a fixed basis, smth like @Kan mentioned below
@Jeff No, I'd think by formula Chris means something like: $T(x)=\text{blah}$
oh.... the 'blah' operator :D
Hello!
13:06
Hi @ymar.
@KannappanSampath this is right. i recall that from LA class.
@Ilya: So if 2 linear maps, have the same image, they don't have to be equal, right?

@Jeff: We haven't worked with eigenvectors until now :S

Is there any theorem I can use? (any source, like wikipedia, where I can study sth related to my question?)

@KannappanSampath: When you say BbbF?

About the formula I mean sth like the $T(x)=\text{blah}$ :P
OK, That was an unfortunate typo: $\Bbb F$.
No, I'd not suggest looking into books about this. Most book are crappy when it comes to explaining finer details. You must think slowly.
@chris you don't e-vals/vects. use @Kan's description. You are familiar with the basis of the vector space, right?
@Chris Kan says BbbF when he is frustrated :)
13:12
@KannappanSampath: Then, is there any entry in wikipedia that I can use as source?

@Jeff: Yes, I'm familiar with the basis of the vector space
OK. @Chris. Forget Wikipedia. Let's think slowly.
2
Let's consider $T$ a linear map between two vector spaces $V$ and $W$ over the same field $\Bbb F$.
Now, you know that every vector $v \in V$ can be written uniquely as a linear combination of basis vectors.
Let's designate the ordered basis $\langle e_i \rangle_{i=1}^n$ for $V$.
So, let's take a vector $v \in V$: Then, $$v=a_1e_1+a_2e_2+\cdots+a_ne_n$$ where $a_i \in \Bbb F$.
@KannappanSampath you're assuming finite dimension here, which is unnecessary.
@ymar Hah, so, I'd let $n$ be possibly infinite?
@KannappanSampath I think I just solved a problem in AM
Thought about it the whole damn day
then just as I was about to go to bed
@Jeff: you back?
13:18
boom
I could not believe my eyes!!!
@KannappanSampath: Ok then, let's..
Yes I know that about the vector $ v \in V $.
Ok then, I understood everything up to here.
@BenjaminLim What happened? (I am happy to hear that a problem that took a whole day has finally been solved. Congrats, Ben.)
This is a problem on proving that if every prime ideal of $A$ is maximal
then the nilradical is absolutely flat
@KannappanSampath If you want. :) Perhaps proceeding in finite dimension would be better for Chris to understand first, but it's not necessary to prove what you want to prove.
@ymar But, thank you for bringing this to my notice. I'll proceed with finite dimension, OK?
13:21
sure :)
@BenjaminLim I see, I'll get back to you on this, I'll just tell Chris a few more things so he'd do good. :)
user19161
@kan I see your new avatar! I have one too!
@Chris So, you agree that $v$ can be written uniquely in such a fashion. Now,
@JasperLoy :)
@KannappanSampath Wow the problem took sooooo long....
.....
Sorry, we don't need that.
Now, how does $T(v)$ look? @Chris
(Hint: $T$ is linear!)
13:25
@KannappanSampath: When you say "look" you mean the formula of T? i.e.: T(v) = ...?
Or, what is the best you can say about $T(v)$ from the fact that $T$ is linear and $v=\sum_{i=1}^n a_i e_i$?
@BenjaminLim One full day is surely long! But, the happiness knows no bound, you know! :)
@Jeff: I have to leave for a while. Here is my answer.
Considering [this path](https://i.sstatic.net/mj8RA.png)
$$
\begin{align}
\int_\gamma\frac{\mathrm{d}z}{z^5+a^5}
&=\left(1-e^{2\pi i/5}\right)\int_0^\infty\frac{\mathrm{d}x}{x^5+a^5}\\
&=2\pi i\text{ Res}\left(\frac{1}{z^5+a^5},e^{\pi i/5}\right)\\
&=2\pi i\lim_{z\to ae^{\pi i/5}}\frac{z-ae^{\pi i/5}}{z^5+a^5}\\
&=2\pi i\lim_{z\to ae^{\pi i/5}}\frac{1}{5z^4}\\
&=\frac{2\pi i}{5a^4e^{4\pi i/5}}
\end{align}
$$
Therefore,
$$
\begin{align}
\int_0^\infty\frac{\mathrm{d}x}{x^5+a^5}
Was it for Jeff @robjohn?
@KannappanSampath indeed :-)
13:30
@KannappanSampath bloody hell man
I was thinking the whole damn day
got caught up in other problems too
and then just as I was about to sleep
Oh, I am not sure :-/
That its equivalent matrix is a diagonal matrix whith elements $ a_i $ where i is the number of the row.
No, I'd say that is good; one of the qualities my mentor would want me to have! @Ben
boom
@KannappanSampath What are you talking about
@KannappanSampath The whole day I was frustrated man
and then all of a sudden
Could not get anywhere
@Chris No, OK. What is $T(v)=T(\sum_{i=1}^n a_i e_i)=?$
13:32
@Ben :-D I had an experience like this only once.
hahahahahah @ymar
But I was so confused about other things too
@Ben I was saying that: My mentor would like me to think about one single problem even for months!
concerning tensor products as well
@KannappanSampath Yeah that is a good quality to have
I love how you said "boom" three times already!
@Ben Would you like a nice trick in CA?
I learned it like yesterday.
@KannappanSampath: Sorry, but again, you mean the formula?
Or that the $T(v)=T(\sum_{i=1}^n a_i e_i)= u \in W $ ?
13:36
Right, my question is, what is that $u$? You know $T$ is linear right? And, $a_i \in \Bbb F$. @Chris
Recall that $T$ is linear if $T(av+bw)=aT(v)+bT(w)$ for $a, b \in \Bbb{F}$ and $v, w \in V$
@ymar yeah ok
@BenjaminLim cool, let's go to the CA room then
$u \in W$ is the image of $v \in V$ right?
Yes, sure, it is.
@Chris So shall I tell you what I am interested to see you tell me?
Missed that^.
13:45
@KannappanSampath: Yes, because I don't think I will answer properly :-/
OK.
No worries.
$\displaystyle T(\sum_{i=1}^na_ie_i)=\sum_{i=1}^n a_iT(e_i)$
@Chris, See that ^ to see if you agree?
leo
leo
@KannappanSampath :-)
Feeling stupid now... :-/ yes, totally agree! Since T is a linear transformation!
@KannappanSampath: Forgot to tag you :P Then where do we end up?
13:53
@JM speechless
@Chris Now, $v$ was a arbitrary vector in $V$.
And, you described its image completely in terms of the images of the basis vectors, namely $T(e_i)$
So, $T$ is completely determined by what it does to the basis vectors, right?
@KannappanSampath: Ok, it's determined - we don't know what it does precisely to the basis vectors - but we have described it, so yes!
So, now we have proved the following "Theorem" if you want to put it that way:

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