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12:20 AM
@LeakyNun Thought of an easy task, if you're in the mood anytime soon: help mop up rings with cardinality still unspecified: ringtheory.herokuapp.com/dimensions/… if possible, we can assume more to make sure they're countable.
If you just shoved a list of things that could be countable into a suggestion and things that could be determined to be continuum cardinality into another, that'd be great :)
 
12:36 AM
@rschwieb "if possible, we can assume more to make sure they're countable"?
Also who uses $\Bbb Q_8$ for the quaternion group?
@rschwieb what to do with this?
also 24
also 41
19 is non-trivially excluded
 
 
6 hours later…
7:11 AM
@rschwieb You could use $\mathfrak c$ for the cardinality of $\Bbb R$, it's more standard than $c$ (or maybe even $2^{\aleph_0}$)
 
@AlessandroCodenotti given a set $X$ in a topological space $(S,\tau)$, $\overline X$ consists of exactly the points $x \in S$ for which every continuous function $S \to T$ that sends $X$ to a constant also sends $x$ to the same constant
 
Is this a question or an assertion?
 
$f: S \to T$ are noninjective continuous functions that sent $X$ to a constant $c_f$?
Thus $\overline{X} = X \cup \bigcup_{f} \{c_f\}$?
 
7:39 AM
@AlessandroCodenotti a question
 
That's false in general then I'd say, any function with trivial codomain is continuous
If $T$ is hausdorff then any function which sends $X$ to a constant must send $\overline X$ to the same constant, but non hausdorff codomains are problematic I think
 
The codomain of $f|_X$ is trivial, but $f$ itself may not be
 
@AlessandroCodenotti interesting
aha, I need $T$ to be T1
or rather, consider the continuous functions from $X$ to two points with the Sierpinski topology. $\overline X$ consists of the points $x \in S$ for which every function considered that sends $X$ to the closed point also sends $x$ to the closed point
 
Hmm...
$\forall f : S \to T$. $f(X) = c_f \to ? f(S)=c_f$
$f(S-X)=c_f$?
$f(c_f)=?$
wait a minute, the requirement for $\overline{X}$ means this is an iff not just an if
So we are dealing with all those $f$s such that $f(S)=c_f$ thus the codomain is trivial
$f(c_f)=c_f$
If $\tau$ is non hausedoff, then there could be more than one point that get mapped
 
8:26 AM
Does it have to?
f can be such that $f(X) = 0$ yet for some $x \in S-X$, $f(x) = 1$ or $f(x) =0$
Then $f^{\leftarrow}(1)$ can be open, closed or neither
and there are also $f^{\leftarrow}(0)$ that can be open, closed or neither
 
Note that $f^{-1}(\{0\})$ and $f^{-1}(\{1\})$ partition $S$ and the latter is open.
 
ah, so $f^{\leftarrow}(\{0\})=X$ is closed by continuity of $f$
and thus we have $\overline{X}=X$ in this case
Something unrelated:
Consider the following topology (NB have not drew all the open sets as otherwise the whole figure will become very cluttered, it is to be understood that all open sets are given by finite intersections and unions of what is being drawn here)
What I noticed is that the more number of open sets a point is being enclosed in, the "harder" it is to converge to if it is a limit point since there are less possible nets that can have said limit
So the point at the furthest left will involve mostly nets that are eventually constant
whereas points that will converge to the topmost point, said net can have more variations in the middle
This also seemed to fit quite well with the intuition on large vs small topology
So the trivial topology is one where everything converges to everything, and the discrete topology is one where the only convergent nets are the eventually constant ones
I wonder, whether there is a notion of "difficulty of convergence" can be defined for each point by the cardinality of all nets that converge to set point (probably not useful if the underlying set is infinite, because then every such set of convergent nets will be of infinite cardinality for each point)
[Random]
But can some generalisation of topology forgets even the details of the limit points, and instead can only answer the question of "Given a set X, is its closure itself" with yes/no (meaning that the set of limit points is either empty or nonempty)
The purpose of wanting this kind of generalisation is it might provide me a framework for exploring this philosophical thing:
in The Symposium, yesterday, by Secret
The destination of the Path of The Unknown cannot be planned in advance, nor it is possible to plot, but the destination is reachable in principle, as what used to be part of the unknown, with understanding, becomes part of the known
Which to give a simple example on what this means: Recall how when one is engaging in artistic activity, one does not always have an idea what the product looks like, but there is always going to be a product regardless on what happens in the process of making the artwork
Thus in a sense, the outcome of the artistic activity is unknown, until the artwork is finished. The best we can say is that the product exists, but we cannot specify further what the product will be
I think in order to get a formalisation of this idea, I will need to apply some forgetful functor on the category Top such that it forgets the details of the limit points, and instead reduce that into a question on whether limit points exists given a set
 
9:28 AM
It's very impolite to call someone a goldfish — mathrookie 7 mins ago
is "goldfish" the new insult?
 
The word twat is widely used as a derogatory epithet, especially in British English, referring to a person considered obnoxious or stupid. It is also used informally as a verb in British English to mean "to hit or punch a person". In British English and Commonwealth English, it is pronounced to rhyme with that, or sometimes , to rhyme with hot. In North American English, it is pronounced , to rhyme with squat. Twat is also used in British English as vulgar slang for the vulva or female genitals in general. == Historical usage == Robert Browning famously misused the term in his 1841 poem "Pippa...
lol wut?
 
lol wut
 
 
1 hour later…
10:45 AM
when is (-1)^p real? Is it only when p is even and at least 0?
 
11:17 AM
What about when $p = \frac{1}{2}i$ for example?
And what do you think it's when $p$ is odd.
 
I got it in the end :)
thanks all
 
11:52 AM
@LeakyNun, I can't think how $A^n$ can be expressed as a multiple of $Ae_1$ here.
 
12:10 PM
@Silent $Ae_i \in \operatorname{span}(e_1, e_2, \cdots, e_{i-1})$
so $A^2 e_i \in \operatorname{span}(e_1, e_2, \cdots, e_{i-2})$
proceed by induction
$A^{i-1} e_i \in \operatorname{span}(e_1)$
 
Thank you very much!
 
 
4 hours later…
4:13 PM
it isn't getting any upvotes
hi @LeakyNun
 
If $1 \to G_1 \to G_2 \to \cdots \to G_n \to 1$ is exact then $\prod_{i=1}^n \lvert G_i\rvert^{(-1)^i} = 1$; I think I can show this in the case where $i = 3$ (i.e. a short exact sequence) because if such a sequence is exact then $G_2 \cong G_1 \rtimes G_3$ (right?) I guess I can just extend this inductively to prove the full statement?
 
4:29 PM
@ÍgjøgnumMeg split the long exact sequence
that's the standard trick to go from short exact sequence to long exact sequence
 
4:51 PM
Whats a good introductory textbook about Network theory?
 
 
1 hour later…
5:55 PM
Is there such a thing as taking a graph with edges and nodes and rotating it in discrete steps?
to form a new graph
that's symmetrical
 
 
2 hours later…
8:03 PM
graphs have no notion of angles, much less of rotations
 
well then
did you see my question? i'll attach it here
@mercio
0
Q: Rotating a planar graph

geocalc33While reading about graph theory I wondered if there is a process of rotating graphs in graph theory. The image I found on google images is a perfect example of what I am looking for. Imagine every intersection as a node at all times, and every line as an edge. As you can see the graph is duplica...

0
Q: Rotating a planar graph

geocalc33While reading about graph theory I wondered if there is a process of rotating graphs in graph theory. The image I found on google images is a perfect example of what I am looking for. Imagine every intersection as a node at all times, and every line as an edge. As you can see the graph is duplica...

 
If $R$ is a PID and $a \in R$ nonzero, how does one show that $R/(a)$ is torsion free as a left $R-module?
 
8:21 PM
@user193319 do you mean not torsion free?
The statement as you wrote it is not true
 
Oh, whoops. I misread my book. It claims that $R/(a)$ is a torsion module.
 
Ah, that makes more sense
 
But what exactly does that mean? I can't seem to find the definition in my book.
 
Suppse that A is an integral domain for simplicity, then an element m in an A-module is called torsion if it is annihilated by sone non-zero scalar, i.e. am=0 for some non-zero a
A module is called torsion if every element is torsion
 
Ah, and $a$ annihilates every element in $R/(a)$, right?
 
8:27 PM
Right
 
Ah, thank you very much.
 
@Mathein hallo!
 
@ÍgjøgnumMeg hi
 
@Mathein Ich studier doch nicht in Deutschland; anscheinend hat man in Heidelberg keine bewerbung von mir gekriegt und die uni in Frankfurt hat mir gesagt ich habe die frist verpasst für so ne deutschprüfung :(
 
8:54 PM
@ÍgjøgnumMeg Was? Oh Mist
 
@Mathein jo :/ Tu halt in England studieren <.<
 
Das wird wieder teuer, oder?
 
ja, 7695 pfund kostet das masterstudium und man kriegt dafür 10609
also muss ich mit so 3000 pfund klar kommen
hahaha
 
(Also in Heidelberg oder Frankfurt zu leben ist jetzt auch nicht gerade günstig)
Aber blöd, dass es wegen so Formalien nicht geklappt hat
 
ja gut aber wenigstens muss man keine lunge verkaufen
ja :/
naja selber schuld dass ich mich nicht früher organisiert habe
lol
 
8:57 PM
Kannst du in Frankfurt nicht die Deutschprüfung noch nachreichen?
 
nein ich hab angerufen
die haben gesagt die frist sei am 15. juni gewesen
und man kanns nicht nachreichen
 
So was blödes. Wenn man in HD den Bachelor macht, wird man einfach, wenn man das Zeugnis abholt und die Noten gut genug sind, gefragt, ob man auch den Master hier machen will
 
:/
naja wurst
machs halt in Nottingham lol, da gibts voll die gute gruppe
 
Okay klingt auch gut
Hatte mich schon gefreut mit dir bei einem Fleischkäsebrötchen über Zahlentheorie zu reden :/
Aber wie kann das sein, dass die in HD deine Bewerbung einfach nicht gekriegt haben?
Also was du machen könntest wäre halt ein Semester in Nottingham zu studieren und dann nach HD zu gehen und dir die Sachen anrechnen zu lassen
Ist halt die Frage, ob du das willst
Ach nein, das ginge wahrscheinlich nur zum Wintersemester, also wäre gerade so mitten drin
Das ist dann natürlich blöd
 
9:15 PM
jaaa wär schon nice gewesen! Aber deutsche bürokratie halt... lol
Die forschen Iwasawa Theorie und "höhere klassenkörpertheorie" in Nottingham
hört sich interessant an
 
10:04 PM
Ah cool
@ÍgjøgnumMeg Ist Fesenko nicht in Nottingham?
 
Genau
hahaha
 
Den kennt man auf jeden Fall
 
sicher
 
Der hat ja höhere Klassenkörpertheorie maßgeblich mitentwickelt
 
Anscheinend schon
obwohl ich mich überhaupt nicht auskenne
 
10:06 PM
Ich weiß nur ganz grob worum es geht
Aber bald kannst du dann ja sicher mehr dazu erzählen :)
 
Ich weiß nur worum Iwasawa Theorie geht weils was mit FLT zu tun hat hahaha
 
Ein Prof hier in HD ist einer der führenden Experten in nicht-kommutativer Iwasawa-Theorie
 
Ha nice, ich find es hört sich extrem interessant an
 
Ja auf jeden Fall
ich bin gerade noch bei so banalen Sachen wie Klassenkörpertheorie
 
hahaha ich bin nichtmal dabei
naja in Nottingham werd ich algebraische zahlentheorie nehmen
aber
das wird so
Dedekindringerweiterungen und so sein
 
10:11 PM
Ah okay, das hatten wie letzes Semester
 
Ja das hab ich dieses jahr für die bachelorarbeit gelernt
hab dann halt mehr zeit für andere themen
lol
 
dieses Semester ist lokale Klassenkörpertheorie, damit sind wir jetzt auch fertig, jetzt haben wir globale angefangen, aber da werden wir erst nächstes Semester fertig
 
nice, viel spaß dabei
Ich mach
errr
 
Algebraische Zahlentheorie, "Höhere Zahlentheorie", Komplexe Analysis, Algebraische Geometrie, "Further Topics on Rings and Modules"
und
noch ein modul
lol
dessen namen ich vergessen hab
 
10:13 PM
das klingt ziemlich cool
 
Höhere Zahlentheorie heißt Modulformen
hahaha
 
aber was isr höhere Zahlentheorie?
 
ah genau, Elliptische kurven gibts auch
 
Modulformen mach ich auch gerade :)
 
Dann kann ich dich fragen wenn ich was nicht verstehe ;D
lol
 
10:15 PM
Wow, das sind echt viele coole Themen. Klingt, als wärst du in Nottingham auch sehr gut aufgehobeb
 
hoffe ich hahaha
 
ja, frag mich gern zu Modulformen, ich kann versuchen zu helfen, aber ich kenn mich da nicht so gut aus wie mit Algebra oder so
 
Ich glaub man macht im ersten semester AG/RingsAndModules/KomplexeAnalysis
und dann im 2en semester die anderen moduln
würd sinn machen
 
Wird das jedes Semester alles angeboten?
 
glaub schon
das geht aber nur 1 jahr
 
10:18 PM
also du solltest auf jeden Fall komplexe Analysis vor Modulformen machen
und AG vor elliptische Kurven würde auch Sinn machen
 
und man schreibt im 2en semester die Masterarbeit
Ja hab ich mir auch gedacht
 
halt im sommer nach dem 2en semester schreibt man die masterarbeit
hahaha
 
im 2ten Semester? Krass
1 Jahr ist echt kurz irgendwie
 
ja ich finds auch brutal kurz
 
10:33 PM
Oh, look, it's @Mathein and @ÍgjøgnumMeg
 
Wus gooood
@Ted hey
 
Heya @Ted, chat
 
Heya @Fargle
 
Hi @Ted
I've asked my advisor for a recommendation letter for a free EMS membership (that's the EU equivalent of the AMS) and wow... I didn't expect a recommendation letter like that
 
I haven't read it, obviously, Mathein, but you know from what I've said that I wouldn't be surprised.
 
10:45 PM
I have to make sure to thank him properly
 
Most of us consider writing recommendations (competently) an important part of our jobs. Some of my erstwhile colleagues couldn't be bothered to spend more than a few minutes at it. That always made me furious.
2
 
"In all my interactions with him, he displayed a deep understanding of the material. He is curious about the concepts behind the material discussed, he does independent reading and he often came up with surprising and creative solutions to homework problems. He frequently asks interesting questions in class that go much beyond what has been covered. [MatheinBoulomenos] is an excellent student in mathematics, certainly among the strongest that I ever had in my classes."
 
Oh, that seems quite factual. Less over the top than you realize. But I'm 100% sure he's being totally honest.
And it seems quite right to me, based on our interactions here.
 
Thanks :) it makes me glad and proud
 
I've commented numerous times about how some of the students who've hung out here are quite exceptional. Pedro has disappeared, but he was, quite, too. Now in second year of grad school in Dublin.
 
10:52 PM
@LeakyNun It might just have been a reflex. You're saying $Q_8$ is enough?
@AlessandroCodenotti I don't understand. I believe I've used \mathfrak c everywhere. If you spotted a place where I didn't, please link me and I will fix it.
 
$\Bbb Q$ is reserved for rationals, @rschwieb. I, personally, liked to call the quaternion group $\mathscr Q$. :)
well, actually, a different script font.
 
@LeakyNun For this one, I think letting $k$ be countable, the whole thing is countable, right?
@TedShifrin yeah, but $\Bbb Q_8$ isn't reserved for anything as far as I know.
 
No, but it has nothing to do with the rational numbers, hence no Bbb.
$\Bbb Q_p$, on the other hand ...
 
8-adic numbers?!
 
There you go :P
 
10:56 PM
lol
 
Maybe so! I wouldn't know.. i never work with $p$-adics..
 
@rschwieb In number theory it's used for the unique unramified extension of $\Bbb Q_2$ of degree 3 (nobody uses 8-adic numbers, lol)
 
maybe someone missed a trick with composite-adics
 
Mathein just explained the notation. I don't recall seeing it, I admit.
 
not sure how standard this is, but I've seen it from two different lecturers
 
10:58 PM
I switched it to mathcal
 
Hey everyone!
 
@TedShifrin I've seen $\mathbb H$ used for things that aren't the quaternions, so I don't quite share the sense of notational absolutes :/ But anyhow, I'm happy to make it something better than $\mathbb Q$.
 
The notation is justified since the ring if integers in $\Bbb Q_8$ are the Witt vectors for $\Bbb F_8$, so it relates to $\Bbb F_8$ like $\Bbb Q_2$ does to $\Bbb F_2$
 
Yes, it's used in geometry for hyperbolic space.
Hi, Demonark.
 
Oh do you guys want to see one of the best uses of notation I've ever witnessed?
 
11:01 PM
@Daminark hi
 
@Daminark oh please oh please yes yes yes
 
@rschwieb: Since you were the only person inventing your own notation, that makes it more of a singular event.
 
How's it going Mathein and Ted?
 
@Daminark sorry I missed our discord thing, but I didn't have access to my laptop this weekend and I don't have discord on mobile
 
I suppose that text has a convention of $K$ being the integral klosure of $k$?
 
11:03 PM
@MatheinBoulomenos ah it's fine, thank you anyway! I'll just keep working through some Neukirch
 
Did you actually take a little holiday, Mathein?
 
@TedShifrin I don't know what has prompted you to take this tone with me, but I promise I will not talk with you anymore if it continues. Thanks.
 
@Daminark pretty well thanks, my advisor wrote me an amazing recommendation, you can see it if you scroll up a bit
 
Whoa. No sense of humo(u)r? Fine.
You're the one taking tones.
 
@TedShifrin I'm visiting my parents and simply forgot to bring my laptop
 
11:05 PM
LOL, that is a bit of a holiday, Mathein :)
 
@Daminark we can still do it tomorrow or something
 
@TedShifrin I will assume after this conversation you were just not paying attention to your tone, but I will have none of this reflection business. First you lecture a phd in ring theory about what $\mathbb Q$ stands for, then you imply I'm the only one who has ever used it, as if I had should have known better. I guess we'll chalk it up for a day and not talk about it again.
 
Hey
guys
 
Oh that's fantastic! Also wow the prospect of seeing a recommendation letter from an advisor is cool but also slightly scary. Like, I'm gonna ask for letters from people who hopefully think I'm not a complete dumbass but still shivers
 
Demonark, you've never been a complete dumbass. I promise.
 
11:09 PM
@Daminark just don't ask them to write about your puns
 
Well, yeah, that's a different matter.
 
I think to date I've only made one professor at my school cringe at a joke
 
I found a cool pattern that I want to calculate how many primes there are in it up to 1000 terms
any idea
how i could
do
that
?
 
@TedShifrin not to comment on just being 50%
:P
 
No comment.
 
11:11 PM
But yeah do people usually see their rec letters?
 
No.
 
I know in high school applying to college you don't
 
And when you apply to grad schools, make sure you check the box that you do not wish to see what's written. That way people will trust the letters.
Some faculty do volunteer to show them. If a student really wants to read what I've written, I ordinarily have shown it. But very few asked.
 
Hmm, alright I'll keep that in mind
 
@LeakyNun I think I see your point about 24 and 41: I could make them finite! I really should double check that I didn't accidentally leave out an assumption on the field in the original source. And it also looks like i mixed k's and F's in that one :/ Thanks for the eagle eyes as always
 
11:15 PM
This is not for grad school or anything, just a puny EMS membership, so I'm more happy that my advisor thinks so highly of me than the actual membership. Still thanks for the advice @Ted
 
My advice was to Demonark and the folks applying to American schools. I have no idea how the European system works with regard to recommendations, Mathein. But, of course, you should be happy/proud. You've worked hard.
 
I'm not really sure how it works here either, but I know whom to ask when it matters
 
Oh actually so, I was talking to a grad student the other day and he told me this story about someone named Mark Kisin. Basically this guy's PhD advisor, I think he might've switched from algebraic to analytic number theory? But yeah Kisin seemed to have some kinda difficulty from the switch and his advisor letter said he didn't know what he was doing
 
Lol wow
 
He didn't get any jobs here and had to go to Australia (where he was from), but while there everyone realized he was really good and he very soon got a job back in America
 
11:26 PM
Don't know that you should make his name public here ...
 
Hmm, that is probably a fair point, though it's late for me to edit it now :/
 
A friend of mine spent over half a year trying to verify a special case of a conjecture that his advisor made in the 90ties as his masters thesis. He eventually disproved it, but then the prof looked through his old notes again and realized that he gave him the wrong function
 
Oy.
 
Wow, that's unfortunate
 
christ
 
11:33 PM
Might as well throw in the apocryphal story I heard while an undergrad about an MIT grad student who'd written a 100-page PhD thesis about Hölder-continuous functions with Hölder exponent $>1$. After his presentation, one of the people at the defense asked him for a non-constant example.
 
Riiiiip
 
@TedShifrin I heard that one
 
Ooof, that's rough.
 
This is why I always tell you guys to be prepared with examples :P
Non-trivial examples.
 
My friends and I from complex analysis have this joke going that all holomorphic functions are constant because they're too nice to be otherwise
 
11:36 PM
Um ...
 
In my LA course our prof once gave the "example" of the column space K^n as an example for a finite-dimensional K-vector space (where K is any field)
 
To be fair, if they're also holomorphic at infinity that's a bit less incorrect
 
You mean holomorphic as a map from the Riemann sphere to the Riemann sphere, Demonark?
well, Mathein, he could have disguised it.
 
he said it's more concrete than "Let V be a n-dimensional K-vector space"
 
Riemann sphere to C
 
11:39 PM
blah@ Demonark
 
But we don't quite have that in mind in the joke, it's just ironic. Also wow I have quite the name
 
I had a typo.
 
@Mathein I've heard that a professor once was asked for an example and responded "Take anything satisfying the axioms!"
 
I don't like these kinds of professors.
 
@Daminark lol
 
11:40 PM
My favorite example of an $n$-dimensional $k$-vector space is $\{f \in k[X]\;:\;\mathrm{deg}\;f < n\}$.
 
I was going to mention that earlier, @Fargle.
The professor should have.
 
Of course we covered other examples in the exercises including that with polynomials @Fargle
 
I had wondered for a long time why we ought consider different examples of vector spaces if they're all isomorphic, and finally realized after that example that being isomorphic as vector spaces doesn't mean that they're isomorphic as more specific types of structures.
 
Good realization!
 
lol, yeah. It took me longer than I'm proud of--it's a bit trivial, but very important to notice.
 
11:44 PM
"Let V be any vector space where all subsets of size n+1 is linearly dependent and which is not spanned by any subset of size n-1."
 
Counterexample: $n = 0$ ($\mathrm{Span}\;\emptyset = \{0\}$)
 
I mean 0 is linearly dependent
 
We did some weird examples actually (well I thought they were weird at the time). Like the power set of a finite set as a vector space over the field with 2 elements (that goves a proof that the power set has 2^n elements, as singletons form a basis) or positive reals as a vector space with multiplication as addition and exponentation as scalar multiplication
 
Well, I guess that doesn't quite work: no set has cardinality $-1$.
 
You confuzled me, Fargle.
I think I'll need a double cocktail if this conversation goes on too much longer.
 
11:49 PM
I think my favorite example of a ring I've ever seen is the ring of all subsets of a set under symmetric difference and intersection.
I mean, that's just a Boolean ring, but I didn't know that type of fancy stuff when I was shown it.
 
Can you chat just anyone?
 

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