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5:00 AM
If my field had four elements and they were $0,1,\omega,\omega+1$ then I could have all the elements defined, and I get $0+0=0,1+1=0,\omega+\omega=\omega,\omega+1+\omega+1=\omega$ and that is fine, but apparently I should obtain $x+x=0$ which implies that $\omega=0$ and now I have only two elements again, e.g I am working with $Z_2$
I have a different keyboard now :)
 
no... why do you think $\omega+\omega = \omega$?
 
What would you make it?
0?
 
yes
 
Why $0$?
 
because otherwise $\omega=0$
as you noted above
 
5:05 AM
So really I can define it however I want, as long as it works out
 
in a field of characteristic $p$, $px = 0$ for all $x$, not just $x=1$
 
is $p$ meant to be prime?
Here my characterisitic is $2$ right?
 
yes and yes
there's no field of characteristic $n$ for $n$ not prime
 
Because that wouldn'
't allow all elements to be invertible
?
thank you for your help @mike you are now one of my favourites alongside ted,kaj,balarka and others I forgot right now
 
heh
and yes, if $n$ was the not-prime characteristic, let $n = ab$ where $1 < a < n$ and same with $b$
then $a,b \neq 0$, but $ab = n = 0$
so it has zero divisors
 
5:13 AM
I can trivially see that $\forall a,b \exists c : a|c, b|c$, but why did they label this as meaning that the positive integers are 'directed'?
They being Cohn
 
that's the definition of a directed set
it's a set with a partial order $\leq$, where for every $a,b$ there's a $c$ in the set with $a \leq c, b \leq c$
in this case our set is $\Bbb Z^+$ with the ordering $a \leq b$ if $a \mid b$
 
@beginner I just got back online. Your keyboard problem sounds like a driver issue. Next time you plug that keyboard in, check your device manager to see that it properly recognized the device and has a current driver for it
 
now you don't need to know the more general definition of directed set; but that's where the name comes from
it's called a directed set, i guess, because there's a distinct "upwards" direction that everything goes in? maybe?
nothing gets stranded
 
the hasse diagram for nonnegative integer divisibility?
 
0 is a great number.
 
5:29 AM
Oh very cool, thank you both
 
A quick mention @beginner about the diagram I linked, arrows are pointed upwards in each case, and there are also directed edges to neighbors of neighbors which aren't drawn. For example, there is an undrawn edge from 3 to 30 since $3|30$
 
How come it is fine to say all fields of size $p$ prime are isomorphic? I feel this is true, and I am using the word closer to the meaning of graph isomorphism? I can see that they certainly have a bijective mapping, but I don't know how I would prove the homomorphism
Yeah that makes sense @Jmora
 
Because finite fields of prime order can be generated with a single generator, causing them to be isomorphic to $(\mathbb{Z}_p,+)$
 
Is it just because the homomorphism refers to multiplication and addition which are both clearly commutative
Generated?
 
I've lagged out. >_<'
 
5:36 AM
I think I saw something about generators, it is a single element that reaches all elements of the field, e.g. $1$ for addition, and something else for multiplication. Yeah @JMora I am always lagging out hehe
 
5:57 AM
I can see it is true that for $\Bbb F_p$ where $p$ is an odd prime I have exactly half of the non-zero elements as squares for the first few $p$ odd prime, but I am unsure how to prove this. Has this also got something to do with generators?
Oh I got this mistletoe hat without trying haha
I thought the hats thing was going to be a purchasable hat that was MSE oriented that you really bought and wore
 
6:21 AM
@beginner Consider the $p-1$ elements $-\frac{p-1}2, \ldots,-1,1,\ldots,\frac{p-1}2$
Those are all the nonzero elements of the field.
If you square those, you get $\frac{p-1}2$ squares.
Now, every nonzero square is a root of $x^{\frac{p-1}2}=1$ by Fermat, and since we're in a field, this has at most $\frac{p-1}2$ roots.
Thus, there are exactly $\frac{p-1}2$ nonzero squares.
@Behaviour My comment was of course mildly facetious.
 
@PedroTamaroff: Congratulations on becoming a mod. You were my first choice :D
Also, happy new year.
 
@Nick I hope I don't disappoint you.
 
@PedroTamaroff Superman never disappoints. :D
@PedroTamaroff Quick question: How do I prove $x + a^x < b$ where $a,b$ are constants.
 
6:37 AM
@Nick That is not true for arbitrary values of $x$.
For example, if $a+1>b$, it fails for $x=1$. If $b<1$, it fails for $x=0$, &c.
 
@PedroTamaroff Ok, to put my question right. How do I find the right vals.
 
@Nick Ah, you want to interval where that holds, yes?
 
@PedroTamaroff yes :D
 
So the question to make is "For which values of $x$, does $x+a^x<b$ hold, where $a,b$ are constants?"
 
yes :)
 
6:41 AM
[7,23.2]
 
Well, suppose $a>1$. Then $x+a^x$ is increasing. Thus, the answer is "the interval $(-\infty,x_1)$ where $x_1$ is the unique solution to $a^x=b-x$.
 
oh! ohk, I got it.
1
Q: Solve $x+a^x<b$ algebraically

user272651The answer to $x+3^x<4$ is $x<1$ by plotting the graph of $y=x+3^x$ and $y =4$. Is there a way to get to the solution algebraically? Updated: is there a way to get to the solution of $x+a^x<b$ in general, algebraically?

@PedroTamaroff: ^ the above is the source.
Thanks. Now, I can head off to the New Year festivities in peace.
Toodles.
@PedroTamaroff: I'll know you'll make a great mod.
 
 
1 hour later…
8:14 AM
@robjohn Any progress on that question I had given to you + book?!
I am talking about that integral.
@BalarkaSen Hey
 
@Pedro You there?
 
@BalarkaSen Maybe.
 
@Pedro Sanity-check : Isn't Spec C[x] with Zariski topology just homeomorphic to C with the cofinite topology?
Because prime ideals of C[x] are all <x-a> for a in C and since these are maximal, these ideals are also closed as points of Spec C[x]
So (considering (0) along the way), the only open sets are (0) and the whole space Spec C[x] with finite number of points removed.
 
@BalarkaSen The cofinite topology is always $T_1$, but $\Bbb C[X]$ has a generic point: since $(0)$ is prime, we have $\overline{(0)}=V(0)={\rm Spec}(\Bbb C[x])$. So $(0)$ is not closed.
 
ahh
So I guess Zariski topology on Spec C[x] with (0) removed is equivalent to cofinite topology on C*?
 
8:22 AM
Don't guess. Prove.
 
We've got lots guys over here, but nobody is something in Fourier theory.
I had one question, even math overflow folks minds has overflowed!
 
@FreeMind I know a tiny bit of Fourier Theory.
Actually, about Fourier Series, not Fourier Transforms.
 
uhum
 
I fear your question is about Fourier Transforms. But I kinda should know about those, since I've taken complex analysis.
 
@PedroTamaroff Tell me your opinion, is there anything that you need further info in this question?
.
I had asked this question in MSE but I deleted it.
I got no answer indeed.
 
8:28 AM
@Pedro Well. You can construct a bijection from Spec C[x] to C using (x-a) $\mapsto$ a.
 
Doesn't seem to be a good fit for MO, @FreeMind.
@BalarkaSen That's true.
 
@PedroTamaroff So where should I ask it then?!
 
Then Zariski topology on Spec C[x] says every point (x-a) is closed, as they are maximal.
 
@FreeMind Here on MSE. Please stop screaming at me. =/
@BalarkaSen Yes.
So you need to provide a homeomorphism, not just a bijection.
 
@PedroTamaroff Screaming? I had asked on MSE, nobody answered!
 
8:30 AM
@FreeMind Sometimes it takes time for someone to answer.
 
It has been there for a long time, about 28 days
 
@PedroTamaroff Actually the correct bijection is Spec C[x] - (0) to C using (x-a) |-> a
And this is a homeo if you equip C with the cofinite topology.
i.e., make every point closed
 
@BalarkaSen Proof?
@BalarkaSen That's not precise: the discrete topology has every point closed too.
 
Well, yeah, I need the open sets.
Wait wait let me sketch this out.
 
You want to define: "The closed sets are the finite sets."
 
8:37 AM
Hmm, OK. Define the map f : Spec C[x] - (0) to C. Equip C with the topology whose open sets are C minus a finite number of points.
Then inverse image of some point a in C goes to (x-a)
An open set U around a in C is complement of a finite number of points along with a.
So taking inverse image maps that to the subset of Spec C[x] - (0) which consists of the whole space minus a finite number of points (ideals) along with (x - a)
So that's an open set around (x - a).
Hence f is continuous, and since it is bijective, it is a homeomorphism.
 
Continuous + bijective =/=> homeomorphism.
But it is easily seen $f$ is a closed map, since it sends finite sets to finite sets.
So it is an homeomorphism.
 
Well, duh f^{-1} is also continuous.
I need to get used to this weird topology. Never worked with spaces where open nbhds around points doesn't even contain the point.
 
@BalarkaSen What...?
 
For some psycological reason I like to think nbhds around a certain point consists of the point.
 
hello
 
8:45 AM
@BalarkaSen What does "...nbhds around a certain point consists of the point." mean?
 
hi @Mike. shouldn't you be asleep right now?
 
@BalarkaSen Please clarify what you mean.
 
happy new year, now that it's actually new year here
 
Well V(I) is apparently the definition of nbhd around I in the Zariski topology. But by definition V(I) doesn't contain I :P
 
@BalarkaSen No.
 
8:47 AM
I feel weird about that
 
Wait.
$V(I)$ is a closed set.
 
Redo : The definition of open nbhds around I is the "collection of ideals that doesn't contain I"
 
$V(I)=\{\mathfrak p\supseteq I:\mathfrak p\text{ prime}\}$.
 
I am talking about the open sets, @Pedro. Typo.
Didn't really mean V(I)
Meant $X_I$
 
@BalarkaSen Well, your problem is $I$ is generally not a point of the spectrum.
OK?
 
8:51 AM
Given a point $\mathfrak{p} \in \text{Spec}(A)$, the open neighborhood $X_\mathfrak{p}$ around $\mathfrak{p}$ in $\text{Spec}(A)$ by definition of Zariski topology doesn't contain $\mathfrak{p}$
^That is my problem
 
@BalarkaSen Oh. That's not a neighborhood of $\mathfrak p$.
 
It isn't?
 
Well, no.
What's the definition of neighborhood of a point?
 
If I haven't forgotten everything, then an open set in X containing x.
 
@BalarkaSen Well...?
 
8:55 AM
But then in this topology, there are no open neighborhood around $\mathfrak{p}$...?
Because all such sets would be closed.
Not?
 
One sometimes call set $N$ a nbhd of $x$ if it contains an open set $O$ containing $x$.
 
I'm confus.
 
If the nilradical is prime, the spectrum is irreducible.
So the notion of "neighborhood" is kinda fuzzy.
 
It looks like as if there can't be two disjoint open sets.
 
Well, yes.
 
9:00 AM
It's not... Hausdorff?
WHOA
 
Why are you impressed?
Open sets are very big in the spectrum. You can think of them as "complements of curves."
 
Not impressed. Awed.
 
@BalarkaSen I edited something above.
A topological space is called irreducible if every pair of nonempty open sets have nonempty intersection.
 
So there are spectrums which are Hausdorff?
Oh. Just Spec k for some field k.
Hehe.
@Pedro Interesting. Thinking about how to prove that.
 
@BalarkaSen Prove what?
 
9:06 AM
6 mins ago, by Pedro Tamaroff
If the nilradical is prime, the spectrum is irreducible.
 
@BalarkaSen It's an iff, actually.
 
For every subset of the specturm, you can define $I(Y)=\bigcap_{\mathfrak p\subseteq I}\mathfrak p$.
Then one shows $Y$ is irreducible as a subspace iff $I(Y)$ is prime.
But $I(X)={\rm nilrad}(X)$.
In general, the irreducible components of the space are in bijection with the minimal primes of the ring.
 
I'm going to leave chat and think about it.
Thanks for the help and informative hints, @Pedro.
 
9:14 AM
lolwut @Mike
 
love your hat, btw, pedro
 
He originally had a question. Perhaps he found the answer.
 
@MikeMiller Yes, it fits perfectly.
 
@Gustavo!
 
@BalarkaSen. How are you? Happy new year.
 
9:16 AM
Happy new year. May this year suck more than the year before it and the year before the year before it and so on.
What've you been thinking about lately, @Gustavo?
 
Hahaha! Hopefully it doesn't suck too much! Well, not much. Philosophy.
I've taken an interest in that.
Yourself, @BalarkaSen?
 
Oh, great.
@GustavoMontano Nothing much. Started algebraic topology, done Munkres and now beginning Hatcher. Meaning to do commutative algebra a bit along with it too.
 
hello, happy new year
 
@BalarkaSen You read all of Munkres?
 
Fantastic! I'm going to start Munkres tomorrow! The bookstore repons :).
How are you finding topology?
 
9:21 AM
@PedroTamaroff Mostly. I skipped the classification of surfaces bit.
Sounded boring to me.
But other than that, all of it, along with exercises.
 
@BalarkaSen Did you also skip the definition of "neighborhood"? ;)
 
Munkres has puny exercise sections.
Nah @Pedro.
I can't help it if my space is messed up.
 
Please, can someone how to do to see that $\frac12(|x|+|y|)\leq |x-y|$ please
 
@BalarkaSen Definitions don't change with spaces.
 
My visualizations does.
I don't stick up definitions in my head, usually. I'll understand them only if I can visualize them.
In the case of Zariski topology, everything is way too huge for me to visualize.
 
9:25 AM
Visualisations help, don't they.
 
Sure does.
 
I'm a very visual person - it sucks to learn when it can not be visualised well.
 
[Exactly why I don't like group theory... waits to get smacked by @Pedro]
 
@BalarkaSen I'll just shake my head.
 
9:27 AM
Please, can someone how to do to see that $\frac12(|x|+|y|)\leq |x-y|$ please
 
Wha..., @Vrouvrou? You were asking questions about algebraic topology 6 months ago and then doing general topology (homeomorphisms and stuff) only a week ago and now you're onto fundamental inequalities?
Are you studying stuff backwards?
 
Hahah.
 
Galois theory is very easy to visualize, on the other hand, @Pedro. Think about larger fields as their positions in the lattice and think about Galois groups are the edges joining the nodes (nodes corresponds to fields) in the lattice ;)
 
What is a splitting field?
 
@BalarkaSen You said groups are not easy to visualize.
Just ponder this for a second.
Groups act on stuff.
 
9:31 AM
@BalarkaSen sometimes i see nothing and my mind stop thinking
 
@beginner If you have a polynomial over a field F, splitting field of the polynomial is the minimal extension of the field F which contains all the roots of the polynomial.
 
What does minimal extension mean?
 
@PedroTamaroff Well, Galois groups are.
 
that's why i asked on the chat because i know that is very simple but i don't see how to do
 
@beginner The book you're reading, if it's using these words, should be definining them.
 
9:33 AM
Trying to understand someones answer to my question
 
@Vrouvrou You need to say more, because it's not true as stated. Let $x=y \neq 0$, say.
 
@PedroTamaroff Groups are not easy to visualize. Doesn't mean that applies to Galois groups too.
 
They are helping me find the elements of $F_p$ where $p$ isn't necessarily prime.
 
Note also that groups were fundamentally discovered as abstraction of galois groups.
So historically, galois theory comes before group theory
 
They also use Lagranges Theorem, is that easy?
 
9:34 AM
Where I can find a reference that the pullback of the metric of the exponential map can be expressed by Riemannian curvature tensor? Also why must this be true?
 
@beginner What is the problem?
 
Yes, so? How is a symmetric group not visualizable? A dihedral group?
There are a lot of groups that are "visualizable."
At any rate, I'm busy.
Goodbye.
 
@Pedro Eh. Groups are OK. But Sylow theory over Groups is... sheesh.
 
The problem was finding all the elements of the field $F_p$ where $p$ isn't necessarily prime
 
Bye.
 
9:35 AM
What is your definition of $F_p$, @beginner?
 
Cya @mike
Field with $p$ elements
$\Bbb F_p$
 
OK, so what do you mean by "finding all the element"?
 
Finding what all of the elements are
 
It's just isomorphic to the field structure over integers modulo p.
 
like $\Bbb F_4$ has $0,1,\omega,\omega +1$
That is for $p$ prime?
 
9:37 AM
Eh? What the hell is $\omega$?
 
@MikeMiller yes you are right, but i missed to say that i want to prove that $\alpha(|x|+|y|)\leq |x-y|$ for $x\neq y$
 
But for $\Bbb Z_4$ we are a commutative ring, but not a field
 
What's $\alpha$?
 
@beginner Equip it with the multiplicative structure
Oh Z_4
 
Actually, I need to sleep. Sorry.
 
9:37 AM
It isn't multiplicatively invertible
 
No there are no field of composite order
 
Thanks for your help today @Mike
There is a field with 4 elements though
It just isn't isomorphic to $Z_4$
Or it is, but it isn't $Z_4 $ sorry
 
Tss.
Sorry.
 
So I was wondering how I would find the elements of $\Bbb F_6$ without trial and error
 
I mean there are no finite field of order anything other than p^n
 
9:39 AM
Whereas for $p$ prime $\Bbb F_p$ is $Z_p$
 
@beginner There is no such field
 
Hiya
 
Because it doesn't have a generator?
Hey @Alec!
 
Hi
 
@beginner What generator?
By the way, beginner, what book are you using to study these?
 
9:41 AM
@Balarka It doesn't have a cyclic generator?
 
"generator" in the context of fields doesn't make sense
 
@Balarka I am using Cohn Classic Algebra and random things I find
@Balarka Only in groups?
 
random things
 
:(
 
Do the book you have rigorously.
 
9:42 AM
I would use Dummit and foote but it is 44mb
I'm only allowed 10mb a day in direct downloads
 
Just buy a hard-copy.
 
My parents won't buy me textbooks because they are 'too grown up and exspensive'
 
such nonsense
 
@MikeMiller any $\alpha>0$ because i want to prove the equivalence between two distance
 
When I have kids I will buy them anything for learning
 
Huy
9:44 AM
@beginner: How about you buy them yourself?
 
How??
I get $10 a week if I behave correctly
Which I apparently never do
 
Huy
@beginner: Then you can buy books worth $520 every year. That's a lot of books.
 
I have successfully gotten my money a few times in the last year
 
Try saving them @beginner
 
This week I let my friend bring a rail gun over so I won't get money for a month probably
 
9:46 AM
I usually don't care about pocketmoneys. Stupid.
 
Give the poor kid the knowledge that he wants. Just let him download.
 
It's 44mb, so I cant download it, but I have Cohn which is apparently terrible
 
Huy
@GustavoMontano: One could argue the same way about the homeless. Give the poor guys the food that they want. Just let them steal.
 
What is DJVU?
 
9:48 AM
Greetings
 
CHRIS!
Hi
 
@beginner Hi
 
@BalarkaSen just a hint please
 
I haven't seen you in ages @chris how has the book been?
@balarka what is DJVU file type?
I think I can only do pdf which is 33mb there
 
@beginner I'm working on it. Well, finishing a book is not that easy as some might believe.
 
9:51 AM
Are you working through the holidays normally? Or did you take a break?
 
@beginner I have no break. I work day and night ceaselessly.
 
Someone downvoted one of my questions :(
@Chris me too!
But I have been learning general relativity recently because it is really fun
 
I see.
 
Do you do any general relativity??
Did you guys know it is impossible for a pendulum to point straight down when it has zero velocity??
What I meant was it is impossible for a pendulum to have zero velocity**
Hence it can never point straight down
 

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