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11:00 PM
What is an algebraic variety? Why would I care about its geometry, or any geometry for that matter?
 
Then why care about visual interpretations?
 
QED
does this algebraic variety stuff relate to diophantine equations
 
@QED: Sure, a diophantine equation defines an affine scheme.
 
QED
what's the simplest one you can get some good information about using these tools?
 
@ZhenLin It's cool. It also gives me a guiding intuition about how to move through the space. Much like how when you turn your monitor pi/2 radians the mouse starts moving to the wrong directions.
When you know where you have to go, then you know where you can pee and how to get there.
 
11:05 PM
@QED x^n + y^n = z^n :p
 
QED
that's the simplest??
 
Peeing is one of the most important things of the day, so it's a good skill to have a visual interpretation.
@QED: He is trying to scheme you out of your money, watch out :-)
 
@Asaf: There isn't really any geometry in the Zariski topology. It's like asking what the geometry of the space that is Stone-dual to a Boolean algebra.
@QED: There are easier ones, like elliptic curves. But Fermat's equation is more famous.
 
QED
Do you know which theorem about elliptic curves makes interesting use of this?
(although I'd rather study something other than elliptic curves)
 
All of them. Elliptic curves are geometry.
 
QED
11:10 PM
none of the theory I've done about elliptic curves uses this
 
@ZhenLin You know, "geometry" means the measurement of land. Of course in mathematics we change that to "measurement of space". The fact that you are unable to comprehend the Zariski space does not mean it cannot be measured.
 
I have to go out for a few hours. I have started something that I hope turns into a decent answer. Comment on it if you wish.
 
It's just hiding in the background, to avoid scaring number theorists. :p
 
Zariski topology is like taking a sigma-algebra on acid.
 
@Asaf: I understand the Zariski topology well enough, thank you. I simply know not to interpret it geometrically.
 
11:12 PM
@Zhen: I'm sure that you understand it much much better than I do, or will ever do. I just don't see why you geometers are so adamant not to try and interpret it visually.
Just because it doesn't make sense doesn't mean that it doesn't make sense. Much like this very sentence.
7
 
QED
maybe because the human visual cortex is so limited that it doesn't help?
 
It's quite easy to interpret geometrically, in the case where A is a finitely-generated integral algebra over the complex numbers. Then in that case we do have a genuine geometrical object which is amenable to geometric intuition.
For example, if A = C[x], then Spec A is essentially just the complex plane.
The trouble is that the Zariski topology is essentially the cofinite topology on the complex plane.
 
I suddenly understand. You are just insisting that everything that has the name geometry will look, at least somehow, at least locally enough, as real vector space.
 
Hah, if you were talking to me last year, perhaps. But I've graduated from that. :p
 
That's good.
 
11:17 PM
Now geometry is anything that has a reasonable notion of sheaves/bundles and cohomology. :p
 
It is interesting, I am thinking about it and I try to move my hands around to somehow make shapes and movements as in my mind... but there simply too many dimensions. Perhaps if string theory works...
How many dimensions are they claiming now? 26 or 12?
 
My mind is feeble: if it can't be embedded in 3-space, then I can't see it. So I don't use visual intuition very much.
 
QED
@Zhen, I don't think you're the only one...
 
It's funny, you know. My geometrical intuition is crap. However I can imagine incredibly complicated structures so vividly that most of my intuition in set theory comes from actually seeing in my mind how the set I am working with is ordered.
 
QED
11:22 PM
that's kind of scary Asaf :D
 
The first time I learned that about myself was when I was reading about Suslin trees, the book kept saying that the underlying set is omega_1, and I kept thinking it means with the usual ordering and how could that be a tree.
Then I realized that it is actually just the set, and the order of the tree is different.
At the same moment, I will never forget this, the line I had in my mind for omega_1 just splitting into a huge and complicated tree.
 
Well then, the importance of the Zariski topology is not that it is a topology, but that it is a sup-complete semilattice.
 
QED
for what reason are you using the Zariski topolgy?
 
For passing the course in commutative and homological algebra, so I won't have to take any course work during the next semester and I could actually work on my thesis.
 
Essentially, the frame of open sets in Spec A is just the opposite of a certain sublattice of the lattice of ideals of A.
 
11:27 PM
The one generated by the prime ideals?
 
Actually, maybe not the opposite. Hmmm. I need to think this through more carefully.
But yes, the inf-complete semilattice generated by the prime ideals is the lattice of closed sets in the Zariski topology.
 
Yes. The moment they told me "It is simpler to describe the closed sets" I knew that this topology was made by a bunch of wusses.
 
It's a very weak topology, yes.
 
You should instead define a class of "spineless topologies".
 
Why are topologies defined in terms of open sets anyway?
 
11:35 PM
Funny you should mention that, there's a notion of a hedgehog topological space...
@Srivatsan: Because Bourbaki said so.
 
@ZhenLin Is that a serious answer? [Does any text do topology via closed sets?]
 
Well, it's half-serious. From what I gather the modern definition of topological space is due to Bourbaki.
 
Anyway, I am going to sleep.
I have seven hours before the world starts again.
 
mcb
$test$
 
@mcb Don't worry; it does not work yet. =)
 
11:38 PM
If you look at the old historical definitions you will see definitions that are much closer to geometrical intuition.
 
mcb
@Srivat Oh. OK :)
 
@ZhenLin How old? And what definition do you have in mind?
 
I'm thinking of Hausdorff's original definition in terms of neighbourhoods and Kuratowski's definition in terms of closure operators.
 
The origin of most definitions is N,Z,Q and R.
 
@Asaf: Funny you should mention that, those symbols are due to Bourbaki too...
 
11:39 PM
@ZhenLin I am not sure about that, but perhaps.
 
He could've chosen based on French words =)
Someone's making sure the Koenig's name dominates the side board. =)
 
Well, Z stands for Zahlen, which is notably German...
 
Bourbaki were notably French.
 
Is that a good thing?
 
I have shown my protest =)
 
11:44 PM
Shall I remove the star from the TeX test? :-P
 
Yes.
 
But I have to go to sleep.
 
Why?
 
Long day tomorrow.
 
@AsafKaragila That sucks.
 
11:50 PM
Yeah. I know. Seven hours of sleep, slowly dissipating into the night... I should leave. Farewell.
 
Bye, Asaf
 
@AsafKaragila Good night.
 
Too sad. No comments to star for a few hours... =)
@Jonas, I have a «stupid» question on convergence of integrals. See here: math.stackexchange.com/q/82975.
Question: If an integral int_{- INF}^{INF} f(x) dx converges, then it is not the case that f(x) to 0.
 

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