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16:00
:)
@SBM $\displaystyle \int \int_D f(x,y) \ \mathrm dx\ \mathrm dy =\int \int_D f(x,y) \ \mathrm dy\ \mathrm dx$
SBM
SBM
Um
didn't know that had a name though
What theorm is useful for proving global solutions of a system of ODE's
2
Q: Global solution for ODE

GobiSuppose a smooth vector field is given on plane. I want to find a global solution. Is the following procedure ok, or is there a problem? Since the vector field is Lipschitz continuous, we can find a unique local solution $\gamma(t)$ for an initial point $P=\gamma(0)$. If the solution is defined...

2
Q: Global Solutions of Ordinary Differential Equations

orlandoweberBackground Let $f: [0, \infty) \times {\mathbb R}^n \rightarrow {\mathbb R}^n$ be a jointly measurable function satisfying, $f(t, \cdot)$ is locally Lipschitz for every $t \geqslant 0$, for every compact $K \subseteq {\mathbb R}^n$ and every $b > a \geqslant 0$, $$ \int_a^b \|f(t,\cdot)\|_K\,dt <

too analytic I guess
@Zophikel
THanks @BAYMAX
solving a difflclut one I think i'll only be able to prove there exists one solution
16:10
I think
There is something related to eigenvalues
BAYMAX it's possible to use LA techniques but both my books haven't covered that yet
Hello
Uhm if someone could help me out a bit
I have a very small question
0
Q: Multipling surface equation by -1 gives different gradient

Mohamed Ahmed NabilSo while trying to find the gradient of the surface given by the equation x+y=5 The gradient was <1,1,0> And then I multiplied both sides of the equation by negative 1 giving me -x-y=-5 The gradient became <-1,-1,0> So the gradient changed although it is the equation of the same surface... ...

If any of you could take a look because im struggling a bit
16:27
@BAYMAX I have a system of ODE's that I magnaged to turn into two linear ODE's now i'm trying to show that there exists one unique solution
oh nice,now you can apply uniqueness theorem I guess@Zophikel
@BAYMAX ahh ok so I was on the right track
so I guess applying a Uniqueness Theorem to each ODE would work ?
Hey everyone!
yeah , uniqueness we can apply
@AlessandroCodenotti buongiorno
16:30
hola@Daminark
Does anybody know what pi-one refers to in geometry?
Fundamental group @benjamin
Hi @DHMO
In mathematics, homotopy groups are used in algebraic topology to classify topological spaces. The first and simplest homotopy group is the fundamental group, which records information about loops in a space. Intuitively, homotopy groups record information about the basic shape, or holes, of a topological space. To define the n-th homotopy group, the base-point-preserving maps from an n-dimensional sphere (with base point) into a given space (with base point) are collected into equivalence classes, called homotopy classes. Two mappings are homotopic if one can be continuously deformed into the...
what sucks is i'll have to trap the problem and associated function in a rectangular region $R$ and assume $R$ $\subset$ \$mathbb{R^{n}}$
16:32
@AlessandroCodenotti what do we call $\Bbb R^3$?
How's it going @Baymax and @Alessandro?
I know that $\Bbb R^2$ is the Euclidean plane
Euclidean space?
Or 3 dimensional euclidean space if you want to be sure
heh, $\Bbb R^4$ is also a Euclidean space
@AlessandroCodenotti Really, what is the relation to pi?
16:32
@BAYMAX another approach I did think of was defining a Maximal Interval of Existence and then extending to $(-\infty - \infty)$
I usually don't give names beyond 2
@Zophikel,you can post this as your new question.stating uniqueness part
you might get creative way!
@Benjamin no idea where the notation comes from
@BAYMAX yeah it will be a great question i'll have to see if I can get in the case of proving it in one solution
@Benjamin it's just another Greek letter
16:33
@BAYMAX it will suck if I can't solve the problem myself
You also see $\pi$ being used for projections
@Daminark I'm on holiday for a few days so great
What about you?
I think the nice part is learning new thing and especially learning those things which we don't know!
@Zophikel
@DHMO What would the pi-one value for a circle be? A friend of mine is wondering and it isn't my area of interest.
by that we grow higher!
16:36
@BAYMAX good point
@Benjamin $\Bbb Z$
so you should try it asking a question and don't forget to put the link to the differential equation room, it would be beneficial for others too including me :)@Zophikel
right, $S^1$ is the circle
Because intuitively loops turning around the circle a different amount of times or in opposite direction aren't homotopic
I have a 2 day holiday, the weekend :P
16:38
(Can't be continuously deformed one in the other)
but $\pi_1(S^2)=0$
@Daminark we get a Friday and Monday off sincd it's Easter
Ah, makes sense
Lol our calendar is such that between the first day of class and the last day of exams is 11 weeks
Regardless of holidays within
fora days holaday
So I wouldn't be terribly fond of days off
16:40
@AlessandroCodenotti As in the set?
Last quarter we had 3 days off, all on MWF, so that cut into analysis, for example
Yes, with usual addition as group operation @benjamin
@AlessandroCodenotti Great.
Which was sad, we would've done a lot of really cool stuff otherwise. This quarter I think we just have memorial day
@AlessandroCodenotti what values might the output of $\pi_1$ take?
16:42
@Daminark yeah, we'll have 3 Mondays off (next one, 25th april and 1st June because they're national holidays) and we'll have to have a couple of analysis lectures to make up for the misse ones
@BAYMAX do you know what a general solution to a system of Differential Equations is ?
@DHMO every group is the fundamental group of some space (of a CW-complex even)
@AlessandroCodenotti Is $\Bbb Q$ a group?
sorry no,there must be as I have seen somewhere that there are numerical methods to solve systems of ODE@Zophikel
@DHMO It's a field, in fact
So it forms a group under addition, and its non-zero elements form a group under multiplication
@Alessandro In fact, a 4-manifold
In difftop that's how we showed that 4-manifolds cannot be classified
Because if you could classify them, you would be able to solve the word problem in groups, which is undecidable
16:45
@Daminark Mike told me that but I don't know how to prove it
@AlessandroCodenotti is $\Bbb Q$ a fundamental group of any space?
Yes, every group is
you posted your question@Zophikel
?
@AlessandroCodenotti what space?
I don't know how to do so either, like I still don't know any algebraic topology
We're only doing basics of homotopy and all in difftop
16:47
A rather messy one, you'd need a group presentation of $\Bbb Q$ to begin with and I don't know if there is a nice one
what is a group presentation?
A set of generators and relations
Thing is, presentations are really crazy
(Which is a way to write a group as a quotient of a free group)
Like, the problem of taking a group presentation and finding out whether it presents the trivial group is undecidable
never mind
16:49
But yeah, it just blows my mind that complexity theory meshes in with manifold theory
Like, 2 subjects that aren't supposed to have anything to do with each other end up combining like that
Honestly that's the kind of work I want to do
Anyone know the name of a chart thats a horizontal bar chart but has a vertical line in the center? Where the bars extend left or right based on their distance from the center value?
17:04
Hi
How are you?
First year filing taxes. I feel weird.
17:22
Hey @Zach and @Fargle!
Hey @Dami
Hi @Daminark, @Fargle, and other people I missed
How's it going?
Bored. Trying not to go out like Capone.
Let's do algebraic topology!
17:31
no
Lol I mean, it seems like everyone here knows the subject, so we should probably learn it too
Peer pressure Zach!
I don't know that much algebra
Only basic group theory
To be fair I only really know a bit of group theory plus linear algebra
I'll be doing actual analysis soon...
how to generate conjugates algorithmically?
17:41
Lol next week in measure theory we're gonna do integration
e.g. the conjugates of 1+r2 is 1-r2
but the conjugates of $1+\sqrt[3]2$ are more complicated
I might go play some BP...
So I'm finally gonna actually learn it
Like, first quarter we didn't really do integration at all
He just said "Yeah cut up the region into boxes and then do sup minus inf like normal"
And we did change of variables
Now we're gonna actually do Fubini's theorem and all
@Daminark chapeau for using "Fubini's theorem"
17:58
Lol, I mean I've only heard it as such, what have other people called it?
They don't even know that it has a name
Weird
Demonark: That doesn't mean you have any idea how to actually compute integrals in dimensions higher than 1 :P
Hi @DHMO, @Fargle, Zach.
bye @Ted, @Daminark
Um, bye.
18:05
See you @DHMO!
And shrugs
Hi chat
Hi @Astyx, @Alessandro
If a sequence of functions converges pointwise to 0 and are all bounded by an integrable function, I can at least compute that
crosses fingers that this suffices
Nope, it doesn't.
18:08
@Daminark wow, it took us a couple of months at least to go from the basics of measure theory to integration
You need to get over this attitude that calculating/computing is something sullied and dirty in mathematics.
I feel quite guilty of this too
@Alessandro All we did so far was just Caratheodory extension and define Lebesgue measure, then do a bit on Radon measures to get the characterization that a Lebesgue measurable set was locked in (within a measure zero set) by an $F_{\sigma}$ and $G_{\delta}$ set
I wouldn't say it's dirty so much as painful
But yeah after Lebesgue integration I think we're gonna backtrack to measure theory
Vitali/Besicovitch coverings, density topology
The engineers are going to lose their jobs if we start doing calculations! braces for Ted's smack
(This is based on last year's notes, and we're more or less following what they did)
18:13
@Alessandro At least we do them carefully
@Daminark I have no idea what those are :P
@Alessandro Yeah! That'd be rude! Exactly, see it's a good thing that I screw up computations so much... hides
Hi @Ted
Jokes apart I should start doing exercises on integrals in more than one variable soon since there will be a few in the written part of the analysis exam in June
Yeah, I'll have to figure out that stuff for the GRE
I've been told to start taking it early 3rd year so as to have a couple opportunities in case the first goes badly
18:17
@Daminark We can try to work through Hatcher's book together if you want to learn algebraic topology
What's the GRE?
It's this test that you take when you're applying to grad school
And lol it's probably a good idea to actually learn point-set first
Woops, I thougth you already had had a point-set topology course
Oh lol, nope
All I've had is calculus first year and analysis + differential topology this year
Problem: If H and K are subgroups of G, then |K : H \cap K| \le |H v K : H | , where H v K = <H \cup K > denotes the join of H and K. Fact: If H and K are subgroups of G, then |K : H \cap K | \le |G : H|. Am I correct in thinking that the problem would follow trivially from preceding fact, since H and K are subgroups of H v K?
@Daminark how can you do diff top without knowing (at least a bit of) point set?
Like how do you define a manifold?
18:23
I mean I know a bit of point-set of course
We all do topology of metric spaces in analysis
Alright! Taxes complete.
Our book defines a k-manifold as a subset of $\mathbb{R}^n$ which is locally diffeomorphic to $\mathbb{R}^k$
Ah, makes more sense now
@Daminark that's GP, right?
Yup, we're also using a bit of Milnor
I see
I have to go now, I'll be back later
18:26
Alright, I should probably get back to psets, and get food eventually
Congrats @Fargle!
And yeah, see you guys around!
I'd prioritize food :P
Take care @Dami, and I agree with Alessandro.
attempts to eat a theorem
... so multitasking doesn't work, it seems
Why don't you prove some spaghetti?
A math student is a machine for turning ramen into theorems.
18:30
You can measure how good they are by dividing the noodles per day by the proofs per day, and you'll get noodles per proof, an accurate measurement
What about turning theorems into ramen ?
19:00
A comath student is a machine for turning cotheorems into coramen, but I don't know if the morphism is iso.
4
19:26
$\langle comaths , maths \rangle =\delta$ ...?
Cauchy Ramen Theorem Let an open pot of ramen. The ramen is soft enough and ready whenever it can be stirred and when stirred, it rotates without interruption in the pot
There is always ramen which sits at its original position in the ramen pot, no matter how it is stirred
That's true
Oh goodness, what have I begun.
19:48
soo I'm going buy the pro version of Cold Turkey today morning, an app which enables me to block all distracting websites
hope the 25 dollars are worth it
I feel like I'd buy it, and then just uninstall it every time I feel like procrastinating. :I
Sounds exactly like something I would do
20:04
There, a list of most known complexity classes and most of their known inclusion relations (dotted arrows denote containment, solid arrows denote strict containment):
Current link (4.7 MB)
20:24
Rehi chat
Hi ale
@Fargle you can't unblock any websites you've blocked by uninstalling it
it will remain blocked until the timer you had previously set runs out
other than having a system restore on windows I don't think there's any way to revert the blocking of a website before the timer runs out
they built the app keeping every possible way for the user to wiggle out in mind
Nothing will prevent me from procrastinating, you hear me ? NOTHING
man I really look up to you guys
please be a better role model for me
@SoumyoB pfff, I'll just boot up ubuntu from my usb key
Nothing can stop a student who wants to procrastinate
20:37
I haven't kept any such usb's so yeah
I'm gonna buy that
the app's pro version I mean
I sometimes wished my mother put me in a military boarding school instead of regular school, I'd be way more disciplined in my life than I currently am
21:35
Hi
22:10
8
Q: Is there a set of axioms governing the properties of derivatives in calculus that include this particular axiom and what would it be?

TheGreatDuckMoved from Math Overflow due to not being regarded as a high degree of research Note: I am looking in particular at real valued/real input functions at all values regardless of differentiability. In this question a series of axioms or postulates governing calculus are proposed. Granted, that is...

^ Old question, and Community user bumped it to the main page a few days ago, so I thought I'd bring it here.
22:23
9
Q: The first few values of Rayo's function?

Simply Beautiful ArtRayo's function defined in English: "$\operatorname{Rayo}(n)$ is the smallest positive integer bigger than any finite positive integer named by an expression in the language of first order set theory with $n$ symbols or less." More formally, we make use of the following second-order formula (Sa...

^ Also an interesting question for those who can understand it.
22:35
@KevinCarlson There is a fibration $\text{Map}(X, X) \to \text{Map}(A,X)$ given by restriction; the fibration is obviously acyclic since the inclusion $A \hookrightarrow X$ is an equivalence. The fiber above the obvious map is the set of maps $(X,A) \to (X,A)$ that are the identity on $A$, and we've just seen that this fiber $F$ is contractible. Then the space of deformation retractions of $X$ onto $A$ is a pathspace whose first point lies on the identity and final map lies on a retract to $A$.
So clearly this space has the same homotopy type as the space of retractions of $X$ onto $A$, but again this is easy to compute as the fiber of the acyclic fibration $\text{Map}(X,A) \to \text{Map}(A,A)$.
Hi @Ted
23:01
Hi Zach, @MikeM ... Looks sorta dead in here today.
@MikeM: As if I didn't have enough headaches from sinus garbage, Rafe just sent me an exercise on Lie-algebra valued 1-forms to unwind. Ugh.
1
A: The first few values of Rayo's function?

JDHYour question is about what I view as the definable-in-set-theory analogue of the busy beaver problem. I had previously posted an answer on MathOverflow to the corresponding question concerning what I view as the definable-in-arithmetic analogue of the busy-beaver function. The main conclusion th...

What's wrong with those?
I guess I wouldn't want to write out any bases.
I saw moving frames in a paper for the first time yeste stay.
It's about damn time :P
23:57
Hi @TedShifrin

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