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Zee
1:16 AM
Guys I wanna prove that a finite integral implies finite function on null set, can someone listen to my proof?
 
Ok.
Though your proof yesterday was shoddy.
 
Zee
my proof yesterday was incomplete, I was distracted
But anyway, my proof requires measure theory, do you know that?
 
@BalarkaSen ping, check Hangouts
 
1:50 AM
@Zee ugh, doing analysis when you could've been doing infinity topoi
:P
 
2:19 AM
@Daminark i think group theory won't end, every time i think i finished something i find that there is stuffs i should know. what your opinion about this?is that because group theory is huge?
 
It's a very big topic, yeah
 
 
2 hours later…
4:03 AM
@LeakyNun Thanks.
How do I solve: $ x^2 - 5 \lvert x\rvert +6 <0$
 
Split it into two cases, $x<0$ and $x>0$.
 
@Semiclassical But why when |x| is always > 0?
 
Because $|x|=x$ for $x>0$ but not for $x<0$.
...actually, though.
Simpler way (which avoids cases) is to note that $x^2=|x|^2$.
Then you've got a quadratic in $|x|$.
 
@Semiclassical yes
then?
 
Well, what's the most obvious thing to do with a quadratic?
 
4:13 AM
@Semiclassical solve it.
 
Right. Though in this case the most useful is more specifically to factor it.
And $|x|^2-5|x|+6$ isn't hard to factorize, happily.
(So, uh, do that.)
 
@Semiclassical understood this question but in general too why do we take cases?
 
Because one usually prefers to work with $x$ rather than $|x|$.
 
@Semiclassical But |x| is always positive so why take a negative case
 
Because $|x|=-x$ if $x<0$.
You could equally well say that one should write $x=-|x|$ in that case, and I wouldn't disagree.
 
4:27 AM
@Semiclassical Nope. It's always positive right?
 
The left-hand side is.
 
|-2| = 2
 
The right-hand side is not.
|-2|=-(-2)
|2|=(2)
With the second, one has $|x|=x$. But for the first it's $|x|=-x$.
In both cases, the left-hand side is positive. But the right-hand side is positive for the first one only when x>0, and positive for the second when x<0.
 
@Semiclassical Oh. I get it.
TY
 
np
Another way to get at the same point is that $|x|=x\,\text{sgn}(x)$
But happily $|x|^2=x^2$ means that one can dispense with that.
 
4:32 AM
@Semiclassical sgn?
 
sign function
(for some reason it gets abbreviated like that)
 
ok
 
so sgn(x)=+1 if x>0 and -1 if x<0
 
k
 
4:50 AM
hey how the fixed points varies for $\dot{x} = -ax + x^3$
like fixed points are $0, +\sqrt{a} , -\sqrt{a}$
Computationally i see that there is a bifurcation when $a$ changes its values from -ve to =ve
but first of all i am not convinced that alue of $a$ can be negative as it will then result into a complex fixed pint which is not possible!
 
@BAYMAX The thing to notice is how the sign of the right-hand side varies with $x$.
Suppose, for instance, that $a=1$.
 
ok
 
Where is the right-hand side positive and where is it negative?
 
then $f(x) = -x + x^3$
when
$$-1<x<0
then $f$ is positive
 
That's one instance, yes.
 
4:56 AM
and when $0<x<1$
 
What happens for $x<-1$?
 
$$ is negativef
for $x < -1$
 
I'm not sure I know what you quite said, it was a bit unclear.
 
it is negative
 
For which intervals is $f(x)$ negative?
 
4:57 AM
$x < -1$
and $0<x<1$
@Semiclassical
 
Right. I imagine you can see when it's positive as well.
Note, then, that that means that $\dot{x}<0$ when $x<-1$ for instance.
So $x$ should be decreasing with time; it's already negative and it's only going to get more so.
 
Hey
 
yes
 
So if you've got $x(t)<-1$ at $t=0$, you should expect $x(t)\to-\infty$ as $x\to\infty$.
hey @dodsy
What about if $-1<x(t)<0$ for $t=0$?
 
So if you've got $x(t)<-1$ at $t=0$, you should expect $x(t)\to-\infty$ as $x\to\infty$. ?
"$x(t)\to-\infty$ as $x\to\infty$. "
 
5:01 AM
Woops
$t\to\infty$ at the end.
 
t right1
actually i am on another system today
so i have not got chatjax with me
please bear if some mistake !
 
thanks
why $x(t) \rightarrow \infty$
sorry $$- \infty
got it1
 
Well, if $\dot{x}<0$ whenever $x<-1$, it'll be decreasing as a function of time.
 
:)
yes
 
5:04 AM
mmkay.
 
next how we proced!
 
Well, that was $x(0)<-1$.
What's the next relevant interval in $x$?
 
yes
(0,1)
$x(0) \in (0,1)$
 
Right.
 
then $x(t) < 0$
and $x(t) \rightarrow \infty$
 
5:06 AM
Sure. But what about $\dot{x}$?
 
$\dot{x} < 0$
right?
 
Well, remember: $\dot{x}=-x+x^3$
Oh, wait.
I misread you.
Right, it's negative. So $x(t)$ should be decreasing.
So if it starts in $(0,1)$, where will it go?
 
sorry
$-\infty$
 
Well, keep in mind: It'll be decreasing so long as $x$ remains in (0,1).
But if it continues to decrease, eventually it'd have to cross $x=0$.
What's $\dot{x}$ at $x=0$?
 
gotcha
$0$
and will remain there right for ever!
for all time1
 
5:12 AM
Right.
So it'll decrease only down to $x=0$.
 
yes
 
In fact, though, it will never actually reach $x=0$.
 
nice oone there
why
 
It'll get closer and closer, but $\dot{x}$ will also get smaller and smaller (though still negative)
so it'll decrease more and more slowly.
To back this up further, suppose we have x(0) as some small but positive initial value.
 
like in b/w 0 to 1?
 
5:14 AM
Well, closer to 0 than 1 lets say.
 
ok
 
Then x^3 is even smaller, so we can approximate the right-hand side of $\dot{x}=-x+x^3$ as just $x$.
And then the resulting differential equation should look familiar.
 
$x = x_{0}e^{-t}$
 
Right. Which you'll notice never actually reaches 0 as t goes to infinity, though it gets arbitrarily close.
How about if -1<x<0 ?
 
so we say limit x tend to 0 as t tend to infinity
and never x = 0
right1
 
5:16 AM
right.
 
and when $x(0) \in (-1,0)$
then $\dot{x}$ is positive
and hence $x$ increases with t upto 0
 
Right.
 
like aymptotically
 
That makes $x=0$ an attractive fixed point. If $x(0) $ is nearby on either side, then it will move towards x=0.
 
yes
sure
 
5:19 AM
By contrast, what's happening near $x=-1$?
 
and also it is visible if we calculate $f^{'}(x)$ at the fixed points
yes
they donot come near -1
rather they diverge from -1
 
Right. So that's a repelling fixed point.
 
so -1 is an unstable fixed point
ok
 
Yeah, that's the other terminology. Probably better tbh.
 
now can we see how the stability of fixed points changes when parameter $a$ varies ?
:)
 
5:22 AM
Well, one last bit.
 
ok
 
One convenient way to encode this information is to draw the graph of $f(x)=-x+x^3$
 
yes
 
And then draw right/left arrows on the x-axis wherever $f(x)$ is positive/negative.
 
yes
 
5:23 AM
From there it becomes obvious that x=-1 will be unstable (the arrows point away from it) and x=0 will be stable.
 
nice one !
yup
 
There's one other fixed point, though, since f(x)=0 has three roots
 
yes
x = -1,0,1
 
Right. Is x=1 stable or unstable?
 
unstable
 
5:25 AM
Right.
Now, suppose you have $f(x)=-ax+x^3$.
 
yup
 
There are two aspects to that curve. On the one hand, if $x$ is large (positive or negative) then x^3 is the dominant term. Since this doesn't depend on a, we should have that the plot of $f(x)$ won't change much if we change $a$.
Agreed?
 
Suppose the fourier series coefficient of Y(t) is Cn then what will be the fourier series coefficint of Y(2t+1)?

My doubt regarding this is if we do first shifting and then scaling ans will be ejnwCnejnwCn but if we do first scaling and the shifting then it will be e0.5jnw)Cne0.5jnw)Cn
why is my answer changing and what is the correct procedure?
Please help me with this?
 
wii ll be back @Semiclassical shortly
spoory
sorry
 
5:41 AM
mmkay, i'll have to head out unfortunately. But think about what happens to the graph of $f(x)=-ax+x^3$ as $a$ goes from being positive to negative.
 
Zee
5:54 AM
Hey @Semiclassical
Can you take a look on my measure theory proof?
If anybody else can help, that would be nice
So I wanna show that the lebasgue integral of (measurable function on the positive real life) is finite implies f is finite on null set.
proof: By a theorem, we know that there is a sequence of increasing simple functions that converge to f point wise
By the monotone convergence theorem, we can say that the integral of f = lim (integral (sequence of simple functions as above))
If the sequence of simple functions is infinite only on a null set, then since they converge to f , the result is proved
No le I only need to show that the sequence is infinite only on a null set
Which follows from the slandered representation of simple functions as a linear combination
 
but that is the problem how can we take a negative
If $a$ is negative then $x^2 = a$ giving us complex fixed points which is not possible
@Semiclassical
?
but at the same time when i plot it I see that the stability of fixed points is changing that is bifurcation occurs
so there is some logical error in my analysis
><
 
6:26 AM
@Abcd one must realize that $x$ can also be negative.
@BAYMAX so that means the only real fixed point is 0. I don't see how that is not possible.
@BAYMAX why not?
2 hours ago, by BAYMAX
hey how the fixed points varies for $\dot{x} = -ax + x^3$
 
for fixed points $x(-a + x^2 ) = 0$ implies $x^{*} = 0 , +\sqrt{a} , -\sqrt{a}$
 
Does $\dot{x}$ mean $x'$?
 
@LeakyNun
yes $\dot{x} = \frac{dx}{dt}$
 
@BAYMAX so?
 
1 hour ago, by BAYMAX
now can we see how the stability of fixed points changes when parameter $a$ varies ?
so here we need to change the parameter $a$
 
6:30 AM
I fail to see why "fixed point" isn't $f(x)=x$ instead of $f(x)=0$, but maybe it's my problem
go on
 
oh
actually we have two things here
in case of discrete time we use maps
and in cont. time we use flows
so in discrete time we use $f(x_{n}) = x_{n}$
for fixed point
and for cont. case
$\dot{x} = f(x)$
we use $\dot{x} = 0$
for calc. fixed points
 
alright, move on
 
so here
I checked the stability of fixed points for he cae $a = 1$
fixed points are 0 ,1,-1
 
agreed
 
1 hour ago, by Semiclassical
There's one other fixed point, though, since f(x)=0 has three roots
1 hour ago, by BAYMAX
x = -1,0,1
 
6:35 AM
yes?
 
Hey there everybody!
 
@Daminark hi
 
Zee
Hey leaky , did you see my proof above? Maybe you should flag it...
 
@Zee where?
 
Zee
At 1:57
 
6:50 AM
I don't know measure theory
 
Zee
Be honest, did you flag me that time?
No hard feelings, I swear
I knew it was you...
Damn algebrists
 
How's it going @Leaky?
You call @Zee?
Also yo @Astyx
 
@Daminark quite nice
 
Yo
 
Zee
@Daminark your kind makes me sick
 
6:57 AM
yesterday, by Leaky Nun
6 mins ago, by Leaky Nun
@Zee don't jump to conclusions?
 
Get some Tylenol
 
Zee
By Tylenol you probably mean some kind of an algebra over a ring
Or a scheme
Stack
 
> Tylenol is a pain reliever and a fever reducer used to treat many conditions such as headache, muscle aches etc.
 
I'm not much of a schemer yet
 
Zee
@LeakyNun stop acting like your rational
If you can't understand what's happening now, then how can you expect yourself to discover the secrets of knowledge?
 
7:00 AM
sorry i was out a bit,Now by plotting $-ax + x^3$
 
I mean I don't think Leaky's transcendental so... probability of being rational is not trivial
 
and by varying we see that fixed points collapse
and stability changes
but still
we ask how can we take negative values of $a$
why
because one of the fixed points ids $x = \sqrt{a}$
and if a is negative we see the problem!
@LeakyNun
so i am stuck here
 
@BAYMAX there is no problem. there is one real fixed point x=0 and two complex fixed points.
 
but analytically i am facing the square root thingy
oh
but graphically it is ok
gotcha
my mind got fuzzy
thanks
 
Zee
@Daminark I haven't given up on you yet
 
7:04 AM
With respect to being rational? Or something to do with algebra?
 
Zee
@Daminark with respect to your pursuits of the truth
Mathematical or otherwise
 
Well, I thank you. Hopefully this hope shall never die
 
Zee
Well you seem like a smart kid, id hate to see you waste your time on dank stuff, like topos theory or stacks
Speaking of wasting time
I gotta go learn some math
 
7:20 AM
I don't know anything about stacks or topoi, I just do it all memetically. I don't know what I'm going to end up going for, but I will say that I have less concern than most about things being practical to society, nice as that may be, and more about cool ideas coming together. And see you!
 
@SoumyoB So take an random variable $X$ such that $|X|\le1$ and $E(X) = 0$. First we see that $E(e^{tX}) \le e^{t^2/2}$ for all $t\in \Bbb R$ by convexity of the exponential.
Now take $X_1, \dots, X_n$ centered independant random variables such that $|X_i|\le a_i$ for some $a_1, \dots , a_n$, and finally let $S_n = X_1 + \dots + X_n$. From what we just did we deduce $$E(e^{tS_n}) \le \exp\left({t^2\over2}\sum_{i=1}^n a_i^2\right).$$
Thus by Markov inequality, for $\epsilon,t\in \Bbb R_+^*$, we have $$P(|S_n|\ge \epsilon) \le 2\exp\left(-{\epsilon^2\over 2\sum_{i=1}^n a_i^2}\right).$$
We then apply this more specifically to a sequence $(X_n)_{n\ge 1}$ of random variables following the same Bernouilli law of parameter $p$ to find strictly positive reals $\epsilon_n$ such that $$P\left(\left|{S_n\over n}-p\right|\ge \epsilon_n\right) \le {2\over\sqrt n}$$ for $n\ge 1$.
This is just one example among many in an exercise sheet
Forget that $t$ in the second message btw
 
Hey there @Steamy!
 
Morning
 
How's it going?
 
7:36 AM
Okay :P
 
:thumbsup:
 
What about you @Dami ?
 
And you? Got through your AlgTop troubles yet?
 
Doing well @Astyx, thanks!
@Steamy Yeah I finally figured it out with the help of Balarka
So, the goal was to prove that $[\Sigma X, Y] = [X,\Omega Y]$
The idea is that you can think of a map $f:\Sigma X \to Y$ in terms of $f(x,t)$, where the relations we mod out by guarantee a few things
First, that $f(x,0) = f(x',0)$ for any $x$ and $x'$
(Similar for 1)
And that $f(x_0,t) = f(x_0,t')$ where $x_0$ is the basepoint
So you have a loop of based maps, and thanks to the compact open topology, you get a homeomorphism
 
what's $\Omega Y$? @Dami
(and what to the square brackets mean as well?)
 
7:45 AM
Loop space, so the set of maps from $S^1$ to $Y$, equipped with the compact open topology
Square brackets mean the set of maps from the first space to the second, modulo homotopy
 
just looked it up, that's a weird topology
 
The point of that topology is that it makes the currying a homeomorphism
You know there's a bijection between the continuous functions from $X\times Y$ to $Z$ and those from $X$ to (the space of continuous functions from $Y$ to $Z$) via this currying
So that's kinda the point of the compact open topology, to make that natural bijection into a homeomorphism
 
makes sense
is that the coarsest topology turning that bijection into an homeomorphism?
 
I guess it makes sense... definitely not my branch of algebraic topology though :P
 
Actually maybe
 
8:00 AM
\o
 
Though the fact that this topology doesn't seem to be defined as such makes me feel like the answer is likely no
Like, product topology was basically defined such that projections are continuous
But this is more, there's a definition of some sort floating around which feels more like it just happens to work
 
yeah, that's why I'm asking
 
I mean that's my heuristic for believing that it's probably not the coarsest one, it just doesn't seem like that definition leads to it naturally, like it'd be a huge coincidence if it were
 
8:16 AM
@AlessandroCodenotti it's a generalization of the uniform metric topology
 
8:31 AM
My recent question hasn't received any feedback at all: math.stackexchange.com/questions/2334119/…
Wondering if someone would have a look and let me know of any problems.
0
Q: How to convert system of univariate functions into bivariate function?

Web_DesignerI have a bivariate function $f(a,b)$ that takes 2 positive integers as input and gives another as output. I do not know the "inner-workings" of the function — I can only see the value it returns when I give it any 2 variables. I would like to represent this function with an equation. My naive at...

 
 
2 hours later…
10:21 AM
Have you tried making a table of $f(a,b)$ for $a=1,2,3,4,5...$ and $b=1,2,3,4,5...$ and then looking up the values in the inverse symbolic calculator: https://isc.carma.newcastle.edu.au/advanced

The OEIS might have some values too.
 
 
1 hour later…
11:38 AM
Good morning chat.
 
Hi Farglem how's it going ?
 
12:11 PM
It's going. Quite tired.
Stayed up all night having an unnecessarily deep conversation with a friend.
Yourself, @Astyx?
 
What about if I may ask ?
I'm going through a low-moral phase, but I'm getting better, thanks
 
@Astyx An endlessly winding set of topics, from the question "do mathematical objects exist" to the nature of human consciousness, and how we as two individuals interface with the world around us.
And I'm glad to hear you're getting better. Low morale is something I struggle with frequently, I wouldn't wish it on anyone else.
 
I'd say you're quite lucky to have friends wanting to discuss these kinds of things :p
Yeah, it also happens quite frequently to me, but I find it particullarily low right now, both because of personal reasons and cause I'm sick
I'm thinking about seeing a psychiatrist next year
 
I'd recommend it. Help always helps, or at least can't hurt.
Also hi @Daminark.
 
That's my line of thought. I tend to think I overestimate how bad I actualy am, which is why I didn't bother at first. But these keep on happening regularily, so I might just as well try
 
12:43 PM
@Astyx do you think you need medication?
 
I have no clue
 
why wait?
 
Maybe I'm just whiny and everyone experiences this
I have to take exams first
 
Perhaps it's stress
 
I have been feeling a little bad today too.
 
12:47 PM
are you physically active yet?
 
Me ? I go for a run quite regularly
 
a little exercise can't hurt
 
I do the walking thing. I don't do it to "cure" (whatever that means) myself "psychologically".
 
Let's say that I feel like my feeling crappy is a little more complicated than depression. There's a lot of things going on.
 
12:49 PM
Is the walking thing the one where you put one leg in front of the other and use this to go forward ?
 
Yes.
 
a regular sleeping pattern may help
 
I fixed it (temporarily) two days ago, because it interfered with my work-schedule on the morning. But also, this was not intended as a "cure".
 
it will take time for your body to adjust
then you'll feel better
 
What I am saying is, one of the things that's going on is that I don't want to feel better (presumably, better means being happy and functioning), if that's what lies on the complement of feeling bad.
 
12:56 PM
why do you not want to feel happy?
 
For a multitude of reasons.
lol that escalated quickly
no
 
ok
do you meditate?
 

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