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00:00 - 19:0019:00 - 00:00

00:10
what does it mean for a metric to take on finite many values
?
The domain of the metric is finite?
00:28
I would interpret that to mean having finite range, but as always, this depends on context
 
1 hour later…
01:41
Hey, guys, I posted a question about Integral Transforms, but no one answered that. The thing is: can I create a Integral Transform? Can I set K(s,t) the way that I want? Does any of you have a good book recommendation about ITs? I do know the applications, but wanted to try things out.... And also I have no one to talk about this, so here I am. Any help would be great
01:52
@topologicalmagician No. The discrete topology comes from a metric that takes on two values. What happens if you try three values? Can you make up a metric?
That's a good question.
What is the name of this inequality (x_1)^2 + \dots (x_n)^2 \leq ( |x_1| + \dots + |x_n| ) ^2?
02:08
cauchy schwarz
Oh, no the squared is in the wrong spot.
 
1 hour later…
03:16
Would the statement "For every field $\mathbb{F}$, the only solution to $3x = x$ is $x = 0$" be FALSE since in a field with only 2 elements such as $\mathbb{F} = \{0 ,1\}$, $x$ simply cannot be equal to $0$ since every element has to be distinct?
I don't quite understand why $x$ can't be 0 in the case for the field with 2 elements.
because if we are given that the field contains only 2 elements, isn't every element supposed to be unique?
What do you mean by "isn't every element supposed to be unique"?
Every element in the field?
What does that mean mathematically?
03:22
because if we have a field with two elements only, if one of them is equal to the other, then it contradicts the fact we have 2 elements in it, isn't it?
Wouldn't it just be clearer to say that both x=1 and x=0 satisfy that equation, and thus the statement is false?
This "uniqueness of elements" thing is vague and doesn't seem to be needed.
Yes, it will be much clearer indeed! So let me just make sure I understand the reason x is also 1.Would it be a solution because in a field with 2 elements such as $\mathbb{F} = \{0,1\}$, 1 + 1 = 0 and 1 corresponds to the multiplicative identity, thus x = 1? Or am I missing something?
03:44
I think you might be overcomplicating it.
Let's just think about the real numbers, R.
What real numbers $x$ satisfy the equation $x^2 = 2$?
$\sqrt{2}$ and $-\sqrt{2}$
How do you know that those are solutions?
I just took the square root of both sides
to isolate for $x$ and find what numbers will make $x^2$ = 2
Nice, so that gets you to arrive at the classification of the types of x. But is there an easy, naive way of checking if something is the solution of an equation?
I think I got it. Is that what you are basically trying to imply $3x = x \iff 2x = 0 \iff x(1+1)=0 \iff x(1+0) = 0 \iff x(1) = 0$?
and in the $x^2 =2$ example we could simply just $x^2 - 2= 0 \iff (x - \sqrt{2}) \cdot (x + \sqrt{2}) = 0$?
04:37
I was more or so just thinking "plug in $x=0$ into the equation, and plug in $x=1$ into the equation".
05:04
@Abwatts Remember that the crucial point in both your examples is that if $ab=0$ and $a,b\in F$ ($F$ a field — or more generally an integral domain) then either $a=0$ or $b=0$. So if $2=0$ in $F$, then $x$ can be arbitrary (and the field could have lots more than 2 elements). If not, then you must have $x=0$.
@TedShifrin So, in my example, since we're given the equation $3x = x$ and are told that for every field, the only solution would be $x=0$, could we just do the following to prove this statement is false? $3x = x \iff 2x = 0 \iff x(1+1)=0 \iff x(1+0) = 0$ so, $x=0$ or $1+0=0$ and by the additive identity axiom this becomes $1=0$. Thus we get $x=0$ or $x=1$?
I'm not entirely sure I fully comprehend how we can show this statement is false..
@Abwatts if you plug x=0 into the equation, what do you get?
05:23
$x+x+x = x$, so $0+0+0 = 0$ or $0+0 = 0 \iff 0=0$?
So what does that mean about $x=0$ and this equation?
Since plugging in $0$ into the equation leads to $0=0$ which is always true, would that be the reason $x=0$ is a solution?
Yes.
So then what happens when you plug in $x=1$?
$1+1+1 = 1 \iff 1+1 = 0$ which is also true in fields with 2 elements?
So to find all the solutions if we're allowed to work in all fields, we basically need to factor and solve for $x$ and check (by plugging the result back in) if this actually results in something that we know is true?
Take a step back and look at what you just did.
You just found two solutions for $3x=x$ in the field of two elements.
The statement is "for every field, the only solution to 3x=x is x=0".
Have you disproved this statement?
05:34
I see! So, since we found that $x$ also equals $1$, then this claim must be wrong
So was it a valid approach to factor $x$ like that $3x = x \iff 2x = 0 \iff x(1+1)=0$ to find all the solutions for the equation?
I don't quite see what the last if and only if gives you that you didn't have at 2x=0.
The idea is that in the field of 2 elements, "2=0"
Got it, I think I understand now. Thanks a lot!
05:49
:)
Can anyone help me learning a concept, actually I have an equilateral triangle does there exist a specific method to directly find the coordinate of ortho Centre of triangle, like we have in right angle triangle that ortho Centre lies on the mid point of hypotense.
06:28
Hi, I have a question about derivative of exponent in power. I want to find the limit of the following: $(2/3)^n$ as n goes to infinity, it equals to 0. But I still want to prove this fact. How to proceed?
(2/3)^n = ln 2/3 * (2/3)^n if we use L'Hoptial's rule and it doesn't work if you continue..
Now, if you use the e^ln, then e^{n * ln 2/3}
 
1 hour later…
07:40
What's the general notation for "abelianzation" ?
For example, for a group $G$, sometimes I see $\tilde G$, sometimes I see $G_{ab}$
 
2 hours later…
09:22
@Andrews I would write $G^{ab}$ but it’s a matter of taste and consistency
Which is coincidentally also the reason I hate mushrooms
09:34
that was unexpected
looks like I stumbled into European morning time
Can you use consistency like that in English? I thought texture would be the correct noun
09:48
@AlessandroCodenotti he's English so one wouldn't doubt him
also yes
Aye one can use it like that
Also trains are lame in this country
@LeakyNun Good points
@EdwardEvans is "this country" Germany or the UK?
Germany lol
The train was over 30 mins late
And now I’m gonna miss the start of a Seminarvorbesprechung
I agree then, but they still seem much better than the Italian ones to me
Wow really
I was ready to firebomb the train out of anger
10:05
@EdwardEvans Are you sure you're not from northern Ireland?
Hahaha
Alas, I am from England
The crucible of Brexit
I thought everything is on time in Deutschland
grave misconception
German efficiency is such a lie
I have no idea how they managed to convince the rest of world
10:24
operation paper clip went very well I'm guessing that might be one reason
well results wise anyway naturally it was a human rights violatey kind of nightmare depends how you look at it really
anyway if I can get out a proper proof for this ive got a very solid proof for the tpc I mean I think there are already lots of proofs but anyway this is the last bit really
$${\Biggl\{ \frac{2n^2\pm9}{9}-\Bigl\lfloor\frac{2n^2\pm9}{9} \Bigr\rfloor:n \in \mathbb N }\Biggr\} = {\Biggl\{0,\frac{2}{9},\frac{5}{9},\frac{8}{9}}\Biggr\}$$
10:41
0
Q: $ \int_R | \frac{ \cos(x) + \cos( \sqrt 2 x) + cos( \sqrt 3 x)}{1 + x^2} | dx = ? $

mickIntegrals where the integrand contains an absolute value can be very hard or impossible to express in closed form. Computing the zero’s of the integrand might help. But what if the zero’s have no closed form or there are infinitely many ? We can express any single zero by a contour integral but...

Hi all. Any ideas ?
What does it mean for a ring to embed into another ring?
i.e. " ring R embeds into ring S"
@user injective ring hom
Thanks!
So R embeds into S iff there exists a injective ring homomorphism sending R to S ( not necessarily surjectively). Is this correct?
@LeakyNun
Many thanks
11:18
when people knew you are a philosopher, they actually treat you as a philosopher
all jokes dropped and even if the question seemed to have no real consequences or in some cases, mathematical consequences, a fruitful discussion follows
after all, deep down, we are all philosophers
12:10
@Alessandro I have a seminar topic! Brauer Groups of local fields
I guess
As long as you like it!
I think it’ll Be cool
I also have a seminar on local class field theory and my talk for algebra is right at the end so I’ll have some experience with local fields by then lol
Nice
I won't any seminar in the next semester
I might not do any course at all to be fair
12:29
So you can concentrate on your thesis?
 
1 hour later…
13:57
Yep
Also the courses next term are awful since there's no more logic or set theory
I might do geometric measure theory though
14:35
You may be interested in the following question.
3
Q: How to estimate the capacity of a neural network?

jaeger6Is it possible to estimate the capacity of a neural network model? If so, what are the techniques involved?

15:29
@Alessandro fair enough!
@EdwardEvans local fields are easier than global ones
the dream
Can integral curves always be turned into geodesics?
I hope the talk on completions won't be too advanced for me, lol
hahaha
I think I asked in the seminar already, but how can projective limits come after infinite Galois theory?
I guess you can define the Galois group of an infinite extension without bothering to mention that it's an inverse limit, but that seems weird lol
15:40
yeah I agree
I should have gone for the Galois theory talk
to go full Grothendieck
Definition 1: A Galois category is a category equipped with a fiber functor satisfying the following properties ...
I like that your comment to me taking local Kronecker-Weber was "Gönn dir"
That feels ominous
lol that'd be a nice first talk
I mainly want to go to the seminar so I have a good excuse not to visit the numerical analysis lecture
a fine reason
Oh yeah I got the final Vortrag in both seminars
damn
15:46
now you have to use the Brauer group of a local field to prove local Kronecker-Weber
which is actually reasonable
:D that'd be cool
I think that's maybe what Vogel has planned for ANT2, among other things
Maybe
I'll have ANT2 and AlgGeo1 in the same semester lol
he's an algebraist, so he probably won't do the L-functions approach to LCFT in ANT2, Lubin-Tate-theory is covered in the seminar, so among the major modern approaches to CFT that leaves only group cohomology
that's cool, some group cohomology will appear in the algebra seminar too
15:51
I have to say the cohomological proof of GCFT was pretty tough
LCFT was alright
Well I'm up for getting psychologically destroyed by ANT so that's fine
if you set up the right machinery, it applies to both cases, so that's nice, it's just that checking the axioms is harder in the global case
also there were so many people for the algebra seminar that we're probably going to end up splitting each of the Vorträge into 2
oh wow
I like your random Germanisms
Lol it just makes it easier
15:53
German is so terribly precise sometimes, how do you even translate "Übungsgruppenleiter"?
Also, Böckle mentioned that instead of splitting the Vorträge, he might end up splitting the seminar entirely and offering a seminar on Darstellungstheorie and another on Galoiskohomologie
Also
Exercise group ladder, obviously
lol, ladder
sounds good, then you can do three algebra seminars in one term
can't have enough of those
hahaha
perhaps
15:54
isn't it a form of TA?
yeah I guess
though we also have "teaching assistants" who create the exercise sheets and coordinate the Übungsgruppenleiter, typically PhD students or postdocs
so there's some confusion
just adapt the German and call those ÜBER-TAs
hahaha
Oberübungsgruppenleiter
Hallo, ich wollte mich melden wegen einer Oberübungsgruppenleiterpositionsbewerbung
hahah I swear this conversation happens once every few months
Sie werden demnächst eine Oberübungsgruppenleiterpositionsbewerbungsverlaufsbenachrichtigung bekommen
@EdwardEvans I think the first three talks of that Böckle seminar or something like that are covered in my blog, lol
16:03
Yeah I thought that :P
I'll cite it as a source hehe
nice, my first citation (apart from some Italian bachelors thesis who apparently cited my blog)
hahaha nice
I actually didn't get the Vortrag I wanted
didn't you get local Brauer groups?
or is that not what you wanted?
Yeah I got that one, which is good anyway, but it was added at the end because of the amount of people in the seminar
How can you not want $\Bbb Q/\Bbb Z$
(spoiler alert)
16:05
I want that one anyway hahaha, it'll be cool, but I originally asked for Br(L/K) = H^2(G, L*)
ah, local Brauer groups are more interesting, I'd say
more arithmetic, too
That's good then hehe
I have no idea what to expect from analytic number theory
except lots of nested logs
not necessarily
you might do some stuff you've seen before, like Dirichlet L-functions or modular forms, too
Kohnen really likes the latter
oo nice
yeah
that's fine then heh, I need to actually finish learning the modular forms material first though lol
the exam was today
how did it go?
16:10
I didn't sit it but apparently it was kinda hard
did you have to prove the Fourier expansion for Poincaré series?
lool
I didn't see the paper
but I assume not
behold the beauty that is the Fourier expansion of the Poincaré series:
$$P_n(z)=\sum_{m \geq 1}\left[\delta_{m,n} + 2\pi \cdot (-1)^{\frac k2} \cdot \left(\frac mn\right)^{\frac {k-1}2} \cdot \sum_{c \geq 1} \left( \frac 1c \cdot \left(\sum_{\substack{d (\operatorname{mod} c) \\ (c,d)=1} \\ d \bar{d} \equiv 1 \pmod c} e^{2\pi i \frac{md + n\bar{d}}{c}} \right)\cdot \left(\frac{2\pi \sqrt{mn}}c\right) ^{k-1} \sum_{\ell \geq 0} \frac{\left( -\left(\frac{2\pi \sqrt{mn}}c\right) \right)^\ell}{\ell! (k-1+\ell)!} \right)\right]e^{2\pi i m}$$
hahaha yeah I've seen it, we proved it in the lecture
really concise formula, I like it
16:11
aye
better than Euler's identity
Kasten said "Now.. I know what you're thinking.. how the f*** did anyone come up with that.."
Easy to remember
which I guess was a nice pedagogical point to make lol
@LeakyNun Normal chess mostly
16:13
@Balarka there's a NT research seminar here next term that has some interesting topology in it: Dehn twists, mapping class groups, intersection cohomology... though I won't give one of the topological talks
only beaten in concision by the formula for degree 4 polynomial roots
little scared since I know nothing about these things
but it's amazing you can use them for NT
wat is a topolgy
what the hell is that?! XD
@LukasHeger Wow cool
I know intersection cohomology can be used to prove Weil conjectures on singular varieties over a field of characteristic p or something.
I should learn IH properly. Soon.
16:15
oh wow, I didn't know that
I have been saying that for a year now
@Lukas Yeah I think this is Deligne-Goresky-MacPherson
@Balarka tbf I've been saying "I should learn something more than definitions" about topology since about my second year in university
I really struggle with topology unfortunately hehe
Aight I'll pop in and out of the chat for the next couple hours. Going to study for a statistics quiz
16:17
ew
actually the seminar uses a crazy amount of cohomology theories: singular, smooth De Rham, algebraic de Rham, étale, crystalline, intersection homology
It's not too bad but I'd rather do something else :P
20th century mathematics = cohomology
I am pretty sure most people will agree
It's just all about building good cohomology theories
Or proving things about them
16:35
What are some interesting probabilistic proofs you came across?
if a dating profile only shows half a face split down the y axis is it a block worthy offense to ask if she smelt toast when she had the stroke? American cartoons corrupted my mind for starters, and it's not like it was offensive, I mean it was merely an expression of needing to see the other side of the axis before consensual intercourse could be considered
I suck. I'm in the wrong ear.
Weierstrass approximation theorem from Chebyshev is old, but gold
One blocks at one's own discretion, if she decided it's block-worthy then it's block-worthy
Although tbf implying someone looks like they had a stroke on a dating site is a suboptimal strategy
If geodesics converge to a point does this always imply some sort of positive curvature?
@Thorgott I've already seen it, I'm looking for something an undergraduate-graduate can understand
16:48
that criterion is certainly met by the example
alternatively, you can look at various applications of the probabilistic method
17:09
@geocalc33 In general terms, yes. With negative curvature, geodesics spread apart.
Hello @TedShifrin can I ask a quick question about an exercise in your textbook?
LOL, sure.
if it's not too much trouble
So at the end of chapter 2, the last exercise #18
Which book?
multivariable mathematics
you ask to show that the solution of $Ax=b$ varies continuously with $b$ in $R^n$
A being an invertible nxn matrix
17:18
Ah, OK. Sorry, other things going on for a minute here. So what's your question?
But I'm not sure what you're asking precisely with "show that the solution of Ax=b varies continuously with b in Rn"
are you asking me to prove the continuity of the linear map $A$?
No, no. I'm asking for a proof that the solution $x(b)$ is continuous as a function of $b$.
If $(X,\mu)$ is some probability measure space, what is the precise definition of a probability distribution on $X$?
17:31
Sorry but I'm still confused Ted.
OK, so talk about it :)
Should I prove that if I find an element close to $Ax$ then I can find an element close to $b$?
No. If you wiggle $b$ a little, why does the (unique) solution $x(b)$ wiggle just a little? What in the problem tells you that the solution is unique?
that the matrix is invertible and thus one to one
right?
@user193319 a probability distribution is a measure s.t. the total mass of the space is 1
17:38
@Simone: Yes, one-to-one and onto. In fact, what's a formula for $x(b)$?
hi @Thorgott
@Thorgott Hmm...really? The source I'm working through seems to mean something different from that. See page 4 of arxiv.org/pdf/2001.04383.pdf
@Thorgott The relevant sentence is "Note that due to the normalization conditions..."
I'm not familiar with that noation, and I fell somewhat embarrassed by that, but I think a formula for $x(b)$ would be $A^{-1}x$
I "suspect" rather than "think"
Well, it has to be solving for $x$, so that can't be right :)
The definition is as I told you. I have no clue about that paper, but if the authors use some non-standard definition, then it's their job to specify that.
You start with $Ax=b$ and you know $A^{-1}$ exists. So what do you do?
17:42
Hey Ted
The question is cooler if you let $A$ vary instead of $b$ :P
That's the next part of the question, @Thorgott, although they don't have enough knowledge yet to prove things in higher dimensions.
I can't believe Rush Limbaugh won the medal of freedom smh
Oh, that's neat then
GRRR @geocalc. The world is crap.
@user193319 ok, I took a look, it's just saying that $\sum_{a,b}p_{abxy}=1$
17:49
@TedShifrin are you asking to demonstrate that because we know the inverse of $A$ exists, there must exist a real scalar constant that is the gradient of linear proportion of $x$ to $b$, and $b$ being in $\mathbb R^n$, concurrently making $x$ also in $\mathbb R^n$?
@Adam: I have no idea what you're saying.
ok well isn't the determinant a real scalar?
It's a polynomial in the ring your matrices are over.
What does the determinant have to do with anything?
as in given the knowledge that the inverse exists, $A \cdot A^{-1}$ must be $I$, and this is suffice to show that $x$ must belong to the same domain as $b$?
17:54
@anakhro: We're talking about a fixed matrix. Hush. This is confusing enough
@Ted I am sorry ;_;
@Adam: Both $x$ and $b$ are vectors in $\Bbb R^n$. So what?
determinant ---> top differential form, hehehe
@Simone: I need to leave soon, so let's get this settled. Everyone else is interrupting.
that's my point if $b$ had non real elements knowing $A$ to be invertible wouldn't be enough to know if it is in $\mathbb R^n$
17:57
@Adam, what does this have to do with the question that was asked?
but because we know $b$ to be in $\mathbb R^n$ and $A$ is invertible, isn't this enough to know the same about $x$?
@TedShifrin I'm sorry but I still don't get it :( I'll keep thinking about it, thank you for your time
btw your book is really cool
If you multiply $Ax=b$ on the left by $A^{-1}$, you solve for $x=A^{-1}b$. Why does this give $x$ as a continuous function of $b$?
because $x$ is a linear continuous function?
Right. $f(b)=A^{-1}b$ is a linear function, hence continuous (that follows from previous stuff in the chapter).
18:02
ok so can $A$ have imaginary elements or are they all real?
ahhhh.
It was easier than I thought
Thank you Ted.
Make sense now?
@Adam: In this case, everything is real. It would work fine with complex matrices, too.
OK, I need to leave for now. Bye, all.
yes it does, thank you.
um it would be polite to share simone seeings im the one still unsure
@SayanChattopadhyay it would be induced by a volume form.
18:07
I'm being asked to prove that the solution to the equation $Ax=b" with $x,b \epsilon R^{n}$ and $A$ an $nxn$ matrix varies continuously as $b$ varies
@Adam
i mean otherwise im probably going to spend the next decade as the last, all about $\mathbb N$ and considering matrices above and beyond being handy not really something I should worry about. really who wants to talk about dimensions of space anyone that does for too long becomes mentally ill
basically b is no longer a constant, but I need a function whose argument is b and.
well that is what I was told anyway. God damn Annetta god damn psychologists full stop actually
and as b wiggle in $A^{-1}b$ we see that the whole function is continuous because it is just the vector $x$ in $R^n$
I just don't see how everyone is able to keep physics and algebra separate in that respect it's so much simpler to observe and relate number patterns and ignore physics tbh
ok well im going to wiggle off to bed
18:11
night night
how do you do the capital R for the reals in Jax?
\mathbb{R}
thanks
@FuzzyPixelz I like Erdos' proof of an exponential lower bound on the Ramsey number using probabilistic methods
Any sort of probabilistic methods idea is great tbh
The general principle is if you want to show certain things exist you randomize it and prove probability of that random event is nonzero
Every time I log on this chatroom I always get intimidated and humbled by the amount of stuff I don't know. most often you guys talk about things that may as well be arab for me.
math is pretty humbling in general tbf
18:18
it is indeed
It's like fighting a 100kg black belt sometimes XD
@Thorgott This Bayesian statistic stuff is super weird, do you know anything about it
Hi Balarka!
Hey anakhro
What's up?
18:23
Just reading statistics
The basic idea seems to be the following: The setup is you have a parametrized family of distributions $\mathscr{F}_\theta$ on $\Bbb R^n$, $\theta \in \Theta \subset \Bbb R^k$, then given data $(x_1, \cdots, x_n)$, you want to ask what is the $\theta$ of the distribution from which this came. This is the goal of statistics, summarized. So in the Bayesian world they assume $\theta$ is also some random quantity, so has some "prior distribution"
Assume the pdf of $\mathscr{F}_\theta$ exists and is $f_\theta$, and assume $\theta$ follows pdf $\pi$, the prior distribution.
Then you setup the thing as follows: $X$ be the random variable whose realization is $(x_1, \cdots, x_n)$. Essentially, we are saying $X \sim f_{\widehat{\theta}}$ for some $\widehat{\theta} \in \Theta$, and we want to find this given that we know the distribution of the conditional $X|\theta$.
Did somebody read the work on Causation of Judea Pearl? it uses (and expands upon) Bayesian analysis.
The knowledge is $X|\theta$ and $\theta$ are independently distributed. So their joint pdf is $f_{X|\theta}(x|\theta)\pi(\theta)$. We want the pdf of $\theta|X$, the posterior distribution, so we can evaluate it near $(x_1, \cdots, x_n)$ to say what is the most likely $\widehat{\theta}$ from which it may come from.
That is why it's called Bayesian, given we know $X|\theta$, we want to infer about $\theta|X$
So, Bayes's theorem for conditional densities?
Yeah exactly
Which says if the pdf of $\theta|X$ is $\pi(\theta|X)$ then $\pi(\theta|X=x) = f_{X|\theta}(x|\theta) \pi(\theta)/\int f_{X|\eta}(x|\eta) \pi(\eta) d\eta$.
I know that, but nothing more in this direction.
18:33
The Bayesian estimate $\widehat{\theta}$ for $\theta$ is just $\Bbb E(\theta|X = x)$.
Which we can compute from above
How does one compactify the first quadrant of the real plane?
You can do it by adding a point at infinity
18:49
Can I also do it by adding 3 points at infinity? one on the y axis, one on the x axis and one at infinity along the line y=x?
Hello everybody!!
Can anyone help with the inverse laplace transform?
@geocalc33 How would you define the topology on that space?
I can not prove that the integral on a small circle tends to zero. I have it tends to infinity))
2
Q: Mellin/Bromwich Integral (Inverse Laplace Transform) problem

EugenSI have a solution in laplace images: \begin{align} &p_f(x,s) = - \frac{1}{s}\frac{b}{a} \frac{1}{\sqrt{\frac{s}{a}+ \frac{b}{a} \frac{\sqrt{ s}}{1+c\sqrt{s}}}} e^{-x \sqrt{\frac{s}{a}+ \frac{b}{a} \frac{\sqrt{ s}}{1+c\sqrt{s}}}} \label{pressure_frac_sol:laplace2c}\\ &p_r(x,y,s) = - \frac{1}{s}...

If anyone helps, it will be very cool!
maple could not
3
Q: Many point compactification

user16015If $X$ is a noncompact LCH space (locally compact, Hausdorff) then its one point compactification is $X^*=X\cup \{\infty\}$ with topology $\mathcal{T^*}$ given by $U \in \mathcal{T^*}$ iff either a) $U \subset X$ is open, or b) if $\infty \in U$ then $U^c \subset X$ is compact What whould be t...

Looks like there is no four point compactification
Interesting
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