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3:00 PM
@TedShifrin yes, I think the law of non-contradiction fits
 
Hi @Ted.
 
ok, I concur, @skull. But, honestly, I have never taught at this level of pedantry :P
 
@skull: How do we do "not" in LaTeX ... using \sim leaves too much space (it's a binary relation).
 
@TedShifrin \lnot $\lnot$
 
3:02 PM
oh, what if I want the centered tilde for "not"? :D
$\tilde{} P$
ah, I got it.
I never, never, never write symbolic mathematics. :P
 
@TedShifrin Every now and then one needs a $+$.
 
I've missed you, @DanielF :D
It's not as fun or as educational here without you and Pedro ... :(
 
agreed
 
Mike can also be fun and educational.
 
Of course, it appears that @Pedro is going entirely over to the dark side (more and more algebra) :D
 
3:05 PM
Ah!
 
Mike has mostly surpassed my knowledge of topology already.
 
@TedShifrin Not so long ago he had some analysis questions in the mod chat room.
 
MichaelAlbanese is a good guy, too.
 
Huy
@TedShifrin: Hm. Maybe I'm fixating on the inclusion map too much but I don't see why its differential should be an isomorphism (to apply inverse function thm), I guess I have to look for a different map?
 
In the mod chatroom? Why are there questions in the mod chatroom?
 
Huy
3:06 PM
Maybe I can do something with the charts?
 
More to the point, think about the tangent space at $p$. What must be true about it?
 
@TedShifrin Well, if he wants to ping me in a lower-traffic room, that is a pretty natural choice.
 
ohhh, he pinged you specifically. Got it, @DanielF.
I try not to interlope in there.
 
@TedShifrin Is there any place in math for pedantry such as this Professor?
 
@TedShifrin I mean the real mod chat, private, unless you get yourself elected, you can't interlope in there ;)
 
Huy
3:08 PM
@TedShifrin: Do you mean in very general or in this specific case?
 
It's really more philosophy of mathematics these days, @skull. Perhaps it's discussed in an introductory logic course. We don't have one of those in the math department here. But some departments do.
oh, that one, @DanielF. Good, I prefer to antelope.
This specific case, @Huy.
But, yes, ultimately, you'll want to work with a chart, along with my comment, @Huy.
 
@TedShifrin Never tried antelope, I stick to beef, duck, mutton and such.
 
steak and potatoes?
 
Huy
I don't really know what should be special about the tangent space in this case. :(
 
Well, @DanielF, you might even like a conch ... on the head.
Think about our conclusion we want, @Huy. What do you know about any $k$-dimensional subspace of $\Bbb R^n$ ?
 
Huy
3:13 PM
It can be projected to a standard one? I have no idea where you're going with this so I'm just guessing really.
 
It projects isomorphically to at least one of the standard ones. Prove it.
I guess @Huy doesn't like my lead-a-horse-to-water teaching style :D
 
Huy
I myself do lead-a-horse-to-water teaching but I feel like you're placing me in the middle of the desert and saying "go to the water, it will help you" :D
 
LOL ... Well, it's very hard to know what wavelength you're on. But I put you next to the water, now, so get to work drinking.
 
Huy
Maybe I'm just overthinking stuff or so, that's why. Give me a second.
 
Yes, I think I'm being more down-to-earth and you're being more abstract :D
 
Huy
3:16 PM
Can I just use a matrix for basis transformation and the matrix will be square and invertible so an isomorphism?
 
Well, I was thinking of a matrix, but it wasn't square. What matrix precisely do you propose?
 
morning chat
 
hi pal
 
heya @Semiclassic
 
question: what behavior does a suspension for "voting irregularities" typically reflect?
(also, don't have a great connection right now, so i'll probably disappear soon)
 
Huy
3:20 PM
Oh, yeah it's not square because we're in a subspace. So it's $n \times k$ and I want to take the basis of the subspace to $e_1, \dots, e_k$. The other entries I'd choose as zeroes, but I don't know if I can call it an isomorphism then if I take a $k \times n$ matrix to get back to the subspace?
 
@Semiclassical @DanielF needs to answer that
 
@Semiclassical serial down-voting
or up-voting
 
So think about the $n\times k$ matrix whose columns give a basis for the subspace. What do you know about this matrix?
 
kk. i noticed it on someone's account and was curious
 
I've been the victim of serial downvoting a few times, @Semiclassic.
 
3:21 PM
kk. i noticed it on someone's account and was curious
 
Huy
It has rank $k$.
 
@Semiclassical Using sockpuppets to inflate one's own reputation, or targeted voting (up or down) towards other users. Those are the usual cases.
 
Huy
Or $\geq k$ just to be sure.
 
OK, which tells you what? @Huy
 
i don't really get why people would do it. a fit of pique?
 
3:22 PM
Can't be bigger, it's only got $k$ columns :P
 
Huy
Right, there was something about column rank = row rank.
 
Correct @Huy.
 
Huy
But that requires Axiom of Choice, no? :P
 
NOOOOOOOOOO
 
Huy
:D
On a more serious note, the rank is $k$ so it's injective.
 
3:23 PM
BTW, I put an apology for the price of the stupid book on that posting you linked me to yesterday, @Huy.
 
Huy
That's why I linked it to you, secretly.
 
Remember what you're looking for, @Huy. Keep the goal clearly in mind :D
 
Huy
I want an isomorphism.
 
To what?
 
Huy
To the standard $k$ dim linear subspace
 
3:24 PM
To a standard ...
 
Huy
a
 
So how do we decide which one?
 
Huy
Use Axiom of Choice.
:D
 
smacks Huy hard
 
Huy
How do we decide which one. Does it matter?
I mean which one?
I'm confused.
 
3:25 PM
How can I find the closed form of the recurrence relation: $a_{n+1}=\frac{1}{1-a_n}$ ?

I found the following values:

$$a_1=3$$


$$n=1: a_2=\frac{1}{1-3}=-\frac{1}{2}$$

$$n=2: a_3=\frac{1}{1+\frac{1}{2}}=\frac{1}{\frac{3}{2}}=\frac{2}{3}$$

$$n=3: a_4=\frac{1}{1-a_3}=\frac{1}{1-\frac{2}{3}}=\frac{1}{\frac{1}{3}}=3$$

$$n=4: a_5=\frac{1}{1-a_4}=\frac{1}{1-\frac{1}{3}}=\frac{1}{\frac{2}{3}}=\frac{3}{2}$$

$$n=5: a_6=\frac{1}{1-a_5}=\frac{1}{1-\frac{3}{2}}=\frac{1}{\frac{-1}{2}}=-2$$
 
No, it doesn't matter, but you need to tell me why there is one.
 
Huy
@evinda: It's quite obviously $$a_n = \frac{-((-1)^{1/3} - 3) e^{2i \pi n/3} -1 + 3 (-1)^{1/3}}{(2+i \sqrt{3}) e^{2 i \pi n/3} + 3 + (-1)^{2/3}}.$$
 
@evinda Since $a_4 = 3$, you get $$a_5 = \frac{1}{1-a_4} = \frac{1}{1-3} = -\frac{1}{2}.$$
 
WTF? @Huy
 
Huy
@TedShifrin: ?
 
3:28 PM
How did you get that formula?
 
Huy
@TedShifrin: A god whispered it to me in a dream.
(the gods name: Wolfram)
 
OK. Back to your task at hand.
 
Huy
Well just take the first $k$ canonical basis vectors of $\mathbb{R}^n$ and take its span?
 
Might not work, though.
 
Huy
Why not?
 
3:29 PM
@evinda try listing out the first 12 terms. do you notice something, and can you prove it?
 
@DanielFischer Oh yes, right!!!
@Huy Wolfram?
 
@Huy What happened to this?
 
Huy
To what, @TedShifrin?
 
I linked it.
 
Huy
Huh?
 
3:31 PM
@Huy <- Reply arrow
 
You mean you don't know about linking to previous comments in chat?
 
Huy
No, I'm rather new in this chat.
 
Wow, I thought I was the chat-stupidest person.
You are not.
 
@DanielFischer @Huy @Semiclassical And how could we find this formula without the use of Wolfram?
 
Click the arrow.
 
Huy
3:31 PM
BTW, I only see your LaTeX formulae in plain text, how can I view it properly?
 
See the link to LaTeX in chat on the right --->>>
 
Huy
That was a joke.
 
2 mins ago, by Semiclassical
@evinda try listing out the first 12 terms. do you notice something, and can you prove it?
 
growls
I'm thinking retirement from here might be a good idea.
 
Huy
Why does the column = row rank cause problems for that space?
 
3:32 PM
It doesn't cause problems. It guarantees you can find a correct standard $k$-dimensional subspace.
 
Huy
I still don't understand why mine doesn't work in some cases?
 
What if the tangent space is spanned by the last $k$ standard basis vectors?
 
Huy
That's what I was just thinking about.
 
@DanielFischer Ok, I will try to
 
Darn, @DanielF, you want us to be like elementary school children and look for patterns?
 
Huy
3:34 PM
Can I use the inclusion to figure out which standard basis vectors span the subspace and then use those?
 
No, you have to use our matrix we've been discussing for hours, @Huy. :D
The subspace will almost never be spanned by standard basis vectors, btw.
This is why I keep saying that linear algebra is the lynch pin for most of mathematics.
 
Huy
Hours? I guess time really goes by faster the older you get. No offense.
 
LOL
 
Huy
Urm.
 
I guess @Huy doesn't appreciate the art of hyperbole.
 
Huy
3:36 PM
So that matrix we've been using.
 
@TedShifrin I just want everybody to practice handwriting. It's an art in severe danger of dying out.
 
I'm still proud of mine, @DanielF, but I concur.
 
I'm not proud of mine. It would probably have been better if I had waited until school, but with elder siblings, you just start writing before.
 
Huy
The matrix we're discussing, why doesn't it already bring us to a standard subspace?
 
How?
 
Huy
3:39 PM
Oh, it doesn't.
 
Suppose our matrix is $\begin{bmatrix} 1 & 1\\1 & 1 \\2&2\\3&4\end{bmatrix}$. What subspace?
 
Huy
$\langle e_1, e_2 \rangle$ I'd like, but I don't know if that works.
 
Well, figure it out!
<--- continues to think most teaching of mathematics is at way too abstract a level ... even though @Pedro will disagree with me :D
 
Huy
I'd like to eliminate to rows to get to the desired space so row reduction would be something I thought about a while ago already but this seems so arbitrary.
 
So column rank = row rank guarantees you what, precisely?
 
Huy
3:42 PM
Two rows will become zero.
(exactly)
 
@evinda with regards to proving the pattern you should see from the first few terms, it's illuminating to try to write $a_{n+2}$ in terms of $a_n$, in as simple a manner as possible
 
But, more specifically, some two rows of the original matrix will ....
 
Huy
Become $I_2$?
 
well, ok, which means that some two of the original rows form a basis for $\Bbb R^2$, which means ...
 
Huy
Um.
 
3:44 PM
I do agree on the abstractness, @Ted. I'm teaching a summer class and didn't have adequate time to prepare, so I mostly used the material I "inherited" from what someone taught last semester. Too much detail into junk they don't need, and I'm definitely regretting it.
 
Huy
Again, I think I know the answer but just don't know where you're going.
 
well, @pjs36, even when I teach advanced courses, I do examples and I give quite concrete homework (along with more abstract homework). But I do like to be on my soapbox :P
Which rows? Well, in our case, @Huy, it's 1,4 or 2,4 or 3,4. The plane will project isomorphically on those standard two-dimensional subspaces.
 
Huy
@TedShifrin: I can check out which rows form such a basis and then take the associated standard subspace?
 
hi @Ted
 
Right @Huy.
 
Huy
3:46 PM
So yeah, I knew but just didn't think that's where you were going. :D
 
hi @Balarka
 
Nah @Ted, your soapbox is fine, I'm just going over my semester regrets now that class time is running out
 
Hi@Balarka
 
All along I was trying to find a guaranteed $k$-dimensional standard subspace, right, @Huy? :D
 
3:47 PM
I got a doubt you free?@Balarka
 
Huy
I think so, yes.
 
What are you teaching, @pjs36?
 
go on @Remember
 
Huy
So what's next, @TedShifrin? I need to leave in less than 30 minutes, so I'll try to think about it on my way.
 
It's called Intermediate Algebra, it's the lowest math class in the department. The students are actually doing much better in 5 weeks than any 15 week class I've taught, but I constantly feel frantic!
 
3:48 PM
So hard to teach that stuff, @pjs36, because they've already not learned it for so many years.
 
Let $A⊂X$ let $f:A→Y$ be continuous ,let Y be hausdorff . Show that if f may be extended to a function $g:\bar{A}→Y$ then g is uniquely determined by f @balarka
 
Now that you have (a) right $\Bbb R^k$, prove that locally your submanifold is a smooth graph over it, @Huy.
 
use what hausdorff means and don't forget the definition of continuity
 
Well, the advantage of these short-term courses is that students do literally nothing else (academically), @pjs36. But there's so little time to absorb the math.
 
@Ted I have two sections, and that's the case for the first section. The second section is part of "running start" program where it's about half kids who didn't even get a summer off after graduating highschool, and they're doing exceedingly well.
 
3:50 PM
Ah, @pjs36, but did they really belong in that class? Did some placement test say so?
 
Okay@Balarka
 
@TedShifrin That's the question, and I have no idea of the answer :) I'd have to guess our good buddy A. Computer probably placed them there.
 
University administrations seem to be trying to dismantle placement mechanisms, @pjs36. Sigh.
 
"Warum verwechseln Mathematiker Weihnachten und Halloween? Weil Oct 31 = Dec 25"
 
@Ted I also have to wonder if they're part of a learning community of some sort, because there's a TA involved (who I never really interact with), and they seem to do a bunch of stuff together. The whole thing is frankly very mysterious
 
3:53 PM
Yes, a lot of those "running start" programs have a lot of supervision and community interaction. Which is all good.
 
why good?
 
Das verstehe ich gar nicht @evinda
Because students do better with support systems and talking about stuff with other students, @skill.
 
I see
 
Even my quite brilliant multivariable math students work together a huge amount.
They learn more and they have more fun. Win/win. Unless there's direct copying, which I managed to convince them not to do.
 
Yeah, it (the "running start" program) certainly seems good for them. I just feel bad because I didn't have a ton of time to get the semester organized before starting, so things feel shakier than I'd like them to.
 
3:56 PM
The lower level a course, the harder it is to teach well, @pjs36, IMHO.
 
Yeah, and it's my first time teaching such a course without a coordinator; so I just frantically tried to steal whatever homework assignments (online, from past instances of the course) I could, and then teach to those. Not my proudest class, but it's going alright
 
@TedShifrin Mit Oct bezeichnet man eine Zahl im Oktal-System (Basis 8),mit Dec eine Zahl im Dezimalsystem (Basis 10).

Und die Zahl 31 (3*8 + 1) im Oktalsystem ist 25 im Dezimalsystem.
 
Huy
@TedShifrin: Oct 31 as in "31" in octal number system
 
@TedShifrin Es ist aber eher Informatik als Mathe
 
ohhhhh ... ich dachte dass vielleicht 31/10 = 25/12 :P
 
3:58 PM
@evinda Nah, just plodding along. Or being "persistent", as Bott would say. :)
@TedShifrin Sie meint, $(31)_8 = (25)_{10}$.
 
Huy
@TedShifrin: And then you checked with your calculator and realized the equation doesn't hold?
 
Precisely. I needed Mathematica for that, @Huy.
 
Huy
Most of my students need their iPhone to compute 67 * 10.
6
 
Oh, someone answered that. My chat had timed out.
 
Huy
(that's not a joke)
 
3:59 PM
@SohamChowdhury Sie :)
 
I thought that was true only in the US, @Huy. :(
Sie ≠ Er @Soham
 
@SohamChowdhury I am a girl :D
 
well. assumptions.
 
Huy
@TedShifrin: It depends probably, but my class majors in biology & chemistry so they don't really care too much about maths.
 
4:00 PM
Sexism at 15 ...
 
I'm sorry, @evinda.
 
@SohamChowdhury No problem :)
 
Well, we know that biologists and chemists need no math and definitely need no thinking skills :P
Anyhow, @Huy, you can report to me later on your proof :)
 
Huy
@TedShifrin: I actually attended oral graduation exams yesterday. There was a question "I flip a fair coin repeatedly. Let $X$ be the coin flip at which heads shows for the second time. What is the expectation value?"
 
I've been helping someone factor polynomials and whatnot, and I'm always a bit surprised when they use their TI-84 for 6 times 4. Is it possible that they're dyscalculic?
 
4:01 PM
OK, a fair question, @Huy.
 
Huy
@TedShifrin: Almost all students started by drawing a table, filling out $X = 2, p(X) = 1/4, X = 3, p(X) = 1/4,$
 
It's damn pathetic @Fargle.
 
There was some post on /r/math where someone from ETHZ was talking about tensor products of modules or whatnot. It emerged that person was a first-year undergrad and was doing a homework problem.
 
Huy
and then the teacher would interrupt and ask "so clearly $p(X) = 1/4$ for any $X$, right?
 
@Huy, confirm?
 
Huy
4:02 PM
and nobody could think of a reason why not.
that was bizarre to observe.
 
expected number of throws is 1/4 ?
I love Bayes's Formula for expectation, @Huy :)
 
@TedShifrin I try to be forgiving of it. No reason to make math look even more unattractive to the outsider by berating them over their times tables. But it is sad; it speaks to how terrible K-12 math ed is.
 
Huy
Or that the sum of the probabilities shouldn't be greater than 1.
The teacher specifically asked "so what will the sum of all the probabilities be?"
 
Yes, @Fargle, berating is terrible, but they'll never be able to do higher-order thinking.
 
Huy
and one student answered "I guess infinitely large. but that seems unlikely"
 
4:03 PM
LOL
Sigh.
Probability is harder than it seems. Even for college students.
 
@TedShifrin I agree completely. If you don't have good arithmetic skills, algebra becomes orders of magnitude harder.
 
Huy
I know
 
Not to mention things built on algebra, like chemistry and calculus and physics.
 
To say nothing of calculus, linear algebra, basic probability, and beyond.
 
Huy
I'll talk to you later about the proof.
Need to go now. Cya all.
bye @TedShifrin
 
4:04 PM
Why does MSE have pretty less questions in topology?
 
So, @Fargle, I keep waiting for more interesting diff geo discussion. Did you get your job?
 
Bye @Huy
 
Relatively few college math students take topology, @Remember.
 
@TedShifrin Unfortunate circumstances have taken place, and I'm not in Knoxville right now.
I was about to get the job, though. :I
 
Oh oh ... are you ok, @Fargle?
 
4:05 PM
I'm fine. I screwed up in school this past year, that's basically it. My family is deciding whether to send me back.
 
They leave such a beautiful subject... I thought topology is some highly taken and talked about subject @TedShifrin
 
You don't seem like a screw-up. I don't want to pry, but let me know if you want to talk privately, @Fargle.
For the most part, in the US, only math majors who want to go to graduate school take topology, @Remember. Not universally, but mostly.
 
@TedShifrin I don't seem like one because I don't present myself like one in here, haha.
 
it also depends a bit on what is meant by topology as well
 
4:07 PM
I found that theorem pretty remarkable.
 
Well, @Fargle, if there are mental health issues, get them sorted out and go to school when you're ready to excel.
2
 
It's some sort of cool indicator function.
 
he means point-set, @Semiclassic.
 
i figured
 
Nice one @Soham
 
4:08 PM
which, as a physics grad student rather than a math one, i never took
 
@SohamChowdhury Is this Urysohn's, but stated for metric spaces rather than normal spaces?
 
The amount of point-set in basic analysis should be enough for you, @Semiclassic.
 
@Rememberme it's just $$\frac{d(x,B)-d(x,A)}{d(x,B)+d(x,A)}$$.
@Fargle ah, wouldn't know yet.
 
@Fargle Yes .. You can use the metric to construct it quite concretely.
 
but the next thing that's proved is Tietze (for metric spaces).
 
4:10 PM
i imagine you're correct. analysis proofs really don't come up in physics
 
Tietze?
 
Alright. I thought it looked sort of familiar. Usually I see $[0,1]$ rather than $[-1,1]$. But yeah, Urysohn's is used for the Tietze extension theorem, so that makes sense.
 
And the One True Source Of Knowledge tells me that Urysohn is used to prove Tietze generally.
 
i've found that some knowledge of algebraic topology, on the other hand, does come in handy
 
Correct, @Soham.
Yes, @Semiclassic, which means you need a dose of abstract algebra.
But not very much point-set.
 
4:11 PM
right. had that in undergrad
 
@TedShifrin I once used to ask how much and got shouted at. :P
 
and you naturally encounter some group theory in physics
namely quantum mechanics
 
math.stackexchange.com/questions/1321577/… I really liked this one though....
 
Depends on what level of algebraic topology ... But what a physicist would need is different from what a homotopy theorist would need.
 
@Rememberme Tietze extension theorem: if $A$ is a closed subset of a normal space $X$, then any continuous function $f : A \rightarrow \Bbb R$ may be extended to a continuous function $F : X \rightarrow \Bbb R$ such that $F(a) = f(a)$ for all $a \in A$.
 
4:12 PM
no doubt about that.
 
Yes, and character tables, which we don't do in undergraduate algebra at all, @Semiclassic.
 
Ahh.. Its like that
 
OK, time for lunch and shopping. You all misbehave without me. @Fargle, email me if you want to.
 
@FargleI have just started quotient topology
 
eh, i never actually had to do character tables in QM. though i imagine i would if i was doing either physical chemistry (spectroscopy and the like) or particle physics.
 
4:13 PM
@TedShifrin I likely will.
 
my representation theory in particular is pretty weak
 
@Rememberme Ah, yeah, that's a little ahead. You're working out of Munkres, right?
 
So is mine, @Semiclassic.
Bubye for now.
 
Yes @Fargle
Bubye @Ted
But thats a supplement chapter in the 1st chapter
@Soham Which book are you using
 
later @ted
 
4:15 PM
I'm quite enjoying Armstrong right now, @Rem. I know, I know, I've probably looked at every intro topology book by now, haha.
 
Not a prob.... Well how did you reach the question I was doing?@Soham
 
So I'll start the connectedness/compactness chapter tomorrow. It's hopefully going to be smooth sailing from there.
 
I think you require Hausdorff spaces
Hello@ᴇʏᴇs long time no see
 
Hi @Rememberme I forgot who you are
 
@Rememberme that limit point business? oh, the exercise I was doing said to prove it for $\Bbb R$, which is Hausdorff (I just learned what that is).
 
4:17 PM
@Rememberme Oh
 
Okay okay.. Have you done product topology@Soham
And quotient
 
Nah, although I have the briefest idea what those are (I've skimmed them once from Lee a long time back).
 
Skimming eesh......
 
Eh, I like knowing what's on the menu.
 
Okay gtg.. dinner calling
 
4:23 PM
@Ted: apart from the cofinite topology, are there any other non-Hausdorff topologies I'll be able to understand?
 
4:42 PM
@Huy i m astonished not because decimal multiplication by 10, but how are students allowed to use iphone in class
 
5:13 PM
hey @Lembik
 
I think this is the classic, Consider $(0,1) \cup \{e_1,e_2\}$ and you will equip $(0,1)$ with the standard topology and the basic open sets of $e_i$ will be $(1-\epsilon,1) \cup \{e_i\}$ @SohamChowdhury
each $e_i$ acts as if they were equal to 1, but there are two 1's!
 
Sorry me, I read the Knuth Concrete Math and it looks to hard for me. Is there an helper textbook which would fill all the shortcuts that he makes?
 
5:33 PM
You should probably say what in particular you are having trouble with or why you are having trouble with it. It basically sounds like you need a new text book that covers things you don't understand.or the background assumptions which you are not familiar with (I am not familiar with the book)
 
I think that I am familiar with all prerequisites. I am just too slow to keep up the levels of complexity in the derivations. That is the trouble. The book is full of blah, which take days to decipher. I cannot struggle days at every page.
 
Math books typically are not made to be read quickly
 
'Tis true, what Paul says. I had a course where we deciphered one chapter of a book and a 9 page paper, in our 15 weeks together.
And hello there, Mr. @Paul
 
They consist of series of barriers you need to take. The ones in the book are too high.
 
@pjs36 Hello
Then find a different book... From googling around it seems like it is a computer science/ intro to discrete math book. There are tons of those around, not sure which are good though, or which don't go to fast for you.
Your question is very vague. What do you need in a book? what about the book makes it to difficult (proofs, techniques, arithmetic manipulations etc)?
If you need to learn everything in that book in a week or whatever (basically any time frame), you are setting yourself for failure...
 
5:59 PM
I forgot, who's the user on MSE who owns zerocollar.blogspot.in ?
 

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