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00:01
@Semiclassical @robjohn la fonction pour vous :D
@AMDG. You designed this calculator?
Yes
Wow great :)
So it draws functions I see
Indeed. Of course, I did not design Desmos; I just made it to work with bitwise logic so that I can see what it outputs because it does not support boolean algebra natively. :D
@AMDG this... this is insight into the deeper meaning of things
00:11
This function repeats itself by $\bmod 2^{\theta_{MAX}}$.
Cette fonction se reproduit par $\bmod 2^{\theta_{MAX}}$.
@AMDG. Great. Keep going
How's my French? lol
Mieux qu’ailleurs!
Noice
I quite like the succinctness of being able to use reflexives with se as opposed to saying his/her/itself in English I must say
lacks 'du coup' and 'euh', and ending with "quoi"
00:20
Trop de chic :P
more filler words = more gaulic precision
with enough style people will be compelled to believe you
00:45
Interesting. As is not too incredibly surprising, each graduation in the graph corresponds to the values in the range defined by $a = \lfloor\log_2(t)\rfloor ; 2^{a} \leq t \lt 2^{a+1}$ it would seem.
And then that is just multiplied by a power of two constant. The graph remains the same overall shape and the y value halves as $m$ increases.
Got it. (Note that $y$ and $m$ swapped places here) desmos.com/calculator/szbll30byy
following a standard math curriculum, do people eventually get good enough at real analysis that, say, thrown any theorem of the single variable material from Baby Rudin, they're able to prove it without any other reference than required definitions?
01:03
@Semiclassical @robjohn And there we have it. Solved for $y$ using the graph and minimal trial and error. desmos.com/calculator/twhups9prm
Now I just need to implement log2...
shin: i dunno if you'd get that only from a 'standard math curriculum', at least at the undergraduate level. even good analysis students tend not to be that proficient the first time around. some of those exercises are quite difficult.
i'd expect that most graduate students could handle it, though. at least those with some interest in analysis. after a second or third trip through that material it becomes very clear.
same with your average calculus book, frankly. the level of proficiency you need to get a perfect grade in calculus is somewhat lower than 'do any exercise in the textbook.' but not unattainable with further experience.
huh, that gives me some perspective. i keep getting stumped by analysis problems that look familiar but end up being soul grinders
@Semiclassical Something interesting I noticed is that you also get a Sierpinski triangle if you do $x \land x^2$ which, you probably weren't here when I mentioned it, but I noticed that the high bits of $x^2$ appeared related to $x - \neg x$ and multiplied by a power of two, but this means we can square things somehow using even simpler bit hacks and faster than O(M(n)).
others may have a different view. i think the ideal for an undergraduate textbook is that it be challenging on a first read-through, but manageable or even easy on a second or third encounter, perhaps after other courses.
parts of rudin are too challenging to be ideal in this sense.
Remember that Rudin is aimed at future math grad students, which is on average 10% of math majors at most.
01:13
rudin tends not to tell you where an argument comes from. i don't like 'chatty' textbooks or textbooks with piles of redundant examples, but one can assist the reader far more than rudin does without doing that.
My book is less arcane by far., but still challenging .
And Rudin was written in the 50s, when the math major was far more exclusive
when i first met rudin, i think two people in a class of about 30 went on to graduate school in math. most people were CS oriented or K12 education oriented and could not get past the expository style.
Even at Berkeley it’s ok for the honors section of 104, not in general.
a lot of people used ross in the normal 104.
Wade is ok. Abbott is now popular. Myself, I never taught the course. Spivak Calc w Theory was more fun for me.
01:28
woah! didn't know so few math majors go into grad school
you'd think that as a major, it would select for people with a theoretical bent
i think at some schools the math major is so small, and the student body sufficiently self-selecting, that it can be assumed a larger fraction of math majors will go to grad school in math.
when i was at berkeley, the computer science major was very selective due to high demand. you had to apply to get in. sometimes people would fail to get in and major in math simply because they'd taken a lot of the prerequisites already, and could still use programming in applied math classes.
sometimes the reason they did not get into the computer science major was low math grades. i knew several people who basically had the following: "oh crap, i got some bad grades in math. now i guess i have to major in math."
makes for odd encounters with rudin, to say the least.
01:46
Wow, Sierpinski is everywhere with AND. desmos.com/calculator/qlxryyppf8
Bruhhhh, graphs of integer functions are so cool.
err discrete functions applied to a wide domain
@shin Some schools have over a dozen options for math majors, only one being pure + proofs. Math bio, actuarial science, etc.
Math education, statistics, various applied math tracks…
The applied math major at UGA the last 7 years has only one slightly proof-based course required.
Good night everyone!
@leslietownes At most schools with small programs, they don’t get into much in the way of top grad programs. Williams, on the other hand, with star teachers and star students has (relative to school size) a huge major.
02:13
i was thinking of places like yale and princeton. are there many people who major in math there who aren't mini-mathematicians?
i don't think of those places as powerhouses of applied math or engineering or anything else that someone might take math for. but i know next to nothing about their undergrad curricula.
Yes, those are elite schools with top grad programs. But even there I would bet under 50%. Berkeley was about 20% in the 80s.
Brown is another, with separate applied department.
much lower when i went. i blame the internet and math bio.
No need to blame. Academic math is mostly a dead end, and COVID sure isn’t helping academia.
I’m just arguing that 80-yr old Rudin curricula need to change, if they haven’t already.
there were only about 40 pure math majors in my graduating class. most of them interested in K12 education.
rudin is not the right book for that audience.
02:30
9 hours ago, by BKJA1
Can we rearrange $\frac{Tan(x/2)(e-1)+\sqrt{e^2-1}}{Tan(x/2)(e-1)-\sqrt{e^2-1}} $ to $\frac{e+cosx+sinx \sqrt{e^2-1}}{1+ecosx}$
right, I guess applied math is also a very popular track
Far, far outnumbering
Not just “very popular”
when i graduated there were about 2x applied math majors as math majors. at time people interested in applications tended to just go major in the other thing.
even then they were the clear majority of math majors.
Applied math is not physics or engineering or biology.
Specializing in applied math is perhaps a more recent trend
Rather than specializing in a field in which certain math is applied
02:35
Yes, the world — politics not included — has evolved.
division of labour
CS at berkeley swept up most people interested in applications. i think it's the largest major in the college.
or was.
all of my knowledge is very out of date
Sometimes the divisions seem a bit arbitrary
An actuary is clearly different than a mathematician, but I feel like a statistician could be different degrees of “applied” vs pure
while i student i had no conception of how big some of the majors were. when i went to my wife's graduation in sociology, i could not stop thinking, "how many people are going to be walking across the stage? when is this over? i'm hungry." while hundreds of people got diplomas.
some schools have very strong actuarial specific tracks. uiowa did when i taught there. berkeley did not.
Look at UCLA’s math major options.
02:39
wow.
berkeley was putting together a 'math for teaching' track around the time that i left
03:19
@leslietownes did you see the arrow video?
AS now has support for linear arrows and curved arrows
I'm coding a unicode support widget now, so you don't have to leave the app to find a common unicode symbol
There should also be a LaTeX to unicode feature so that users can type in stuff like $\sum_{i=1}^n$ and it converts it to unicode for you
04:10
@shintuku I am not a mathematician, so take with a grain of salt. I imagine a better goal would be to be able to prove new things as needed rather than proof-on-demand for well known material. Clearly you need familiarity with the techniques used, but spend energy going forward, not dealing with history.
hey copper!
any suggestions on where I should start topology?
given that I know about metric spaces
noted!
Hi @shin
king koro
who's that ?
04:19
the protagonist, of course
Any suggestions on where i should start topology @shin?
Have you studied topology?
all i've done is metric space topology
yeah me too.
chapter 2 from Rudin
but I have skipped some exercise problems in continuity chapter from Rudin-the problems like completion of a metric space etc.
i did it from the first part of Gamelin Introduction to topology, but can't speak about the book for topology on other topological spaces that aren't metric. the parts i did were nice
In an exam I took recently, they asked this question -Let $C$ be a subset of $R$ endowed with subspace topology. If every continuous real valued function on $C$ is bounded, then $C$ is compact.
And I skipped it the moment I saw subspace topology
04:23
no clue what that is either heh
i would have just treated it as a metric space, maybe that would have been worth some points
@copper.hat I agree, but one needs practice (not rote memorization, however).
I discussed this with Leslie and he told me that subspace topology here is not to be confused and that consider $C$ as any subset of R with usual metric. And I think he's right. Leslie gave a hint to solve the problem and I solved it using that.
But then I thought of knowing topology more.
It’s easy. Nothing intimidating. Just intersect open sets in the big space with the subspace.
Munkres is the standard sophisticated text. Also look at more creative books by Ghrist and Colin Adams.
hello
More focused on interesting applications.
04:28
@TedShifrin hi professor Ted. Thanks for the suggestions :)
hello Euler 2
A bit off topic discussion for this room but does anyone here have experience with assembly? Just curious
I guess many would have
@TedShifrin I agree.
@Euler2 Long time ago. I suspect @robjohn is a better resource now.
I'd like to report one issue of LaTeX in chat bookmark: I'm using opera on macbook pro now. I noted that when I remove the bookmark bar and click on the bookmark, then the bookmark doesn't work. It only works when clicked from bookmark bar.
that is, I have to keep the bookmark bar and this occupies screen
I suspect that should be reported in Meta?
I was thinking to try my hand on it (actually it is for a purpose)
looking at assembly makes me feel like it's something hard to understand
I have no idea
04:34
Professor Robjohn handcrafted the bookmark so he might have a solution to that @copper
@Koro that is universal, Koro …
@Euler2 It is hard to understand without context, the above looks like it is setting up a stack frame for a procedure call.
It only works on desktop and mobile from bookmarks
just found a random image on the internet
On the ipad, I have bookmarks as a separate list and I can click there as long as chat is the main page
04:37
@Euler2 I would only learn assembler if needed. It is pretty low level stuff.
ok professor Ted, I'll keep the bookmarks bar then.
I have nasm and fasm, it has nice tutorials most of the programs work on my machine), but I can't find complete resources
I was looking in to operating systems and C stuff
You would need to get a manual for your particular processor.
I don't think I learn assembly for any other purpose; I use js and c++ most of the time
I don't have too much experience in low level languages, but I am learning
@copper.hat okay I would try to find one
got to go now
@Euler2 please tell me why c++ and not python
or vice versa
04:44
I use python, indeed
I know some of both, and some C also
Can’t you have a bookmarks list to click on?
You know, to cut an apple, you need a knife, but to cut a tree, you need an axe
But I am not from a CS background so I am confused at times why C++ and not python
and why not Java
all do the same thing at the end of the day right?
I love c++ (but I also hate it)
But I like the errors
c++ errors
04:46
@TedShifrin professor Ted, that's when it's not working on opera :(
@Euler2 I see so you get some bug and then like to fix them?
While in python, the bugs are "simpler" to fix?
basically I think, for no particular reason, c++ errors > python errors
and > means better
but the segmentation fault error is a demon
ok, i don't know much about that :( I had C in my first semester of engineering
I used c++ to write a simple compiler, ts to write a math parser, and python for 'others'
js frameworks are quite helpful to make nice looking guis
c++ is good for almost everything, that's why I use it
I am not good at C
I'm not good at any of the programming languages :(
I also won't call myself 'experienced'
04:59
@Koro The choice of language is usually dictated by other considerations. Broadly compiled code is faster but more work to write.
05:09
i use python because it was fast to learn and i could do useful things with it on the first day
also there's sagemath that looks like python
05:40
@TedShifrin Hello
@Koro if you keep it in your bookmarks and select it there, does it not work?
I just called it from a bookmark in a bookmark menu and it works there.
Howdy @JackOhara @robjohn
@TedShifrin evenin'
Ted!
I have a question that is driving me nuts
I may not help, but go ahead
05:48
lets say we are going to play a game of cards, you pick 10 cards and hide them from me
oh oh, not my stuff …. Go on. Probably robjohn will have it :)
I like to hide things
And you”re clever.
trying to formulate it in a good way one second
it is a probability thing
05:50
It works if "Mathjax" bookmark in in bookmark bar
6 yrs ago I taught that, but not my expertise!
so the deck of cards has 52 cards , you chose at random 10 cards, and i pick only 5 cards
and the object is to get as many correct as possible
From what’s left?
no from your hidden list
Correct?
05:52
correct means a match
You guess five cards, or you choose 5 of mine?
i think he means that he chooses 10 cards from the deck, leaving them in the deck, and then someone else chooses 5 cards from the deck, leaving them in the deck
i guess 5 out of 52
OK, you guess.
05:52
@robjohn If I remove the bookmark from the bookmark bar, then clicking on it doesn't work (screenshot 2)
Yes it is like putting all the cars on the table
you make a list of 10 cards
and i make a list of 5 cards
and see how many of those 10 i get
It works if "Mathjax" bookmark remains in the bookmark bar
3 mins ago, by Koro
user image
where are you accessing it to click on it?
So the list of $5$ and $10$ are independently chosen?
05:54
You want the expected value of the number of correct guesses?
Yes !
i want the probality of getting 0 correct to 5 correct
@JackOhara are you looking for the expected number of matches?
Here the LaTex is rendered if I click on "Mathjax" on top left
Sounds like a good time for indicator functions.
1 min ago, by Koro
3 mins ago, by Koro
user image
05:54
ah same question ;-)
am looking to how to calculuate that
let me tell you how i see it
C(52,10) is all the choices
you should get an expected $\frac{50}{52}$, I think
less than one match on average
I dont think so Rob
linearity of expectation should be at play here
lets put it this way, all the possible ways Ted can pick 10 cards is C(52,10)
05:56
ok then
but the thing is i only guess 5 cards
my solution is
The naive countimg isn’t going to work for this sort of problem.
So one has to select 10 cards then they are hidden and then what?
C(10,k) * C( 42, 5-k ) / ( C (52,10)
He guesses five cards at random and wants to know on average how many will match mine?
05:58
but it does not seem to work
How is that counting matches, Jack?
are you looking for expectation or probabilities?
we are counting probabilities Ted
Yes
that was an or question
No. Not the right probabilities.
05:59
I see
I want to think of me guessing 10 cards not 5
C(10,k) * C( 42, 10-k ) / ( C (52,10)
You’re still not doing expected number of matches.
you see C(10,k) gives me k correct guesses
C(42,10-k) gives me 10-k wrong ones
Copper, robjohn, and I all asked you the same thing, but you ignored the point.
@Koro each pick has a $\frac{10}{52}$ of being a match. $5$ picks should give $\frac{50}{52}$ by linearity of expectation.
sorry let me see the definiton of it
Expectation
06:02
@robjohn it is not any 5 picks, the 5 are distinct
Yes all cards are distinct
that is the miracle of linearity of expectation, it doesn't matter.
so if i ask this question
Yup, I vote with robjohn. I made a big deal about this in my course.
pick $52$... you get $\frac{10}{52}\cdot 52$
06:03
what is the probability of getting 5 correct?
If you do it by counting, as Jack wants to, you have a pain in the ass.
Oh, what is the probability of getting $5$? that is a different question
@robjohn For now, I'll pass on this problem :( as I'll have to revise some of these concepts.
yes that is what am looking for
I asked you explicitly … you lied, twice.
06:04
i want to find the probability of getting 0 correct , to 5 correct
I am so sorry Ted, really tired and did not understand
@Koro what happens if you right click (control click) on the item in the sidebar?
Just to be clear again, i want to find probabilites
@robjohn nothing happens sadly :( I have shared the screenshot also
From your question, you can compute what robjohn and I said, but it’s harder.
C(42,10) / C(52,10) is this correct ?
06:06
@Koro if you control click, a menu should come up. What does that menu offer?
for the probability of getting none corrrect
I excluded the 10 cards
seems logical to me
@JackOhara: sorry, let me think a bit. Splitting attention here.
@robjohn "Edit Bookmark, Move to Bin, ...etc"
No worries Robjohn
I am not sure why my intuition is not working for this problem
@robjohn Professor Rob, please ignore this now. I'll keep the bookmark bar. I have no problem in that.
06:07
@Koro none offer to go to that page?
no
seems quite reasonable thing that that formula should work , but if I do add the probabilites ,i do not get 1
Intuition is dangerous with probability. You overcount and don’t have the right sample spaces a lot!
So true Ted
You will never get 1!
06:09
sample space is C ( 52,10) right?
i mean you pick 10 cards out of 52
Do $\sum kP(k)$ and see if you get robjohn’s answer.
but why are they weighted?
That’s what expected value is.
the 10 cards bit is misleading. since the choices are independent you can suppose the 10 cards are the first 10 cards.
06:10
How to pronounce Dirichlet? Is it "Dirishle" or "Dirishlet" or "Diriklet" or "Duruklet"?
Good question. French name for a German.
Yes copper
it is supposed to be to find all the possible ways
lets make a new model of same situation
we have 1 to 100
Do small numbers, not huge.
Ok
1 to 10
the computer will randomly pick 5
and the game is to pick 3 numbers
I think your formula is fine, btw.
06:13
that is good news haha
@JackOhara: $\binom{5}{k}\left(\frac{10}{52}\right)^k\left(\frac{42}{52}\right)^{5-k}$ probability of getting $k$ matches
@Koro wikipedia lists dirishlet and diriklet as acceptable
audible 't'
thanks Rob but let me figure out why you found that solution
it sums to $1$ and gives the expected value of $\frac{50}{52}$
@shintuku I see, thanks. I always pronounced Dirichlet as "dirishlet" (without t) but then I heard someone pronouncing it as "diriklet" that's why I asked
06:16
But what is wrong with my reasoning ?
That’s choosing with replacement. Something’s weird.
the quotient is not correct?
i think he just meant 10/52
not C ( 10,52)
That bothers me, robjohn.
But my probability is rusty and I’m asleep.
haha that makes two of us
i am not convinced that one can ignore the difference between a distinct selection and an arbitrary 5 cards.
06:18
I might need to tackle this problem another day
copper they are all arbitrary
For fun, do the weighted sum with yours. I’m assuming you have a computer or calculator .
you mean you can choose the same card 5 times?
but one a set is chosen, we can do calculation
Nooo, copper.
Noo :))
i gave a similar model with 10 numbers
Ted chooses 5 numbers
we choose 3 numbers out of 10
what are the probality of getting 0 correct , 1 correct etc respectivly
eg Ted picks 1,2,3,4,5
06:21
i understand the model.
and if I pick 1,2,7
then that counts as getting 2 correct
The number of ways I have to pick is irrelevant, actually.
it seems like something out of the ordinary is happning so I should follow Teds advice and robs and read about the linearty thing
i mentioned that above
that is the space we are working in
C(10,5 ) are all the possible outcomes
06:23
Your denominator needs to be $C(52,5)$.
the number of 5 element subsets...
ok now we are mixing numbers
C (52,10 )
forget the 10
that is what you mean right?
the way am thinking of this
even thought the question asks for me to pick 5 numbers
I can pick 10
why don't you think about what Ted wrote for a moment. why does he have 5.
06:25
Copper I am intrested in both the solution and why my reasoning is not true
I will have to work that out also , am not saying it is wrong what Ted and Rob said
it is just strange to me why my formula is not making sense and making sense at same time
Read what I wrote, dammit.
Ok haha sorry
I have two facts:

#1 for all $\varepsilon$ and for all $n$, $x<\delta \implies |f(x) - f(\frac{x}{2^n})| < 2\varepsilon x$
#2 $\lim \limits_{n \to \infty}|f(x) - f(\frac{x}{2^n})| = |f(x)|$
intuitively, I interpret this to mean: if I pick a big enough $n$, and suppose $x<\delta$, I get that $|f(x)| < 2\varepsilon x$. How do I make this rigorous?
@TedShifrin can you explain how you see it ? that the number of picks you make is not relevent?
I understnad the logic behind C(52,5)
but what is the space of outcomes we are working in?
I ask because for an epsilon-delta proof I can't just take the limit as $n$ goes to infinity in the middle of the proof, all I have is specific choices of $n$
06:32
Precisely. I picked mine and it’s done. The sample space is your choices.
Treat my 10 as fixed — they are.
That’s what copper suggested an hour ago. :)
This is one of those $\epsilon/2$ arguments, shin.
oh! right right right
say no more
thanks
okay thanks yall !
I ll do the calculations and come back to you in due time
most likely tomorrow :D
06:52
@JackOhara: ok... without replacement it is $\frac{\binom{10}{k}\binom{42}{5-k}}{\binom{52}{5}}$
$10$ slots on the left and $42$ on the right. When you pick $5$, $k$ are on the left and $5-k$ are on the right.
That sums to $1$ and has $\frac{50}{52}$ expectation as well
Sorry that took so long. I had to take care of something offline
07:21
$ \begin{array}{c|c|c} k&\frac{\binom{10}{k}\binom{42}{5-k}}{\binom{52}{5}}&\text{approx}\\\hline 0&\frac{779}{2380}&32.731\%\\ 1&\frac{205}{476}&43.067\%\\ 2&\frac{615}{3094}&19.877\%\\ 3&\frac{123}{3094}&3.975\%\\ 4&\frac{3}{884}&0.339\%\\ 5&\frac{3}{30940}&0.010\% \end{array}$
07:54
states 1 and 3 are transient. 2 and 4 form a single (recurrent) communicating class and a bipartite graph -- draw this and write out the transition matrix. Hence $p_{ii}^{(2n)}=1$ and $p_{ii}^{(2n+1)}=0$ — user8675309 11 hours ago
I just quickly wanted to ask, if I have f(x)=1/x for x unequal to 0, is it still appropriate to say that x=0 is an infinite discontinuity while also concluding that f is continuous over its domain?
Please, why is the probability equal to 1? Is it because it is an absorbing state?
 
1 hour later…
09:02
2
Q: $p_{ii}^{(2n)}=1$ and $p_{ii}^{(2n+1)}=0$

MohcineI learnt the concept of the period of state i for DTMC and I'm wondering why we have $p_{ii}^{(2n)}=1$ and $p_{ii}^{(2n+1)}=0$? if $d(i)=p$ then $p_{ii}^{(pn)}=1$ and $p_{ii}^{(pn+1)}=0$?

09:30
@JackOhara: does that make sense?
09:42
@robjohn Yes sir
It should be a counter part of that formula that we work from the complement perspective right?
having $$\binom {52} {10}$$ in the denomnator
But i dont see what to get on top working this way
the question is solved now, this is just curiosity :D
it is elegant solution to consider 5 subsets instead 10
You mean consider you've picked 5 cards and we draw 10 and count the matches?
@JackOhara: $\frac{\binom{5}{k}\binom{47}{10-k}}{\binom{52}{10}}$
This is the same thing as above.
@robjohn exactly yes !
thanks so much :D
that is the formula that i wanted to find , but I guess the first one is better since it uses smaller numbers

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