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00:00 - 22:0022:00 - 00:00

00:29
$\sum_{i,k =1}^{3}a_{ik}a_{km}a_{mj}$ Anybody give me a hand of what this would be as a matrix and why?
What is $j$?
Just an index
(Does it look wrong?
im not sure I typed the latex right
It looks good, but you have double sum(run over $i,k$) and you use $3$ variables at the sum
Poop I typed it wrong brb
$\sum_{i,k =1}^{3}a_{ij}a_{kj}a_{km}$
That’s right now
@MatheinBoulomenos mathein :D
@MatheinBoulomenos I hope you are doing well :D looong time no see =P
@MikeMiller hey Mike:D
00:35
Still using $j$(and $m$ I didn't saw before) without defining it, are you willing to say that you run over all variables? or are $j,m$ constant?
hello everyone
Hello
Bugger I forgot to change the sum too
@Holo how are you today
$\sum_{j,k =1}^{3}a_{ij}a_{kj}a_{km}$
That is now fully right
00:37
@hungryWolf good, what about you?
Hey!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
@JakeRose but what are $i,k$ now?
I and k?
I am trying to calculate the probability of a character occurring in a sequence given a database of several similar sequences
i and m*
00:38
@Holo I am well, thank you
so the sequence is something like 'abcabcab_'
Ijust free indices
we need not consider any logic for solving the sequence
but we have conditional probabilites of similar several sequences
like prob of 'a' given previous character was 'a'
etcetra
Do you have the full question? @JakeRose
yes
I am typing it in
It’s as I have typed it there
00:41
Only the sum?
owh sorry
I thought that was for me
It is okay @hungryWolf , you 2 :p
Yeah it’s just an exercise sheet
is there a way to base the probability on all previous characters in a sequence instead of just the previous one
00:42
Does the single sum imply that k=j ie that it’s not a double sum?
as in 'abcabcab_', basing it on occurance of a and then the b after a and so on upto the last character
Well, are m,i are constants?
They’re just the free indices
Is it a right thing to do, instead of just looking at conditional probability of existing letter pairs and selecting the best
They can vary
00:51
Sorry, I am not very focus now(watching the international rn)
@Holo do you think my question is unclear?
@hungryWolf a bit, your dataset is only a,b,c?
for the example, yes
the example that I am working on has only a,b,c
I just want to give the character that is best for the sequence, and I am doubting that considering only the letter before will be a mistake
If it is not sure how the previous data will help, if I through a dice 8 times, what will be the probabilities for the 9th dice?
Unless they are dependents
I'm not sure how you intend to determine the probabilities.
01:00
@KarlKronenfeld probabilities of the sequence?
heya @Karl, stranger!
what if I allow a reducing percentage of importance, starting at then end moving towards the start of the sequence?
Probability that e.g. $a$ follows $x_1x_2\dots x_n$ for any given $n$ and $x_i$, like I think you're saying.
@JakeRose sorry, I can't think about the question now. can't focus on it rn, it will be bettert to ask someone else
@TedShifrin Hello there
01:03
yes, but as all the other probs will stay the same
the result will be same as selecting based on last character
if done that way
But how the character decided? If it does not depends on the previous characters the probability won't change
Oh, yeah, what do you actually want to do?
@Holo I think I confused everyone
I think I am confused too :p
May be I can just run it like conditional probability
like x1 * x2 * x3
@hungryWolf I think economists use a similar approach to calculating utility on a first approximation.
01:06
Is the question "You are given a string 'abcabcabc' taking a random next character what will it be?"?
@Holo yes
@KarlKronenfeld is there a formula for that ?
@hungryWolf Of course. I was remembering something I read a few years back, so I dunno how to reference it.
(I'm not an economist)
owh, okay
It involved a progression of states, so similar to this in outline. Also, Shannon's work introducing entropy to information theory talks about a similar problem (to better compress texts).
I study the something similar in game theory 2 years ago, I don't remember it but I remember it being something something on progression(we didn't use entropy), but it wasn't exactly the same
01:14
I am looking at an entropy article right now
We used weight function
okay
entropy looks really similar to conditional probability
except a few tweaks to prevent very small numbers
I've been reading this article: medium.com/udacity/…
01:34
@Holo and @KarlKronenfeld here's what I meant previously
sequence is 'abca?'
probability that the last letter is 'a' is given by: $P_a = b_a*c_b*a_c*a_a$
probability that the last letter is 'b' is given by: $P_a = b_a*c_b*a_c*b_a$
probability that the last letter is 'c' is given by: $P_a = b_a*c_b*a_c*c_a$
if we consider conditional probabilities ^^^ will be the results
then all we are doing is considering the last term
x_a, x = a or b or c
notation is x_y -> y precedes x
You can strike some sort of balance and consider previous probabilities based on coming after duos or trios (such as $b_{ca}$ or $b_{bca}$). I do not think using probabilities involving arbitrary-length predecessors is even possible w/ or w/o weights since you cannot sample for them, and if you knew them why are you doing this at all.
The reason I thought of Shannon + entropy is that the techniques should resemble what techniques you need here. The overview article only references probabilities of involving a single symbol, but strings of symbols should work equally well.
yes that looks good
yeah, forget about sampling, putting all of that in a DB would make the db enormous
for arbitrary lengths
01:50
Oh, you need to put it in a db? I tried to develop what I did 2 years ago and from the little I remember it will need to keep track of $bn^m$ numbers where $b$ is a large number $n$ is the numbers of possible letters and $m$ is the current length$+1$, so it is no good I guess
does there exist some unique holomorphic bijection to the riemann sphere such that the map is isometric?
actually scratch the uniqueness condition
@geocalc33 you can edit the last message in se chat, instead of asking to ignore, but last message only
okay thanks
@geocalc33 welcome
02:04
do you think I could ask a question like that^^^ albeit in much more detail on SE?
Yes you can, but you should add what do you think(have you tried things?)
do you mean se chat or just se?
I think geocalc means in math stack exchange
02:25
in stack exchange
yes I've tried expressing the embedding in pseudo complex representations but I'm not sure about how to transition to the purely complex cases
So if you post the question show that work
I'm specifically interested as to whether there are substantial applications for these isometric embeddings on the riemann sphere so i might do a reference request
 
1 hour later…
03:42
hello
04:15
hello
04:39
hello
04:59
In similar matrices, is $A=P^{-1}DP$ the same thing as saying $A=PDP^{-1}$?
05:39
@Nebulae yes
@Nebulae broke the chain of hello's. you owe me a theorem
06:21
@geocalc33 haha sorry.
@LeakyNun Thanks. In the same matrix $A^n = P^{-1}D^nP$. What's are the eigenvalues of $A^n$? If we know the eigenvalues of $A$, that's.
the eigenvalues of $A$ are the entries of $D$
if you know the eigenvalues of A, then the eigenvalues of $A^n$ are just the n-th power ofthose of A
the eigenvalues of $A^n$ are the entries of $D^n$
because, if $Av=\lambda v$, then surely $AAv=A\lambda v=\lambda\lambda v$, etc
Makes sense thanks, guys.
I was thinking of calculating $P^{-1}D^nP$ then diagonalising it, thereby getting stuck in a glorious loop of going back and forth lol.
Actually going back to $A = P^{-1}DP$ how do I show that $A = PDP^{-1}$?
06:30
Well not for the same $P$
Oh, okay makes sense.
could someone help me
i need to download that graph
Because I was thinking unless the matrices are commutative it can't be true.
What people thought you meant was just whether the equivalence relation $A = P^{-1}DP$ for some $P$ was the same as that but with the inverse swapped, which is the case
Otherwise I would relabel $P' = P^{-1}$ etc.
Yeah, I wasn't clear. Thanks @Daminark
06:33
No problem!
 
1 hour later…
07:40
no problem? I can tell a you a number of problems the first and most damaging one in Australian society at present day is there is no where that is licensed to sell alcohol yet also allows you to bring your computer for the periods where people are annoying
that way you never get called an asshole because whenever you feel like saying something inappropriate you just go back and do math. its the perfect form of social interaction and it doesn't happen
07:59
Social interaction is over rated anyways @Adam
08:41
@Holo sure social interaction with the idiots is well overrated because it sucks, and then all the interesting people never bother with it and the infinite cycle continues. But conversation with interesting people is actually pretty worthwhile I think you'd agree
In all of my heart I hate small talk face to face, but conversation with meaning and with someone that is not an idiot can be worthwhile(I would still prefer it to be not face to face)
anyway I am going thru my notes that were done at some point where I must have believed copying things of Wikipedia aimlessly was effective, and need help with the questions for things I encounter
sure if that's what you prefer there is nothing wrong with that
I am on the 25h without sleep so my help is not a guarantee but if I can I will help
"A pole of a meromorphic function is a certain type of singularity that behaves like the singularity of $\frac{1}{z^n}$ at $z=0$"
can someone please tell what is used to characterize the degree or "exactness" of this behaviour for me please
ok well definitely stop doing everything after the 48 hr mark if you believe you can keep going at that point, you absolutely can but it will result in a psychosis
possibly anyway. you can go anything up to 10 days without any real consequence but I am just informing you of the risks there
@Adam for $f : B_a(r) \setminus \{a\} \to \Bbb C$, equivalent:
1. $f$ has a pole of degree $n$ at $a$
2. there is $g : B_a(r) \to \Bbb C$ such that $g(a) \ne 0$ and $g(z) = (z-a)^n f(z)$
3. the smallest non-zero term in the laurent series of $f$ is the $-n$ term
08:50
I won't go that far, I am on vacation so from time to time I take a night
@LeakyNun thanks I have 1 and 2 written in, but not 3 which is definitely a pretty important one to miss I'd think
@Adam also if you know that it's a pole, then the order can be recovered from $-\operatorname{Res}\left(\frac{f'}f,a\right)$
I have a series problem where I am unable to identify the sequence. I need to display the sequence in a compact form using $\sum$ or/and $\prod$ form.
\begin{align*}
&g\left(0\right)+\\
&f\left(\{0\},\{1\}\right)g\left(1\right)+\\
&f\left(\{0,1\},\{2,3\}\right)\Big[g\left(2\right)+f\left(\{2\},\{3\}\right)g\left(3\right)\Big]+\\&f\left(\{0,1,2,3\},\{4,5,6,7\}\right)\bigg[g\left(4\right)+f\left(\{4\},\{5\}\right)g\left(5\right)+f\left(\{4,5\},\{6,7\}\right)\left[g\left(6\right)+f\left(\{6\},\{7\}\right)g\left(7\right)\right]\bigg]
\end{align*}
This is for $N = 7$, and N is always of the form $N = 2^n - 1$ for some $n$.
The product terms (inside the square brackets) increase recursively. I can post the series for $N=15$, if someone is interested in having a go at it. I have given the above example for $N=7$ for clarity.
09:18
$$a_n=f([2^{n-1}],[2^{n}]\setminus[2^{n-1}])H(n-1,2^{n-1}),a_0=g(0)$$Where $[n]=\{0,1,...,n-1\}$ and $H(n,m)$ is the sum of $a_0$ to $a_n$ with the inputs "shifted" by $m$(instead of, for example, $g(0)+f(\{0\},\{1\})g(1)...$ you have $g(m+0)+f(\{m+0\},\{m+1\})g(m+1)...$)
It is not exactly what you searched for but maybe this will help @BrownNinja
Ahan, it looks like it.
"Look like it" as "not exactly what you searched for" or as "this will help"?
Definitely as this will help. I am just trying to verify it.
This is what I was looking for. Thanks a lot @Holo :)
You're welcome
:P
This chatroom has never disappointed me and I have always regarded this place as filled with people of great talents :)
10:08
Hey guys, it's been a long time since I've exercised anything math intensive. I have a problem where I want to merge 2 range of numbers that given a condition will equal [0,1]
I could specify if that isn't anywhere clear enough to understand what I need, but what's an approach I could use for this?
You should be more specific about how to merge them and what conditions
To be more specific:
Y is a range from [-2, 2]
Z is a range from [-2, 2]
I need a function that will return 1 if Y < 0 and Z > 0.5, otherwise gradually down to 0
How "gradually" exactly? Remember you have 2 variables
I don't think the magnitude of how it decreases matters too much as long as it's somewhat linear
10:24
So maybe $$f(y,z)=\begin{cases}1&(y,z)\in[-2,0)\times(0.5,2]\\1+\frac y2\cdot\frac{z-0.5}{2.5}&\mbox{Otherwise}\end{cases}$$?
Do I need a plugin for latex to work here?
Also, it would be awesome if you could share how you came up with it
On the right side there is a link($\LaTeX$ in chat: ...) and they explain there what to do
The first case you gave me(Y<0,Z>0.5) for the second case I took what left from $Y$, the set $[0,2]$ and "squeezed" it to $[0,1]$ by dividing by $2$, and then I for $Z$ I took what is left, the set $[-2,0.5]$, shifted it to $[-2.5,0]$ by subtracting $0.5$ and then "squeezed" it to $[0,1]$ by dividing by $-2.5$. This gives a linear map that goes from $0$ to $1$, and to make it go down(from $1$ to $0$) I took $1$ minus the map
 
2 hours later…
12:17
Is "de Moivrer" an alternate spelling of "de Moivre", or just a typo: math.stackexchange.com/questions/2885339/…
 
2 hours later…
13:56
Typo.
14:13
A terminology question, in the characterisitc property of free abelian groups it's stated that given any abelian group $H$ and any map $\varphi : S \to H$ there exists a unique homomorphism $\Phi : \mathbb{Z}S \to H$ extending $\varphi$
But strictly speaking I don't see how $\varphi$ is extended in any literal way, because its domain is the same, its codomain is the same and it's still a map nothing more
Does the author mean that the inclusion homomorphism $i : S \to \mathbb{Z}S$ can be composed with $\Phi$ to yield a homomorphism $\Phi \circ i : S \to H$?
$S$ is a set and $\Bbb ZS$ is an abelian group
If $S$ is two elements the latter is $\Bbb Z^2$
Maps from $S$ are just set maps, not homomorphisms
Okay yeah whoops, it's just a set theoretic inclusion
What do authors mean then by "extending" $\varphi$? Do they just mean set theoretic inclusion of $S$ into $\mathbb{Z}S$ 'induces' a homomorphism $\Phi : \mathbb{Z}S \to H$?
Because $\Phi \circ i = \varphi$, but $\varphi$ is just a map and $\Phi$ is a homomorphism (so it's extending it in this sort of way)
Yes, there is only one homomorphism with that property
Ir is uniquely defines
Once you know $\Phi(s)$, you know $\Phi(ns) = n\Phi(s)$ and $\Phi(v+w) = \Phi(v) + \Phi(w)$
So a general element of $\Bbb ZS$, which is an integer weight assigned to each element of S, sends $v = \sum a_s$ to $\sum a_s \Phi(s)$
14:30
Ahh okay I see
Thanks for that info!
Dear Anon, this question math.stackexchange.com/q/2885401/79069 is related / inspired by your answer earlier, may you have a look?
Dear Anon, this question math.stackexchange.com/q/2885401/79069 is related / inspired by your answer earlier in math.stackexchange.com/questions/2538747, may you have a look?
I'm guessing also that for any basis $S$ for an abelian group $G$ we have $G \cong \mathbb{Z}S$, not really sure how to prove it now though
15:27
how recently did the new MSE theme go in?
I don't actually look at the main site that frequently lately, so I'm not sure how long it's been in effect
15:54
About a week.
Anyone familar with any equation in number theory that looks remotely like that form?
nope
I cannot find any number theoretic quantity that is named K
and K theory is something else entirely
@Secret probably you need to look at analytic number theory
16:09
hi everyone
hmm... while I cannot find anything with K (other than the K function), it seems number theory did use a lot of sums, products and integrals in their formulae
I like the gauge theory equation
I'm just trying to figure out if there's any sense that can be made out of the nonsense that is SU(U(2))
lol
16:25
Not my gauge theory
So in my textbook it says that for some stuff in algebraic topology "we need to extend the notion of rank to finitely generated abelian groups that are not necessarily free abelian". But finitely generated abelian groups have a basis and so are thus free abelian...
Why can't we just define rank for free abelian groups and be done with it?
$\Bbb Z/2$ is finitely generated
$\Bbb{Z}/2\Bbb{Z}$, and that has torsion elements because 1+1=0?
16:47
Any $\LaTeX$ wizards here?
if I wanted to alias the cl_1 part in something like $cl_1(X)$ so that it wasn't italicized, what's the right way to do that?
$\text{cl}_1(X)$?
$\mathrm{cl}_1(X)$ also works
Ahh okay thanks for those examples, so $\mathbb{Z} /2\mathbb{Z} = \{0, 1\}$ is finitely generated but has no basis, because linear independance fails for the only possible basis sets $\{0\}$, $\{1\}$ and $\{0, 1\}$
17:09
@rschwieb you can use the \operatorname command
$\operatorname{cl}$
When dealing with problems that ask for a property that is not there, I often like to handle its contrapositive

e.g. not free abelian is the same as there exists at least one torsion element
there are some non free abelian groups without torsion elements
oops...
but they are not finitely generated I think
But yeah, in general, I found negative abstractions hard to grasp especially when proving something, because for example you can have some set A with some members I, and another set with some members J where J is opposite to I (thus you basically have two objects, A and anti A). However not A is not necessary anti A, it can be B, C, D etc.
17:42
@Secret THe idea is to abbreviate this to \somethingshort X
The _1 is an important part. I think I got it working... i didn't realize you could freely use math in there too in the alias
@rschwieb so you can write something like \newcommand{\cl}{\operatorname{cl}} in your preamble
so that \cl becomes the command $\operatorname{cl}$
Does it work with _1 inside?
I think that's what I essentially ended up doing
it's just at first I used dollar signs inside the declaration
right, because if you do this then you can do things like \cl_1 to give $\operatorname{cl}_1$
and that wasn't OK. I was surprised to find out I could remove them and still get upright text
There is also \mbox that works similarly to \text
17:45
It didn't seem to like numbers in the alias name iether
Avoiding typing _1 is kind of important :)
Thanks for the help tho
you can do \newcommand{\cl}{\operatorname{cl_1}}
@rschwieb so you can use
\newcommand{\cl}{\operatorname{cl}_1}
then \cl will be $\newcommand{\cl}{\operatorname{cl_1}}\cl$
Those two answers are slightly different.. is the last \cl on the right necessary?
17:48
@rschwieb both of those two answers should give exactly the same output
At least, this is how I fix not writing things like \mathbb{Z} when I want to talk about $\Bbb Z$
just replacing it with \Z
I use it like this only for long things, usually I use \Bbb Z instead
18:11
My laziest moment was making a new command \fip that prints "finite intersection property"
My worst(or best) moment was to create this: \newcommand{\f}[2]{\operatorname{\frac{#1}{#2}}} once
0
Q: Motivation behind proof of Behrend's theorem on size of AP free subset

alxchenBehrend's theorem regarding AP states that There exists an absolute constant $C$ such that for all sufficiently large integers $N$ there exists a subset $A$ of $\{1, 2, \cdots, N \}$ with atleast $Ne^{-c \sqrt{\ln N}}$ such that no three elements of $A$ forms an arithmetic progression. Th...

18:50
Hello. I am stuck on an easy problem. Given a bin with 50 balls, 20 red and 30 blue. A ball is drawn at random from the bin and then placed back in it. Another draw is made, and the ball drawn is blue, find the probability that this is the first time blue ball has been drawn. So it implies the first ball must be red wit h probability 2/5 and now blue with probabilty 3/5 so the answer is 6/25. Is this correct?
@AlessandroCodenotti Not nearly lazy enough. The term should be printed as "fip" throughout.
@AlessandroCodenotti This was also done for "zip rings"
(zero intersection property)
@Shobhit no, it's not correct
why @fonini
the question is phrased in this way: "the ball drawn is blue, find the probability that this is the first time"
18:58
it is given that the second ball is blue
you don't need to account for this in your multiplication
the answer is simply 2/5
the question is "given that the second ball was blue, what is the probability of this blue ball having been the first?"
which is just another way of asking "given that the second ball was blue, what is the probability that the first ball was red?"
So if i say let $A$ : the first ball is red and $B$: the second ball is blue. Now i need to calculate the conditional probability $P(A|B) = P (A \cap B) / P(B)$ which gives me $(2/5 * 3/5 )/(3/5) = 2/5$
yes!
@rschwieb I don't like the shortened form, but I don't like having to write the full name either
19:01
Thanks @fonini :)
actually, the conditioning on $B$ doesn't make a difference when you phrase it like "given $B$, what's the probability of the 1st ball being red"
this conditioning was important when the question was still in its original form: "given $B$, what's the probability that the second ball was the first blue ball"
in this original form, your first attempt was to calculate the probability without the conditioning
they both look the same to me. Please explain.
the event "the second ball was the first blue ball" is just the same as "the second ball was blue but the first was red"
which is surely not the same as simply "the first ball was red"
got it
but, when you condition on $B$ (which is "the second was blue"), they become the same
19:11
yes i understand now, thanks
19:29
What does it mean to check some rule of inference for its correctness?
19:47
That they give you true conclusions from true assumption
Or that your logic has the soundness theorem if you prefer
okay so do I check that intuitively?
intuitively, you just need to read the inference and ask yourself: would I be willing to believe the conclusion solely from the fact that I believe the assumptions?
or "is it logically impossible for the assumption to be true and yet the conclusion be false?"
20:20
heh, this is funny (and probably won't last long) . I have exactly 99999 rep ( and a screenshot to prove it)
one too many downvotes I guess
@rschwieb apparently 99999 is rounded to 101000
The chat one shows total rep across all stackexchange websites I think
If so it should have show 105k
@Holo Yeah, I found out the chat number is some bogus total
It is just interesting to see the difference @rschwieb , in general how the forums connected to each other(and to chat.SE) is interesting
Isa
Isa
20:32
What is this equal? $\frac{\partial}{\partial x} u(x,-y)$ ?
What is $u(a,b)?$
$\lim_{a\to 0} \frac{u(x+a,-y)-u(x,-y)}{a}$
(which is to say, it's just differentiation with respect to $x$ with other arguments held fixed)
Isa
Isa
@Holo it's a function, g(x+iy)=u(x,-y)-iv(x,-y)
Yes, but apart from the definition we can't help you without knowing how you define this function
Isa
Isa
ah, it's in $\mathbb C$ and it's analytic. I'm supposed to verify that g(x+iy)=u(x,-y)-iv(x,-y) satisfies the Cauchy-Riemann equations.
@Semiclassical how can I use that to prove g satisfies Cauchy-Riemann equations?
20:47
So $\frac{\partial}{\partial x} u(x,-y)$ is the derivative in respect to $x$ and compare it to $-\frac{\partial}{\partial y} v(x,-y)$ and then compare $\frac{\partial}{\partial y} u(x,-y)$ to $-(-\frac{\partial}{\partial x} v(x,-y))$
Isa
Isa
20:57
@Holo will I use the chain rule?
Yes
Isa
Isa
how ?
could you do just one to see how will be written and then I can do the rest
$\frac{\partial}{\partial y} h(x,-y)=-\frac{\partial}{\partial (-y)} h(x,-y)$
I have to go now
Isa
Isa
did you apply chain rule there?
00:00 - 22:0022:00 - 00:00

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