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00:56
https://math.stackexchange.com/questions/2315356/hints-for-my-try-on-an-induction-proof-that-a-matrix-a-to-the-power-of-k-equ/2315362?noredirect=1#comment4764891_2315362

:(
This is the same matrix as you had before?
If you want to think binomial theorem, you probably should note that this matrix can be written as $A=I+\mu S$ where $S$ is one on the first upper subdiagonal and zero everywhere else. @NaCl
So therefore you'd really interested in $A^k=(1+\mu S)^k$. What's nice here is that $I$ commutes with $S$, so you can expand that using the binomial theorem in the obvious way.
(If you had $A=X+Y$, then $A^2=X^2+XY+YX+Y^2 \neq X^2+2XY+Y^2$. So this really does help.)
good point
What that does leave you with is the task of computing powers of $S$. But that's actually not too bad at all, as you'll see if you compute a few small cases.
Well, I guess that stuff is too "advanced" for our course anyways
Perhaps.
It does seem like they want you to do this in a more low-level way.
Which...seems kind of silly, tbh.
01:03
As said, we don't even know about determinants, 'officially'
"Wow, this is a nasty problem." lol
I mean, from my POV the result boils down to: $I$ commutes with $S$, so $A^k=\sum_{n=0}^k \binom{k}{n}\mu^n S^n$.
> using n for index and k for upper limit
grrr
Hey, I'm not the one who picked $k$ as the power :P
01:07
I'm not the one either, lol
Plus, I didn't want to use $i,j$ since those are being used for matrix entries.
And it's not too hard to show that $(S^n)_{ij}=\delta_{j-i-n}$, so that gives $(A^k)_{ij}=\binom{k}{j-i}\mu^{j-i}$. Done.
Actually, I'm being a bit too quick. That only works if $j-i-n=0$ for some $n=0,1,2,\ldots,k$.
So one needs $j\geq i$.
But that's all. :/
This is definitely an example of the method making the problem harder than it really should be. It's basically making you prove the binomial theorem at the same time as you're doing matrix multiplication. @NaCl
Nice.
So, yeah. Not a fan of how they've suggested it should be proven.
The first exercises were just dumb calculating and now this
wtf
Oh. I really shouldn't have used $n$ for my summation index, since you already have that as the matrix dimension.
So scratch that for...i dunno, $m$.
01:15
How about 肉?
lol
Let's stay roman, at least :P
Well, the induction approach should boil down to this: $(A^{k+1})_{ij}=\sum_{l=1}^n (A^k)_{il} (A)_{lj}$
Multiplying $A$ with $A^k$ yields two "non-zero" cases for the entries:
1) $a_{i,i}\cdot a_{i,i}^{(k)}$ <- very straightforward to get to $a_{i,i}^{k+1}$
Bleh, this is just tedious.
I guess one thing to note is that the sum can be restricted to $l=i$ to $j$.
@NaCl Right, the sum should simplify down to...
2) $a_{i,i}\cdot a_{i,i+1}^{(k)}+a_{i,i+1}\cdot a_{i+1,i+1}$
3) $0$
$(A^{k+1})_{ij}=(A^k)_{ij}+\mu (A^k)_{ij-1}$
01:21
Test: $ŋ n\ng$
Hm.
Since $(A)_{lj}=1$ if $l=j$, $\mu$ if $l=j-l$, and zero otherwise.
@Semiclassical I don't think I may use that, since it's restricted to $A^k=A\cdot\;...\;\cdot A$
Eh. If $A^k=A\cdots A$ (k times) then $A^{k+1}=A\cdot A\cdots A= A\cdot A^k$.
yes
So I don't see why you wouldn't use that.
01:23
That's my first step inside the induction
Okay
I don't see what you're objecting to, then.
I think I object, because I simply don't understand
Sorry
Well.
Just to review what I'm doing.
(and to slightly change what I'm doing)
If $A^k$ has the stated elements, then $A^{k+1}=A\cdot A^k$.
So therefore the elements of $A^{k+1}$ are $(A^{k+1})_{ij}=\sum_{l=1}^n (A)_{il}(A^k)_{lj}$.
01:26
And $A\cdot A^k=A\cdot (a_{i,j}^{(k)})_{i,j=1,...,n}$
But $(A)_{il}=0$ unless $l=i$ or $l=i+1$.
yes
so that becomes $(A^{k+1})_{i,j}=(A)_{i,i}(A^k)_{i,j}+(A)_{i,i+1}(A^k)_{i+1,j}=(A^k)_{i,j}+\mu (A^k)_{i+1,j}$.
Where $j=i+1$, right?
I was hoping to avoid using commas, but eh.
Shouldn't need that quite yet.
If $i=j$, then the induction hypothesis gives that as $(A^{k+1})_{i,i}=(A^k)_{i,i}+\mu(A^k)_{i+1,i}=1$ (the second term dies since $i+i>i$).
01:30
Ah, oops, you inserted the values of $(A)_{i,i}$ and $(A)_{i,i+1}$
Right.
So now we're left with the hard one, where $j>i$.
You might need to further separate out the case where $j=i+1$, since in that case the second term will have $i+1=j+1$.
If $j>i+1$, though, then that becomes $$(A^{k+1})_{i,j}=\binom{k}{j-i}\mu^{j-i}+\mu \binom{k}{j-i-1}\mu^{j-i-1}=\left[\binom{k}{j-i}+\binom{k}{j-i-1}\right]\mu^{j-i‌​}$$
But that sum of binomials simplifies to $\binom{k+1}{j-i}$, so $(A^{k+1})_{i,j}=\binom{k+1}{j-1}\mu^{j-i}$ if $j>i+1$.
Which matches. Yay.
That leaves the case where $j=i+1$ but that's easy enough.
ye
So yeah. Not nearly as bad as that one person was making it out to be.
But still kinda silly.
Ah, I see
Beneath the surface, though, this is really just the fact that $(1+\mu x)^{k+1}=(1+\mu x)^k+\mu x (1+\mu x)^k$.
Which expands via the binomial theorem to $\sum_{l=0}^k \binom{k}{l}\mu^l x^l+\sum_{l=0}^k \binom{k}{l}\mu^{l+1} x^{l+1}=\sum_{l=0}^k \binom{k}{l}\mu^l x^l+\sum_{l=1}^{k+1} \binom{k}{l+1}\mu^{l} x^{l}$
01:43
Where did the $\mu^{k}$ go, though?
Eh?
back in a bit, actually.
@Semiclassical For $(A^k)_{i,j}$, isn't it $(A^k)_{i,j}={{k}\choose{j-i}}\mu^{k-(j-i)}$?
lemme see
Hrm, you're right.
I think that modifies the result but not severely. Oh well.
It shouldn't
Well, it does
Well, it turns it from $\mu^{j-i}$ to $\mu^{k-j+i}$.
01:51
Yes
But you still have the two terms coming out with the same power of $\mu$.
So the second one becomes: $\mu{{k}\choose{j-i-1}}\mu^{k-j+i+1}$
...hrm, that doesn't seem right.
Okay. We have $(j-(i+1))$, right?
Second term should be $\mu(A^k)_{i+1,j}=\mu \binom{k}{j-i-1}\mu^{k-j+i}$
Hrm.
01:54
Where did the 1 in the exponent go?
It's still in the $\mu$ out front.
But $\mu^{k-(j-(i+1))}=\mu^{k-j+i+1}$
Derp.
Yeah.
So actually we get $\mu^{k-j+i+2}$ after multiplying with $\mu$
Right. Which doesn't seem right at all.
01:55
The binomial coefficients work out nicely, but that crap is crap
Something is garbage, yes.
Actually, something doesn't seem right with the proposed inductive form.
Suppose $j=i+1$.
If we had $(A)_{i,i+1}(A^k)_{i,j}+(A)_{i,i}(A^k)_{i,j}$ things would go well
Then what they claim is that $(A^k)_{i,i+1}=\binom{k}{1} \mu^{k-1}$.
But what I get in Mathematica is that the power is $\mu^1$.
Are you sure it's not $\mu^{j-i}$ in the problem?
It is $\mu^{k-(j-i)}$, just looked it up
:/
Then I'm confuzed.
If we look at the first off-diagonal of $A^k$, that proposed form gives $\mu^{k-1}$ as the dependence. But that doesn't seem right.
Could be a typo, but if it is...euurgghh.
Anyways. I'll have to leave you to it.
Out for now.
02:03
I'll ask a tutor tomorrow
Good call.
Good luck with it!
Thank you for your thoughts!
Does $1$ belong to the set $\{n\in \Bbb N : \text {if } n\ne 1 \text { then } n-1\in\Bbb N\}$?
02:21
@Silent Yes
02:37
Huzzah for vacuous truth.
03:10
Combinatorial game theory intensifies @Semi
hmm? @Daminark
That subject exists due to vacuous truth. The definition of a game is an ordered pair of sets whose elements are games
Which would be circular, but THE EMPTY SET
Also I think Conway induction uses it
I thought you were somehow making a roundabout comment about the UK election results.
(which I'm following.)
03:12
Oh is that a thing which is happening?
Kewl
670 seats up for grabs
(snap elections are weird)
Oh this wasn't planned? Huh
Yeah. There's a maximum time that can go between british parliamentary elections, but they can also be done more suddenly.
because reasons
Anyways. The Conservative Prime Minister called the snap election with the belief that she'd be able to further shore up her parliamentary majority.
And at the start, that seemed plausible. Not anymore.
Right now it's looking like they'll lose seats, perhaps to the point that they no longer have the majority of the seats and there's a hung parliament.
03:16
LOL
(The projections from the Associated Press put them at 321/670, just shy of the 336 needed for the majority.)
They'll still have a plurality, but in order to actually get stuff done they'll need to form a coalition gvmt.
And if they can't, then probably Labour will do so instead.
(Labour had actually been leading in seat counts for the time I've been following it, but the Conservatives are now in the lead and I imagine the remaining seats are in their favor.)
And, well, combinatorial game theory sounds like a pretty good description of what's happening in that collection of seats :P
Sounds a good bit like it for sure :P
Right now it's Conservatives 247, Labour 228.
And a few dozen seats for the smaller parties.
I see
03:35
is there a way to identify the domain of a function just by looking at the polynomial?
or is this just a pipe dream
Hi
quick plug. Anyone know how to fill in the remaining holes in my self answer here? It's just giving me hassle to understand it.
0
A: Walking on the surface of a triangular mesh

TheGreatDuckThinking about it on my own, i have determined a series of steps to determine the intersection point where the point hits the transition between the two triangles. Partial algorithm for geodesic motion along a 3D model The displacement of the point Find the reflection of the starting point $p$...

and i better get out of here before I get in trouble for posting
:p
04:04
[Ordinals] Ok, it seems we can define ordinal left tetration (since all ordinal operations are left associative, (and because of the defining property of epsilon numbers being $\epsilon_{\alpha}=\omega^{\epsilon_{\alpha}}$, i.e. a fixed point wrt right associative (instead of left associative) exponentiation of $\omega$), tetration will normally be defined left associative as well).

\begin{align}
{}^0\omega & =1\\
({}^0\omega)^{\omega} & =(1)^{\omega}=\omega={}^1\omega\\
({}^1\omega)^{\omega} & =(\omega)^{\omega}={}^2\omega\\
Your unprovoked and awe-inspiring mathematics never ceases to amaze me, Secret.
\o
How are you chat?
gut
good
04:19
gut ist gut
@Dodsy The function being the polynomial? Polynomial has domain all of R. I don't get the question.
like I'm trying to find a way to find let's say given $2^{3a-2}$ what the domain and range of the exponential function are.
That's not polynomial.
bad example
$x^2 - 2x + 3$ let's say
without looking at a graph
The domain is all of R. It is never undefined for any real value of x.
04:22
what about the range
Okay, there things are (more) interesting.
One thing you can say right off the bat is that the range won't be all of R.
@Dodsy You have to "imagine" the graph. What is the range of $x^2$?
for instance the range of this function is ${y > 2 , y E R}$
Believe it or not, the best way is to complete the square.
Quintics are a headache
04:23
Do you know how to do that in the given example?
I was just about to say
a quintic.
I haven't reviewed that far :C
I'd appreciate the lesson.
Thank goodness what you have isn't a quintic, then :)
for a refresher
The point being any quadratic $y = ax^2 + bx + c$ can be reparameterized to get $y = x^2$.
Which you know the range of, and then you back-calculate from there.
One thing I should point out is that while finding the range of a quadratic isn't that hard, doing it for anything worse can get bad fast.
04:25
right, the parent function @BalarkaSen
Unless you can, er... complete the penteract, then complete the tesseract for the extra terms, then complete the cube and finally complete the square...?
Anything of even degree, at least. (Odd degree, things are a lot simpler).
@Secret Something something Galois theory.
wait here, I'll give you a typical question I'm trying to do.
off of the practice test
that I am having trouble with
Well, the galois group only give you the roots if only it is solvable
The reason I say it gets bad fast is because finding the places where the function takes on a min/max value amounts to finding roots of the derivative
04:27
and I want to be able to sketch this quickly
and the derivative of a nth-degree polynomial is an (n-1)th degree polynomial.
so just finding the possible min/max values is a matter of solving a polynomial, and that's not necessarily easy.
:/
I don't mind computing things, but I have a hard time graphing.
without graphing technology
So while finding the range of a quadratic is straightforward (and finding it for a cubic is trivial on a technicality) finding the range of a quartic isn't going to be easy.
And it gets worse from there.
Could you help me see this more intuitively, semi? imgur.com/a/wSmCl
I imagine the parent function they mean is $y=2^{4x-8}$.
Do you know what the domain/range of that is?
04:30
{x E R} , {y > 0, y E R} ?
Right. It picks any real number and gives back a positive real value.
So now we take the image of that under the function $y=3x-7$.
so then, it's vertically stretched by a factor of 3?
Right, and vertically shifted down by 7.
and translated 4 units downwards
I should say that the value which gives 0 is shifted down by 7.
But that's requires that you take $x=-\infty$ initially, so that just tells you that the graph starts at $-7$ on the far left.
04:34
there is some sort of way to identify asymoptes?
In the middle, of course, you have $2\to 1\to -4$.
right.
Well, where does the parent function have asymptotes?
I am unsure.
I suppose it doesn't
since x E r
Well, there's two ways to have an asymptote.
04:36
I suppose it could have an asymptote at y = 0?
Either things go bad at a finiite value of x, or things become asymptotic for large x.
Yep.
so then, it would have an asymptote at y = -7
right?
Right. Which is to say, a horizontal asymptote.
Where in $x$ do you get this asymptote?
at x = 3
but why
Hang on. Which $x$ are we talking about?
04:38
hm.
I am unsure what you mean
oh
Well, we've got two functions. One is $f(x)=3x-7$, the other is $g(x)=2^{4x-8}$ (giving them useful names).
on the transformed function
I should know an answer to a question on main but it seems I don't after all.
And then you've got the composition $f(g(x))$ which is what you want to plot.
Right. I'm asking where in $x$ the asymptote of $g(x)$ will occur.
So the parent function.
there shouldn't be one
if x E r is there an asymptote?
04:40
Oh, wait, maybe I do.
Again, there's two ways to have an asymptote. At finite values, or at infinity.
The function $g(x)$ is smooth for all finite values of $x$, so there's no vertical asymptotes.
What about large $x$ (positive or negative)?
hm
at x = 0
At $x=0$ it's just changing smoothly. Nothing weird there.
no nevermind
What happens at large negative $x$?
04:43
I am unsure :C
this test is going to destroy me.
Well. Do you know what the graph of an exponential function looks like?
it begins at 0 and increases exponentially
Be more precise.
The way you've said it makes it sound like an exponential function equals zero at zero.
it is equal to 1 at 0.
Right. So when you say it begins at zero, where in $x$ are you starting?
04:45
when x < 0
Closer. But if $x=-1$ then the exponential function is still positive, so it's not zero yet.
How far in $x$ do you have to go for it to be zero?
hm
it depends on the coefficient
Let's be specific and say $y=2^x$.
when x = -11, y = 0
Which function do you have in mind here?
04:47
$y = 2^x$
So, 2^-11 = 0?
no :C
Nope, indeed.
In fact, suppose I pick any real value of $x$.
If I make it more negative by 1, then that changes from $2^x$ to $2^{x-1}=\frac{1}{2}2^x$.
right
so 0 is an asymptote
Right. What kind of an asymptote, and for what values of $x$?
04:49
a horizontal asymptote for x < 0
Again, that's not precise. What you're missing is that $x$ is becoming a large negative number.
Just saying $x<0$ isn't enough.
Oh
as x approaches negative infinity, y approaches 0
Right.
(And it approaches it from above, but that's a detail)
That's at -infinity. Is there anything like that at positive infinity?
I don't think so.
And you'd be correct.
04:52
but it almost looks like it.
As x goes to positive infinity, it just blows up faster and faster.
The more correct thing, I should say, is that there's no linear asymptotes here.
right
okay, so then given these informations
you just look at the transformations
and the asymptotes of the parent function
to curve sketch
okay, that makes sense.
You've got a horizontal asymptote $y=0$ as $x\to -\infty$ in the parent function.
04:53
how can I practice curve sketching?
Do examples. Then do more examples. :P
Right.
Alright, well I appreciate the help.
I know I am very slow to catch on :)
a little pain staking at times.
heh, no worries
I also haven't been taught by a real person in mathematics since 2011.
I've just been reading and doing examples :P
Do you have a tutor?
I guess your previous statement would preclude that.
04:56
No, I would almost describe myself as "self taught" at this point, although I am following a curriculum and having my work graded
Which is why there are far too many gaps in my knowledge of trivial mathematics.
Yeah, that's tough.
I just have to power through this, and hope things get resolved in university.
I think Spivak's Calculus may help me a little bit, I'm hoping to read it after I'm done these courses.
anyways, thanks again, Semi. You're a good friend!
glad to be of help :)
Zee
Zee
05:20
Is there anybody out there?
Zee
Zee
Ha!
Am kinda drunk...
Mmmmm I think it would be better for me to be drunk else where
Yes yes , I'll shall go now!
You'll shall have fun.
is infinity/(infinity-infinity) indeterminate?
05:42
Hi
06:38
@Computer Sure. ax^2/(bx^2 - cx) approaches a/b as x approaches infinity.
so that would mean it is not indeterminate
"Indeterminate" means "can take on any value".
Using this formula, I can make it equal any real number r. Just let a = r, b = 1.
Hi @Fargle
Hi @Balarka, how goes it?
Good. I wanted to write an answer here, but kept from doing it because I don't really like that approach (my answer would be equivalent to the existing answer)
currently thinking about how to do it the way I want to
06:48
Best of luck
my approach is probably not going to work lol
loosely,$xsin(1/x)$ is continuous since we can draw it without lifting the pen?
is the above one correct?
 
2 hours later…
08:41
Hello!!! I have a question.

Let n=pq. Suppose that there is an integer $b$ such that $b^2 \equiv c \pmod{n}$.

The above equivalence has 4 different solutions if (c,n)=1 , two different solutions if (c,n)=p or q or one solution if $n \mid c$.

Why does it hold that there are two different solutions in the case when (c,n)=p or q?
09:07
Chinese remainder theorem
p and q are primes, correct?
@Studentmathyes
Use the ring isomorphism $\Bbb Z_n\to\Bbb Z_p\times\Bbb Z_q$, if you know how to prove that there are 4 solutions in the (c,n)=1 case almost the same proof works here
I think there's something really strange going on when we build our way from $\omega$ to $\epsilon_0$. Recall that $$\epsilon_0=sup(\{1, \omega,\omega^{\omega},\omega^{\omega^{\omega}},...\})$$

By using what we learnt back when going from $\omega$ to $\omega^{\omega}$ via multiplication, it is clear the exponential tower is building from the bottom up, yet the evaluation of it is top down
but once we reach $\epsilon_0=\omega^{\omega^{\omega^⋰}}$, how on earth are we going to evaluate it from the top down as there is no top?
The fixed points of the lower hyperoperators don't suffer from this problem. For example:
$\omega = 1+1+\cdots$ is perfectly fine evaluating from the left (i.e. is left associative)

Also true for
$\omega^2=\omega + \omega + \omega + \cdots$

$\omega^{\omega} = \omega\omega\omega \cdots$
but that works because multiplication and addition are associative, but exponentiation is not
sorry typo, I mean limit ordinals, not fixed points
09:28
@Secret $\omega^{\omega^{\omega^⋰}}$ is abuse of notation
what it really means is $\sup(\{1, \omega,\omega^{\omega},\omega^{\omega^{\omega}},...\})$
@Secret $\omega = 1+1+\cdots$ is not fine either.
What it really means is $\omega = \sup(\{1,1+1,1+1+1,\cdots\})$
think about this: how would you define $1+1+\cdots$ otherwise?
So that means we can always compute the exponential towers from the top down (like how exponentiation is usually defined) since the first time when that occurs (when we reached $\epsilon_0$) every step in that sequence is not a tower with an infinite ordinal of levels thus the tower can get very big, but always have a top to evaluate it from top down?
@Secret No. You can't compute $\omega^{\omega^{\omega^⋰}}$. It is just a notation that makes sense intuitively but not formally.
The notation $\omega^{\omega^{\omega^⋰}}$ is defined to be $\sup(\{1, \omega,\omega^{\omega},\omega^{\omega^{\omega}},...\})$
Just like how $\displaystyle \sum_{i=1}^\infty a_i$ is defined as $\displaystyle \lim_{n\to\infty} \sum_{i=1}^n a_i$
If you don't define it that way, it may not be well-defined.
You can't just think of it as summing infinite terms (cf. conditional convergence).
ah ok
@Secret are you using any book/online material?
Mostly wikipedia, Dave L. Renfro's resources and the stuff that simpleart taught me a few months ago
09:41
@Secret where did you see $\omega^2=\omega + \omega + \omega + \cdots$?
I refer you to proofwiki for the definitions used in ordinal arithmetic.
I kinda try to reverse engineer the notation from $\omega\omega$ since I knew $\omega^2 = \sup (\{\omega,\omega 2, \omega 3,\cdots\})$ and that $\omega n$ for finite $n$ can be expanded into $\omega +\cdots + \omega$ n times due to left distributivity, (and similar phenomenon holds for $\omega^n$

So I thought intuitively $\omega^2=\omega + \omega + \omega + \cdots$, similar to how the notion is used when talking about $\epsilon_0$
@Secret so you mean it's based on $\omega = 1+1+\cdots$?
@LeakyNun yeah, since $\omega = \sup (\{0,1,2,3,...\})$ and $n=1+\cdots +1$ n times
@Secret so you basically invented "$\omega = 1+1+\cdots$"?
@LeakyNun Well, not really, since one of Dave's resources also use something similar as a demonstration
That guy uses $2\omega = 2 + 2 + 2 + \cdots = (1+1) + (1+1) + \cdots = \omega$
though I do agree if this is to be done rigorously, we need to manipulate supremums instead
09:49
@Secret immediately after this line:
> This is not a proof, because I haven’t given you the official definition of how to multiply ordinals.
yeah, and ordinal multiplication for successor ordinals is defined recursively as $\alpha(\beta + 1) = \alpha\beta + \alpha$
@Secret so do we have any questions now?
well, the thing about how the exponential tower should be evaluated is now clarified. But I am still trying to understand how the tetration case fails and whether the fix point equation $\alpha=\omega^{\alpha}$ is very different from the fixed point equations that came before it i.e. $\omega\alpha = \alpha$, $\omega + \alpha = \alpha$
@Secret what is the tetration case?
ordinal tetration (base case) $\;\; \sideset{_{}^0}{}\alpha = 1$ (successor case) $\;\; \sideset{_{}^{\beta + 1}}{}\alpha \; = \; \left(\sideset{_{}^{\beta}}{}\alpha \right)^{\alpha}\;$ if $\beta \geq 1$ (limit case) $\;\; \sideset{_{}^{\lambda}}{}\alpha \; = \; \sup \left\{\sideset{_{}^{\beta}}{}{\alpha}: \; \beta < \lambda \right\}\;$ if $\lambda$ is a nonzero limit ordinal Reader Exercise $\;\; \sideset{_{}^{\epsilon_0}}{}{\epsilon_0} \; = \; {\omega}^{{\omega}^{{\omega}^{{\epsilon}_0 \cdot 2}}} $ (Not the same as usual tetration when finite ordinals are used, however.) — Dave L. Renfro May 13 '13 at 19:51
the idea is to figure out if I continue the pattern on $\omega$ in the tetration case like that in the previous cases (addition, multiplication and exponentiation), where I will be relative to the epsilon numbers
09:59
@Secret I don't understand what you mean by "continue the pattern on $\omega$"

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