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12:27 AM
Oh, this is clever:
7
A: Your favourite maths puzzles

AryabhataThe odd town puzzle. You have a town with $m$ clubs formed by $n$ citizens of the town. The clubs are so formed that Each club has an odd number of members. Any two clubs have an even number of common members. (Could be zero too). Show that $m \le n$.

Hm, question: if $a+b\omega+c\omega^2=0$ (and $\omega$ is a primitive cube root of unity), must $a=b=c$?
 
Okay can anyone here do this:
Prove that forall n > 0: 2^n will contain a digit other than 1 and 0 in base 5
 
Well, hold on, that's $a+b\omega+c(-1-\omega)$, so $(a-c)+(b-c)\omega$, so $a-c=0$ and $b-c=0$, so… yeah.
 
Do you know the pretty nice proof that sum of roots of unity = 0 always holds?
 
@Krijn Is geometric sum not good enough?
 
Maybe it is
Roots of unity are almost always nice
 
12:37 AM
It's not really different than the geometric sum approach, but $(1+\omega+\cdots+\omega^{n-1})\omega = \omega+\omega^2+\cdots+1$ since $\omega^n=1$. @krijin
So you multiply by $\omega$ and get the same number back. Can't be anything but zero.
 
@Semiclassical Yay!
 
Another approach could be to look at it geometrically
 
yep. take an n-gon and turn it through $2\pi/n$ to get the same n-gon back.
 
17
A: Your favourite maths puzzles

Moor XuHere are three of my favorite variations on the hats and prisoners puzzle that I've collected over time: Fifteen prisoners sit in a line, and hats are placed on their heads. Each hat can be one of two colors: white or black. They can see the colors of the people in front of them but not behind ...

O.O Infinitely many hats?!
 
Alternatively:
They form the roots of $x^n-1$
 
12:47 AM
yeah, that's another good one.
 
Their sum must be the coefficient of $x^{n-1}$ in that polynomial (well, negative that I think)
but, unless $n=1$, that equals $0$.
 
Ah yes, that is another good one
 
But yeah, the "it doesn't change when you rotate it" one is the simplest proof
 
Well, it's part and parcel: The only way for the polynomial to remain unchanged if $x\mapsto \omega x$ is if the polynomial is of the form $ax^n+b$.
 
It's all related to how $\oint z^n\operatorname d\!z=0$ unless $n=-1$, as has been discussed countless times here before @Krijn
 
12:49 AM
And for that to have 1 as a root of unity means $z^n-1$ up to an overall irrelevant coefficient.
 
Good morning (?) @Secret
 
@Krijn I think it was Daminark who had a problem whose proof used a generalization
Let $G$ be a finite group, and say we have any homomorphism $f:G\to\Bbb C^\times$
 
the hard generalizations come when you're doing stuff like $\omega^{n^2}$.
Something something quadratic reciprocity.
 
Then the sum $\sum_{g\in G}f(g)$ equals $0$ if $f$ isn't the trivial homomorphism (where it equals $|G|$).
 
Oh, nice lol
 
12:52 AM
That took a lot of edits to get to the right formulation :P
But yeah, same proof; find some $g\in G$ for which $f(g)\ne1$, and note that it's invariant under multiplication by that.
 
1:03 AM
@AkivaWeinberger This is quite the analytic perspective you take here
 
@Semiclassical that conjecture needs galois theory to prove it
we are not equipped to do it
 
Which?
 
if a^n and (a+1)^n are rational and a is real, then a is rational
 
ahhh.
Yeah, galois theory would put it beyond me.
 
me making my question prompted the original to appear
in a comment
along with 15 other comments
 
1:13 AM
@Typhon Is it supposed to be true?
 
i was like "what hath I done!"
 
The solution the guy linked to uses Galois theory, but the one right above it (with a higher rating) does not
 
@AkivaWeinberger still used roots of unity
i was looking for a purely algebraic solution
or something somewhat easy or at an elementary level
 
For completeness, I'll repost the solution over here
We have that $a^n$ and $(a+1)^n$ are rational
Consider the polynomials $x^n-a^n$ and $(x+1)^n-(a+1)^n$. Clearly, $a$ is a root of both of these. Also, these have rational coefficients.
Now, in the complex plane, the roots of $x^n-a^n$ lie on a circle of radius $|a|$ centered at the origin.
The roots of $(x+1)^n-(a+1)^n$ lie on a circle of radius $|a+1|$ centered at $-1$.
Geometrically, then, we see that $a$ must be the only common root between them.
But if $a$ is irrational, all of its conjugates must be common roots as well. Thus, $a$ has no conjugates, and thus is rational.
@Semiclassical
(Perhaps this problem was unfair for you, Typhon, since you haven't encountered the notion of "conjugates of an algebraic number" before)
(This isn't the complex conjugate; this is something like "$2-\sqrt3$ is a conjugate of $2+\sqrt3$")
 
1:25 AM
hmm. reminds me of a greshgorin circles argument
 
(or "the four constants given by $\pm\sqrt2\pm\sqrt3$ are conjugates of each other")
(The relevant fact is that, if $a$ is a root of a polynomial $p(x)$ with rational coefficients, then all of its conjugates are roots of it as well.)
@Typhon
 
@AkivaWeinberger I know the conjugate of any element of $Z[x]$ where $x$ is some solution to $x^2 + ax + b = 0$ and where $a$ and $b$ are integers.
is that what you mean?
@AkivaWeinberger of course! that is genius!
 
Yeah, it's a generalization of that, I guess.
 
i forgot
one of the notes on higher order polynomials that the professor left on the final paper was that a polynomial ring for a third order polynomial needs 3 elements appended to it.
and all the elements relations to each other are the conjugates
of course!
 
The definition is as follows: if $a$ is an algebraic number, then its minimal polynomial is the monic polynomial of smallest degree with rational coefficients that it's a root of. The conjugates of $a$ are the other roots.
 
1:29 AM
@AkivaWeinberger smallest degree with rational?
I was messing with polynomials of integer coefficients.
 
Uh, poorly worded, sorry
 
nonono
i mean shouldnt that be integer?
 
Oh. Uh, I don't think it changes it, actually
 
oh
but the leading coefficient is 1
 
You can always multiply by the lowest common denominator of the coefficients
Conversely, you can divide by the leading coefficient, to go from "integer coefficient" to "rational coefficient"
If you want integer coefficient, you'll have to relax the condition of it being monic
because you can't make something like, say, $2x^2+2x+1$ monic
Doesn't matter what the leading coefficient is, really. The roots don't change when you multiply by constants. All that we really care about is that it's of the smallest degree.
 
1:33 AM
@AkivaWeinberger that's not valid. I meant only things like $x^2 + ax + b = 0$
 
Eh, still works for defining minimal polynomials and conjugates.
 
of course
im telling you what i researched
 
By the way, fun fact: $\prod(x\pm2\pm3)$, by which I mean $(x+\sqrt2+\sqrt3)\cdot(x-\sqrt2+\sqrt3)\cdot($not gonna write out all four terms,
has rational (in fact I think integer) coefficients.
(x-√2-√3)(x-√2+√3)(x+√2-√3)(x+√2+√3)
 
if a polynomial has integer coefficients and a leading coefficient of 1, then the only rational roots are integers
that is what makes them so well for constructing rings
 
Wolfram Alpha says that it's $x^4-10x^2+1$.
 
1:36 AM
every instance of $Z[x]$ is a ring
 
Which, incidentally, is the minimal polynomial of $\sqrt2+\sqrt3$.
 
interesting
 
@Typhon Ah, right. Interesting.
 
@AkivaWeinberger never dealt with minimal polynomials. I took a class called "introduction to abstract math". It was a proofing class involving basic number theory. I just know the games of appending, proving division theorems, integers of fields, greatest common divisors, prime numbers, modular arithmetic, and relatively primeness
know that thing I posted with all the number theory conjectures?
I basically generalized the concept of norm to solutions of second order polynomials with integer coefficients and a leading one of 1
beyond that, I could only speculate as to how we do that with 3rd order.
 
Yeah, I think the norm is just multiplying by the conjugate. Right?
 
1:49 AM
yup
for the second order casew
i dare not think about higher order polynomials rings
@AkivaWeinberger the conjugate is the other solution to the polynomial
 
By the way, you know how for the polynomial $x^2-3$, the ring $\Bbb Z[\sqrt3]$ contains both roots?
 
@AkivaWeinberger heh heh heh
I can top that
 
And in general for $x^2+ax+b$, the ring $\Bbb Z[\text{the first root}]$ contains the second
 
for the polynomial $x^2 + ax + b = 0$, $z[x]$ contains both
gosh darnit
sniped
 
This is false for third order polynomials.
 
1:50 AM
I know
 
Consider $x^3-2$.
 
I wrote a paper on this, fyi.
 
Like, a 10 page paper which contained my attempt to make a 3rd order ring.
the professor left one comment
append two solutions
 
So you know how $\Bbb Z[\sqrt[3]2]$ only contains one of the roots
rather than all three
 
1:51 AM
not in particular. it was the general case
let me find my notes
"Now that we have discussed the concept of second order polynomial rings we will attempt to extend the concept of a polynomial ring to the third order to see whether or not they are still rings. We suppose that $x$ is any solution to the polynomial equation $x^3 + \alpha x^2 + \beta x + \tau = 0$. By the first fundamental theorem of algebra, there are three solutions. We can get the"
"relationship between this two solutions by letting $t$, $y$, and $z$ denote the three solution and plugging them in to obtain $(x - t)(x - y)(x - z) = x^3 + \alpha x^2 + \beta x + \tau$. Rearranging terms we then obtain $x^3 + (-t - y - z)x^2 + (ty + tz + yz)x - tyz = x^3 + \alpha x^2 + \beta x + \tau$. By comparing the coefficients of the polynomials we get that $-t - y - z = \alpha$, $ty + tz + yz = \beta$, and $-tyz = \tau$."
Now we can examine $Z[t]$. We've already established that for any number set formed by appending elements to the integers, we obtain a set for which the negation and addition rules hold true. So we only need to determine whether or not $Z[t]$ is closed under multiplication."
We know by the definition of appending elements that we can write two elements $m$ and $n$ as $m = a + bt$ and $n = c + dt$, where $a$, $b$, $c$, and $d$ are integers. Taking their product we get that $mn = ac + adt + bct + cdt^2$. Using the identity $t = -y - z - \alpha$ we get that $mn = ac + adt + bct + cd(-ty - tz) - \alpha cdt$. Then, using the second identity $-ty - tz = yz - \beta$ we get that $mn = ac + adt + bct + cdyz - \beta cd - \alpha cdt$.
However, we are now stuck. There is no conceivable way to write $yz$ as an integer multiple of $t$. However, while we played this out to see exactly how far we could get, in hindsight it makes sense that this might fail.
Going back to the first proof we know that either a solution is an integer or irrational so when we use the polynomial $x^3 - 3 = 0$ as an example, we can quickly conclude that $(3)^{\frac{1}{3}}$ is an irrational number. In this situation $yz$ would be $\pm (3)^{\frac{2}{3}}$ which cannot be an integer multiple (or even a rational multiple) of $(3)^{\frac{1}{3}}$ as that would imply that the solution to the polynomial was in fact an integer.
However, that does not exclude that $(3)^{\frac{2}{3}} = a + b(3)^{\frac{1}{3}}$, where $a$ and $b$ are integers.
However, the attempt above shows that it is highly unlikely to be the case and so we conclude that not all sets formed by appending solutions of polynomials of order greater than $2$ to the integers or the rational numbers form either rings or fields. Of course, the complexity on trying to reduce the multiplication into the form warranted by $Z[x]$ will only increase the more the more the order is increased.
It is highly likely that there is an order at which the only rings formed by appending solutions is the integers themselves and with the same being true for all higher orders after that.
@AkivaWeinberger read my above quote from my paper.
 
@Typhon …Oh, you actually have the definition of appending things wrong
It works for quadratic polynomials, but it fails above that
 
hmm?
 
It's really, $\Bbb Z[t]$ is the set of things of the form $a_0+a_1t+a_2t^2+\dotsb+a_nt^n$, where the $a_i$ are integers and $n$ is a nonnegative integer
 
yeah, i figured that out
 
For quadratic irrationals, you never need more than $a_0+a_1t$.
 
2:04 AM
the professors note was to do $Z[t,t^2]$
 
i just had to remember
 
That's really the same as $\Bbb Z[t]$, using the proper definition.
But yeah, $\Bbb Z[3^{1/3}]$ is the set of things of the form $a+b3^{1/3}+c3^{2/3}$.
 
well, the professor used $Z[t]$ to specifically imply the one i used.
 
Incidentally, $\Bbb Z[\pi]$ is isomorphic to the set of (integer-coefficient) polynomials.
 
2:05 AM
isomorphic?
 
(We don't allow infinite-length ones, though)
@Typhon Uh, "essentially the same as"
 
well of course
 
There's a bijection between them that preserves the ring structure
 
@AkivaWeinberger so... $Z[\pi]$ is the same as appending all solutions to all polynomials like I described?
 
What do you mean?
It's the set of all things of the form $a_0+a_1\pi+\dotsb+a_n\pi^n$.
 
2:07 AM
yah
 
It gives you a really big ring
 
wait no
:/
I meant is appending pi equivalent to appending all solutions to $x^n + a_{n-1}x^{n-1} + a_{n-2}x^{n-2} + \cdots + a_0$
for finite integer n
 
No, that would give us the ring of "algebraic integers" (I think it's called), which I'm 99% sure is not isomorphic
 
ah ok
so that set is called the algebraic integers?
the set of all solutions?
 
I think so
Something's an algebraic integer if it's the root of a monic polynomial with integer coefficients
 
2:11 AM
@AkivaWeinberger monic as leading coefficient of 1?
 
So $\sqrt2$ (root of $x^2-2$) is good, but $\frac12$ (root of $2x-1$) is not
@Typhon Yeah
 
oh my
 
If you get rid of that condition, you get all algebraic numbers
 
that clarifies a large part of what you've been saying.
 
So $\sqrt2/2$ is algebraic, but not an "algebraic integer", since it's a root of $2x^2-1$.
 
2:12 AM
@AkivaWeinberger so solutions to all monic polynomials with integer coefficients is the algebraic integers, and then the algebraic rationals are the solutions to all non-monic polynomials with integer coefficients?
@AkivaWeinberger it is an element of $Q[\sqrt{2}]$
which I can confirm have no solutions in monic polynomials that are not in $Z[\sqrt{2}]$
i proved it
@AkivaWeinberger regardless, could you explain that proof of the power rational conjecture thingy?
 
 
1 hour later…
3:22 AM
@AkivaWeinberger where did you go?
 
3:33 AM
@Typhon Sorry. I'm back
You want me to go over the rational power thingy?
 
Heya DogAteMy
 
Hi professor
 
Howdy, skull
 
Suppose $a$ is real, and $n$ is a positive integer. Then if $a^n$ and $(a+1)^n$ are rational, then $a$ is rational as well.
I stole that from a Math SE post; the proof there was quite clever.
 
3:35 AM
Hey there Akiva, Ted, and Vegas!
 
hi Demonark
 
(Note that it's false if $a$ is allowed to be complex; take a cube root of unity and $n=6$.)
 
How's it going? @Daminark
 
I gave that as a puzzle to Semiclassical, who shared it with Typhon.
 
@AkivaWeinberger yes
and you're a professor?
 
3:39 AM
can't help but laugh
 
Who, Ted?
He's a retired professor, I thought you knew
 
no, he means you
 
las vegas said hello professor to you
???
 
…to Ted :P
 
no, he said that to me
 
3:40 AM
Ted is here?
 
Do you have him blocked or something??
 
hi chat
 
damnit not again
 
heya EricS
 
hi @EricSilva
 
3:41 AM
anybody done interesting math today?
 
@AkivaWeinberger people keep ending up blocked for me today for some reason and i didnt do it.
ugh
 
@EricSilva See my posts above describing a nice theorem
So, uh, proof.
Consider the polynomials $x^n-a^n$ and $(x+1)^n-(a+1)^n$.
 
Only you, Eric.
 
Are you looking forward to teaching again?
 
Me, skull? Yes. Definitely.
 
3:43 AM
the only thing i did today was hurt my brain over federer
 
$a$ is clearly a root of both of them. And, due to the properties of conjugates, all of the conjugates of $a$ are roots of both of them as well (they're polynomials with rational coefficients).
 
I told you that book is hopeless, Eric.
 
it really is :(
 
@AkivaWeinberger ok back up a moment
 
Note that rational numbers don't have conjugates.
 
3:43 AM
When you're an advanced grad student, then maybe, Eric. :)
 
how is a a root of them?
 
so many proofs are just like "We construct blah as in 2.5.10.1.2.4.2.3 and proceed as in 2.10.34.234.2.34.2.34"
 
DogAteMy: You need to explain what you mean by conjugates. Your audience doesn't know field theory/Galois theory.
 
and how did you get the polynomials?
@TedShifrin i know conjugates.
 
You exaggerate, Eric, but not much.
He doesn't mean complex conjugate.
 
3:44 AM
@TedShifrin i know.
 
@TedShifrin We had a discussion on them earlier
Substitute $a$ into $x^n-a^n$ and into $(x+1)^n-(a+1)^n$. @Typhon
 
OK.
OK. Shutting up.
 
@AkivaWeinberger ah I see. That was stupid.
 
As for where they came from… Hell if I know. But they're the key to solving this.
 
lol, hopefully i'll understand how to read it eventually, but i think im gonna give it another shot tomorrow, i'm reading it alongside Morgan's book which is like incredibly readable
 
3:45 AM
So, all the conjugates of $a$ are roots of those as well. Agreed? @Typhon
 
user84215
When you do not like someone, Would this fact lead to not answer his/her math question in this chat room?
 
Morgan's book is the antithesis ... but he hardly proves anything. Yet he still gives vocabulary and intuition.
 
(The polynomials have rational coefficients, because $-a^n$ and $-(a+1)^n$ are rational by the hypothesis.)
 
@TedShifrin I wrote a paper on the theory of creating rings from appending solutions to integer coefficient monomial second order polynomials in that abstract math class I took last semester. One of the main focuses was conjugates in that context.
so umm... yeah I know conjugates
 
Well, specifically for quadratic irrationals.
 
3:47 AM
OK, ordinarily one needs to study Galois theory to learn that. Cool.
 
@AkivaWeinberger of course. Conjugates are generally defined as the other solutions.
 
yeah that's the thing @Ted, Neves was talking to me about this fundamental result on regularity of solutions to the plateau problem, he wrote out the steps of the proof and was like "yeah so morgan skips half of step 1 and all of steps 2, 3, 4, and 5, so we'll have to do more work"
 
Morgan hardly proves anything, Eric.
 
@TedShifrin the professor handwaved it a tiny bit to define the conjugates as the other solutions.
 
Of the minimal polynomial, specifically.
 
3:47 AM
yeah but it has some nice drawings
 
No, his book is a great service (not so much his diff geo book or his calc book).
 
@AkivaWeinberger never heard of a minimal polynomial but I'll assume that is what you constructed.
 
it helps that he like also refers to the results by Federer's numbering frequently cause it makes it a nice companion to that monster
 
I told you about them earlier. It's the polynomial of the smallest degree that has it as a root. @Typhon
 
Is that an abstract question, @aminliverpool?
 
3:49 AM
@AkivaWeinberger ah. I see.
carry on
 
Eric: Most of us who write textbooks have to refer to things by numbers to some extent. But I tried to minimize the equation numbering game.
 
hi chat
 
But it does have the property that, if $a$ is a root of a polynomial, then its conjugates are roots of it as well. This requires proof, but let's assume it for now.
 
hi @Semi
 
user84215
@TedShifrin Maybe
 
3:50 AM
hi @Semiclassic
 
So the conjugates of $a$ are roots of $x^n-a^n$ and $(x+1)^n-(a+1)^n$. Our strategy is to show that $a$ is the only common root between these two, which then implies that $a$ has no conjugates, which implies that $a$ is rational. Make sense? @Typhon
 
@AkivaWeinberger once again, I define conjugates to be the other solutions but carry on. XD
@AkivaWeinberger oooh! I see.
wait though...
couldn't it be an irrational with the quality of having no conjugate?
 
@aminliverpool: I do not feel obligated to answer anyone's questions here (or on main). It's not like you're paying tuition to take my class and therefore are due appropriate attention and fair treatment. :)
 
@Ted yeahh I get lost in the numbering sometimes but there's a sweet spot with it. I've also been reading this book on gmt from a dude who taught a minicourse at the summer school i attended. Way more analytic than federer and so far has been muuuuuch more understandable to me
 
EricS: I find it hard to believe anything is more analytic than Federer. He obscures all the geometry.
 
3:51 AM
No. Every (algebraic) irrational has conjugates. (Note that $a$ is algebraic, since it's a root of $x^n-a^n$.) @Typhon
 
@AkivaWeinberger I see. That makes sense, but do you need the minimal polynomial in this case?
 
@Typhon (You need to be careful there, because both $\sqrt2$ and $\sqrt3$ are roots of $(x^2-2)(x^2-3)=x^4-5x^2+6$. Thus, $\sqrt3$ is one of the "other roots" other than $\sqrt2$, but they're not conjugates.)
^Re: "I define them to be the 'other solutions'"
 
is it not sufficient to find a rational coefficient polynomial such that a has no other solutions
 
@Ted I guess I mean there's less of the algebraic stuff Federer uses to define currents
 
@AkivaWeinberger Well I think of conjugate from the perspective of creating a ring from that space of solutions
 
3:54 AM
The conjugates are allowed to be complex, remember @Typhon
 
Again, that's pretty much analysis (other than the "algebra" of dual space).
 
but I must point out that I never went beyond 2nd order aside from speculation
 
Are you enjoying Wimbledon so far? @TedShifrin
 
yeah fair
 
$\omega\sqrt[3]2$ is a conjugate of $\sqrt[3]2$ (where $\omega$ is a cube root of unity)
 
3:54 AM
yeah, skull, pretty much ... horrible injury, though, today
 
since they both satisfy $x^3-2$
 
How bad?
 
uuuh
ok
I think of conjugates another way as well
 
this other book uses sets of finite perimeter, idk enough yet to know why you'd use one over the other
vs varifolds or whatever
 
Bethanie Mattek-Sands, the #1 women's doubles player, horribly dislocated her right knee in singles ... may be career-ending. who knows ... excruciating to watch ...
 
3:55 AM
conjugates are another element of the same ring such that the multiplication gives an integer
 
oh, damn
 
but... that is shaky
 
knee injuries are a bad time
 
Do you know the fundamental theorem of algebra? (Every polynomial of degree $n$ has $n$ roots, counting multiplicity)
 
yup
it was how I bullet-proof proved that there are always two solutions and therefore a conjugate
 
3:56 AM
EricS: If you're going to work in GMT (or even in parts of complex geometry), you need to think in terms of currents. You can see nice discussions of this in Griffiths/Harris, even, generalizing the Cauchy kernel in one complex variable to manifolds, etc.
 
and therefore there is always a norm etc
 
The grass is tough on the joints.
 
The idea is that, if the minimal polynomial of $a$ has degree, say, $m$, then that polynomial should have $m$ roots (including $a$), so $a$ would have $m-1$ conjugates. So conjugates always exist.
 
do we need the minimal polynomial here though?
 
Howdy @MikeM (g'night).
 
3:57 AM
finding a polynomial where a is the only solution should be sufficient.
 
@Typhon Is that possible?
You tell me.
Remember, we want this to have rational coefficients.
 
yes
 
giggles at DogAteMy's professorial demeanor :D
 
i meant like you said earlier with using a+1 in another polynomial root
 
3:58 AM
@Ted is griffiths/harris that principles of algebraic geometry book
 
and then showing the intersection had one root or something
 
i think that was the name
 
Yup, EricS.
 
mmk gonna see if i can find that
 
ugh, just watched a video clip of the point in the match where it occured.
 
3:59 AM
@AkivaWeinberger how are the conjugates of $a$ roots of $(x+1)^n - (a+1)^n = 0$? Shouldn't the other roots be conjugates of a+1?
 
Sometime we can discuss the Cauchy integral formula and how you can it interpret $dz/z$ as a current. What's $d$ of it? :P
 
hi @Ted
@EricSilva it's on my coffee table if you want it
 
@Typhon $a+1$ isn't a root of that; $a$ is. Substitute it in.
 
LOL, Mike.
 
@AkivaWeinberger ah
 

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