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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Apr 2, 2025
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0 replies
jlacosta
Apr 2, 2025
0 replies
k i Adding contests to the Contest Collections
dcouchman   1
N Apr 5, 2023 by v_Enhance
Want to help AoPS remain a valuable Olympiad resource? Help us add contests to AoPS's Contest Collections.

Find instructions and a list of contests to add here: https://artofproblemsolving.com/community/c40244h1064480_contests_to_add
1 reply
dcouchman
Sep 9, 2019
v_Enhance
Apr 5, 2023
k i Zero tolerance
ZetaX   49
N May 4, 2019 by NoDealsHere
Source: Use your common sense! (enough is enough)
Some users don't want to learn, some other simply ignore advises.
But please follow the following guideline:


To make it short: ALWAYS USE YOUR COMMON SENSE IF POSTING!
If you don't have common sense, don't post.


More specifically:

For new threads:


a) Good, meaningful title:
The title has to say what the problem is about in best way possible.
If that title occured already, it's definitely bad. And contest names aren't good either.
That's in fact a requirement for being able to search old problems.

Examples:
Bad titles:
- "Hard"/"Medium"/"Easy" (if you find it so cool how hard/easy it is, tell it in the post and use a title that tells us the problem)
- "Number Theory" (hey guy, guess why this forum's named that way¿ and is it the only such problem on earth¿)
- "Fibonacci" (there are millions of Fibonacci problems out there, all posted and named the same...)
- "Chinese TST 2003" (does this say anything about the problem¿)
Good titles:
- "On divisors of a³+2b³+4c³-6abc"
- "Number of solutions to x²+y²=6z²"
- "Fibonacci numbers are never squares"


b) Use search function:
Before posting a "new" problem spend at least two, better five, minutes to look if this problem was posted before. If it was, don't repost it. If you have anything important to say on topic, post it in one of the older threads.
If the thread is locked cause of this, use search function.

Update (by Amir Hossein). The best way to search for two keywords in AoPS is to input
[code]+"first keyword" +"second keyword"[/code]
so that any post containing both strings "first word" and "second form".


c) Good problem statement:
Some recent really bad post was:
[quote]$lim_{n\to 1}^{+\infty}\frac{1}{n}-lnn$[/quote]
It contains no question and no answer.
If you do this, too, you are on the best way to get your thread deleted. Write everything clearly, define where your variables come from (and define the "natural" numbers if used). Additionally read your post at least twice before submitting. After you sent it, read it again and use the Edit-Button if necessary to correct errors.


For answers to already existing threads:


d) Of any interest and with content:
Don't post things that are more trivial than completely obvious. For example, if the question is to solve $x^{3}+y^{3}=z^{3}$, do not answer with "$x=y=z=0$ is a solution" only. Either you post any kind of proof or at least something unexpected (like "$x=1337, y=481, z=42$ is the smallest solution). Someone that does not see that $x=y=z=0$ is a solution of the above without your post is completely wrong here, this is an IMO-level forum.
Similar, posting "I have solved this problem" but not posting anything else is not welcome; it even looks that you just want to show off what a genius you are.

e) Well written and checked answers:
Like c) for new threads, check your solutions at least twice for mistakes. And after sending, read it again and use the Edit-Button if necessary to correct errors.



To repeat it: ALWAYS USE YOUR COMMON SENSE IF POSTING!


Everything definitely out of range of common sense will be locked or deleted (exept for new users having less than about 42 posts, they are newbies and need/get some time to learn).

The above rules will be applied from next monday (5. march of 2007).
Feel free to discuss on this here.
49 replies
ZetaX
Feb 27, 2007
NoDealsHere
May 4, 2019
Romanian National Olympiad 1997 - Grade 12 - Problem 3
Filipjack   0
an hour ago
Source: Romanian National Olympiad 1997 - Grade 12 - Problem 3
Let $K$ be a finite field, $n \ge 2$ an integer, $f \in K[X]$ an irreducible polynomial of degree $n,$ and $g$ the product of all the nonconstant polynomials in $K[X]$ of degree at most $n-1.$ Prove that $f$ divides $g-1.$
0 replies
Filipjack
an hour ago
0 replies
Putnam 2001 A5
ahaanomegas   13
N an hour ago by Levieee
Prove that there are unique positive integers $a$, $n$ such that $a^{n+1}-(a+1)^n=2001$.
13 replies
ahaanomegas
Feb 26, 2012
Levieee
an hour ago
Romanian National Olympiad 1997 - Grade 12 - Problem 1
Filipjack   0
an hour ago
Source: Romanian National Olympiad 1997 - Grade 12 - Problem 1
Let $\alpha \in \mathbb{C} \setminus \mathbb{Q}$ be such that the set $A= \{ a+b \alpha : a,b \in \mathbb{Z} \}$ is a ring with respect to the usual operations of $\mathbb{C}.$ If the ring $A$ has exactly four invertible elements, prove that $A= \mathbb{Z}[i].$
0 replies
1 viewing
Filipjack
an hour ago
0 replies
Something nice
KhuongTrang   25
N an hour ago by KhuongTrang
Source: own
Problem. Given $a,b,c$ be non-negative real numbers such that $ab+bc+ca=1.$ Prove that

$$\sqrt{a+1}+\sqrt{b+1}+\sqrt{c+1}\le 1+2\sqrt{a+b+c+abc}.$$
25 replies
KhuongTrang
Nov 1, 2023
KhuongTrang
an hour ago
Two Functional Inequalities
Mathdreams   6
N 2 hours ago by Assassino9931
Source: 2025 Nepal Mock TST Day 2 Problem 2
Determine all functions $f : \mathbb{R} \rightarrow \mathbb{R}$ such that $$f(x) \le x^3$$and $$f(x + y) \le f(x) + f(y) + 3xy(x + y)$$for any real numbers $x$ and $y$.

(Miroslav Marinov, Bulgaria)
6 replies
Mathdreams
Today at 1:34 PM
Assassino9931
2 hours ago
Pythagorean new journey
XAN4   2
N 2 hours ago by mathprodigy2011
Source: Inspired by sarjinius
The number $4$ is written on the blackboard. Every time, Carmela can erase the number $n$ on the black board and replace it with a new number $m$, if and only if $|n^2-m^2|$ is a perfect square. Prove or disprove that all positive integers $n\geq4$ can be written exactly once on the blackboard.
2 replies
XAN4
Today at 3:41 AM
mathprodigy2011
2 hours ago
sqrt(2) and sqrt(3) differ in at least 1000 digits
Stuttgarden   2
N 2 hours ago by straight
Source: Spain MO 2025 P3
We write the decimal expressions of $\sqrt{2}$ and $\sqrt{3}$ as \[\sqrt{2}=1.a_1a_2a_3\dots\quad\quad\sqrt{3}=1.b_1b_2b_3\dots\]where each $a_i$ or $b_i$ is a digit between 0 and 9. Prove that there exist at least 1000 values of $i$ between $1$ and $10^{1000}$ such that $a_i\neq b_i$.
2 replies
Stuttgarden
Mar 31, 2025
straight
2 hours ago
combinatorics and number theory beautiful problem
Medjl   2
N 2 hours ago by mathprodigy2011
Source: Netherlands TST for BxMo 2017 problem 4
A quadruple $(a; b; c; d)$ of positive integers with $a \leq b \leq c \leq d$ is called good if we can colour each integer red, blue, green or purple, in such a way that
$i$ of each $a$ consecutive integers at least one is coloured red;
$ii$ of each $b$ consecutive integers at least one is coloured blue;
$iii$ of each $c$ consecutive integers at least one is coloured green;
$iiii$ of each $d$ consecutive integers at least one is coloured purple.
Determine all good quadruples with $a = 2.$
2 replies
Medjl
Feb 1, 2018
mathprodigy2011
2 hours ago
IMC 2010 - Problem 1
uglysolutions   9
N 2 hours ago by PikaPika999
Let $0 < a < b$. Prove that
$\int_a^b (x^2+1)e^{-x^2} dx \geq e^{-a^2} - e^{-b^2}$.
9 replies
uglysolutions
Jul 26, 2010
PikaPika999
2 hours ago
Squence problem
AlephG_64   1
N 2 hours ago by RagvaloD
Source: 2025 Finals Portuguese Math Olympiad P1
Francisco wrote a sequence of numbers starting with $25$. From the fourth term of the sequence onwards, each term of the sequence is the average of the previous three. Given that the first six terms of the sequence are natural numbers and that the sixth number written was $8$, what is the fifth term of the sequence?
1 reply
AlephG_64
Yesterday at 1:19 PM
RagvaloD
2 hours ago
50 points in plane
pohoatza   12
N 2 hours ago by de-Kirschbaum
Source: JBMO 2007, Bulgaria, problem 3
Given are $50$ points in the plane, no three of them belonging to a same line. Each of these points is colored using one of four given colors. Prove that there is a color and at least $130$ scalene triangles with vertices of that color.
12 replies
pohoatza
Jun 28, 2007
de-Kirschbaum
2 hours ago
beautiful functional equation problem
Medjl   6
N 3 hours ago by Sadigly
Source: Netherlands TST for BxMO 2017 problem 2
Let define a function $f: \mathbb{N} \rightarrow \mathbb{Z}$ such that :
$i)$$f(p)=1$ for all prime numbers $p$.
$ii)$$f(xy)=xf(y)+yf(x)$ for all positive integers $x,y$
find the smallest $n \geq 2016$ such that $f(n)=n$
6 replies
Medjl
Feb 1, 2018
Sadigly
3 hours ago
Line Combining Fermat Point, Orthocenter, and Centroid
cooljoseph   0
3 hours ago
On triangle $ABC$, draw exterior equilateral triangles on sides $AB$ and $AC$ to obtain $ABC'$ and $ACB'$, respectively. Let $X$ be the intersection of the altitude through $B$ and the median through $C$. Let $Y$ be the intersection of the altitude through $A$ and line $CC'$. Let $Z$ be the intersection of the median through $A$ and the line $BB'$. Prove that $X$, $Y$, and $Z$ lie on a common line.

IMAGE
0 replies
cooljoseph
3 hours ago
0 replies
complete integral values
Medjl   2
N 3 hours ago by Sadigly
Source: Netherlands TST for BxMO 2017 problem 1
Let $n$ be an even positive integer. A sequence of $n$ real numbers is called complete if for every integer $m$ with $1 \leq  m \leq  n$ either the sum of the first $m$ terms of the sum or the sum of the last $m$ terms is integral. Determine
the minimum number of integers in a complete sequence of $n$ numbers.
2 replies
Medjl
Feb 1, 2018
Sadigly
3 hours ago
Question about Riemann Hypothesis
hashtagmath   11
N May 28, 2021 by greenturtle3141
I am just learning about the Riemann Hypothesis and a question that has been nagging me is why can't we just find the zeroes of the functional equation and see if it holds against the hypothesis? It seems to me that finding the zeroes of that equations would easily prove/disprove the hypothesis. But obviously, it's more complex (no pun intended) than that. Can someone provide a glimpse into the difficulty of the problem?

Thanks again :D
11 replies
hashtagmath
May 26, 2021
greenturtle3141
May 28, 2021
Question about Riemann Hypothesis
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hashtagmath
1600 posts
#1 • 2 Y
Y by centslordm, etvat
I am just learning about the Riemann Hypothesis and a question that has been nagging me is why can't we just find the zeroes of the functional equation and see if it holds against the hypothesis? It seems to me that finding the zeroes of that equations would easily prove/disprove the hypothesis. But obviously, it's more complex (no pun intended) than that. Can someone provide a glimpse into the difficulty of the problem?

Thanks again :D
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Tintarn
9029 posts
#2 • 5 Y
Y by MathNinja7, hashtagmath, centslordm, Mango247, Mango247
I suppose that by the functional equation you mean Riemann's Functional Equation for the Zeta Function:
\[\pi^{-s/2} \Gamma\left(\frac{s}{2}\right)\zeta(s)=\pi^{-(1-s)/2} \Gamma\left(\frac{1-s}{2}\right)\zeta(1-s).\]If so, I don't understand what you mean by "finding the zeroes of that equation".
If we ignore for the moment the powers of $\pi$ and the Gamma Function (which we can assume to be well-understood, in particular regardings its zeroes and poles), the functional equation relates the values of $\zeta(s)$ and $\zeta(1-s)$.
In particular, it tells us that a zero of $\zeta$ at $s$ gives us a zero at $1-s$ (unless we are at a pole of the Gamma Function which would cancel the zero, this happens exactly at the even negative integers, giving the so-called "trivial zeroes").
So we do know that the non-trivial zeroes of $\zeta$ possess some structure: For $s$ a zero, also $1-s$ is a zero.
We also know that since $\overline{\zeta(s)}=\zeta(\overline{s})$, for $s$ a zero, also $\overline{s}$ is a zero.
So the zeroes are symmetrical around $s=\frac{1}{2}$ in several ways.
This explains naturally why something interesting could happen at the line $\text{Re}(s)=\frac{1}{2}$ but of course it does not tell us at all whether the zeroes are on that line or not.
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hashtagmath
1600 posts
#3 • 1 Y
Y by centslordm
I see, thank you for your thorough explanation.

I was actually thinking about the equation $\zeta(s) = 2^s\pi^{s-1}\sin\left(\frac{\pi s}{2}\right)\Gamma(1-s)\zeta(1-s)$ (which I assume is a manipulation of the equation in your post). The trivial zeroes are completely obvious in the $\sin$ part of the equation. But I now I understand the difficult part about proving the zeroes lie on one line is the part such that $\zeta(s) = \zeta(1-s)?$.
This post has been edited 2 times. Last edited by hashtagmath, May 26, 2021, 4:18 PM
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Tintarn
9029 posts
#4 • 2 Y
Y by hashtagmath, centslordm
Indeed your version is just a rearrangement of mine (the other one is more natural, for instance because it generalizes more nicely to Dirichlet L-functions). However I don't understand what you mean by "the part such that $\zeta(s)=\zeta(1-s)$".
Of course, when $\zeta(s)=0$, the equation $\zeta(s)=\zeta(1-s)$ is true, but I don't see why you would study exactly this equation.
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hashtagmath
1600 posts
#5 • 2 Y
Y by centslordm, Mango247
It may be a misunderstanding, but wouldn't finding the zeroes of this functional equation directly prove/disprove if the hypothesis is true?

Also, I meant to say, wouldn't studying the behavior of $\zeta(1-s)$ help in finding when the equation is equal to zero? Intuitively, I would expect that part is the most nontrivial as it's almost like a recursive function
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Tintarn
9029 posts
#6 • 2 Y
Y by hashtagmath, centslordm
Well, as you maybe can see better from my version of the functional equation, this is really not a recurrence, but a symmetry:
You can relate $\zeta(s)$ to $\zeta(1-s)$ and vice versa, but applying it twice really does not give you anything interesting, but a tautology!
It doesn't simplify anything to work with $1-s$ instead of $s$, this is just a change of variables.
Of course there are regions where $\zeta$ is well-understood. For instance, when $\text{Re}(s)>1$ we have the Euler product from which one can read off that there are no zeroes. One can then use the functional equation to "translate" this to the region $\text{Re}(s)<0$ where there will only be the trivial zeroes.
This leaves you with the so-called "critical strip" $0<\text{Re}(s)<1$. Again you could translate from $\frac{1}{2}<\text{Re}(s)<1$ to $0<\text{Re}(s)<\frac{1}{2}$ and vice versa. Indeed, if you can prove that one of them does not contain any zeroes, then the other won't as well. The point is that we understand neither of the two regions and neither of them is a priori simpler than the other!

Maybe I should also mention that it would already be a major advance (most likely winning you a Fields medal, unless you miss the age restriction) to prove that there are no zeroes with $0.999999<\text{Re}(s)<1$.
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naenaendr
639 posts
#7 • 4 Y
Y by hashtagmath, Mango247, Mango247, Mango247
Not sure if I'm misunderstanding your notion, but just finding the zeros to $\zeta(s)$ is not sufficient. Computers have been able to detect over a quadrillion non-trivial zeros, all of which have a real component of $\frac{1}{2}$. Just one counterexample would disprove the hypothesis, but until we can mathematically prove that all of the non-trivial zeros have this property, the hypothesis will remain hypothetical.
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hashtagmath
1600 posts
#8 • 2 Y
Y by centslordm, Mango247
Tintarn wrote:
Well, as you maybe can see better from my version of the functional equation, this is really not a recurrence, but a symmetry:
You can relate $\zeta(s)$ to $\zeta(1-s)$ and vice versa, but applying it twice really does not give you anything interesting, but a tautology!
It doesn't simplify anything to work with $1-s$ instead of $s$, this is just a change of variables.
Of course there are regions where $\zeta$ is well-understood. For instance, when $\text{Re}(s)>1$ we have the Euler product from which one can read off that there are no zeroes. One can then use the functional equation to "translate" this to the region $\text{Re}(s)<0$ where there will only be the trivial zeroes.
This leaves you with the so-called "critical strip" $0<\text{Re}(s)<1$. Again you could translate from $\frac{1}{2}<\text{Re}(s)<1$ to $0<\text{Re}(s)<\frac{1}{2}$ and vice versa. Indeed, if you can prove that one of them does not contain any zeroes, then the other won't as well. The point is that we understand neither of the two regions and neither of them is a priori simpler than the other!

Maybe I should also mention that it would already be a major advance (most likely winning you a Fields medal, unless you miss the age restriction) to prove that there are no zeroes with $0.999999<\text{Re}(s)<1$.

Thanks for the explanation!
naenaendr wrote:
Not sure if I'm misunderstanding your notion, but just finding the zeros to $\zeta(s)$ is not sufficient. Computers have been able to detect over a quadrillion non-trivial zeros, all of which have a real component of $\frac{1}{2}$. Just one counterexample would disprove the hypothesis, but until we can mathematically prove that all of the non-trivial zeros have this property, the hypothesis will remain hypothetical.

I was originally thinking to find ALL zeroes to the Riemann zeta function, it may be sufficient to set each individual part of the function equation (i.e. $2^s$, $\pi^{s-1}$, etc.) equal to zero and find the values of $s$ such that it is true. But it seems like the $\zeta(1-s)$ creates a symmetry which makes it more difficult to solve. Is my understanding correct?

And yes, you are correct about the large amount of non-trivial zeroes; in fact, Hardy proved that there are infinitely many zeroes on the critical line.

A follow-up question that I may ask about this is, would all the zeroes of the Riemann zeta function arise from the functional equation? In other words, is every zero check/verified/found through finding when the functional equation equals zero?
This post has been edited 3 times. Last edited by hashtagmath, May 27, 2021, 4:41 PM
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naenaendr
639 posts
#9 • 2 Y
Y by centslordm, hashtagmath
That would be an extremely challenging way to calculate zeros. Instead, computers input complex numbers into the function and notice if they are trending closer towards $0$. It's just like how computers calculate limits. So they are fund randomly, not from actual calculation. The zeta function is much more irregular than a polynomial like $f(x)=x^3-2x^2+1$. We currently do not have a way of calculating zeros of zeta directly. They are just detected
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hashtagmath
1600 posts
#10
Y by
Interesting, is there a known pattern for the distribution of zeroes on the critical line? Or are they irregularly distributed?

Also, separately, is the Riemann functional equation the only possible analytic continuation of the Riemann zeta function $\left(\zeta(s) = \sum_{n = 1}^{\infty}\frac{1}{n^s}\right)?$ In other words, does there exist a possibility for a different equation to extend the domain of the Riemann Zeta function?
This post has been edited 6 times. Last edited by hashtagmath, May 27, 2021, 6:31 PM
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Tintarn
9029 posts
#11 • 1 Y
Y by hashtagmath
Yes, any meromorphic function defined on a non-empty open set has at most one analytic continuation (Proof: The difference of two such analytic continuations vanishes on a non-empty open set hence is identically zero.) Nothing special about $\zeta$ here.
Regarding the zeroes: I think there is some fundamental misunderstanding. We don't (and we cannot!) "find" the zeroes from the functional equation. Again, regarding the zeroes inside the critical strip, the functional equation tells us that if $\zeta(s)=0$ then $\zeta(1-s)=0$. No more, no less. The other factors (the power of $\pi$ and the sine) are completely irrelevant for that purpose.
Instead there are clever methods that actually allow you to check for any finite number of zeroes whether they are on the critical line or not. But as already mentioned several times in this thread now, there are infinitely many zeroes so we will never prove the Hypothesis in this way.
Regarding the distribution: There is no obvious pattern, but there is some regularity. One can estimate how many zeroes roughly are there up to a certain imaginary part. Also, there are very deep conjectures about the "fine-scale statistics" i.e. how the distances between successive zeroes should behave, but this is all very speculative and certainly at least as hard to prove as the Riemann Hypothesis. (At this point we cannot even decide whether there could be a double zero, or whether the imaginary part of some of the zeroes is rational etc... we certainly expect that all of this does not happen and that these are essentially "random" numbers on the critical line.)
This post has been edited 1 time. Last edited by Tintarn, May 27, 2021, 6:49 PM
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greenturtle3141
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#12 • 5 Y
Y by hashtagmath, PhysKid11, centslordm, Mango247, Mango247
Just to elaborate on the above re analytic continuation, there is a theorem in complex analysis that I like to call "rigidity" this post became longer than i thought it would be, so treat this as an advertisement for learning complex analysis because it's SO cool:

Definition (Accumulation Point): Let $S$ be a set of complex numbers. Then $z_0 \in \mathbb{C}$ is an accumulation point of $S$ if for every $r > 0$, I can find $z \in S$ with $z \ne z_0$ such that $|z-z_0| < r$.

For example, the set $\{1,1/2,1/4,1/8,\cdots\}$ has an accumulation point, namely $0$. (The definition of accumulation point generalizes to arbitrary metric spaces, which you'll encounter very often in real analysis.)

Definition (Holomorphic): Let $\Omega \subseteq \mathbb{C}$ be an open region. Then $f:\Omega \to \mathbb{C}$ is holomorphic if it is "differentiable"; that is the limit $\lim_{z \to z_0} \frac{f(z)-f(z_0)}{z-z_0}$ exists for all $z_0 \in \Omega$.

This may sound boring for such a fancy word, but the fact that the limit holds over complex numbers actually spawns a crazy amount of properties that makes the word "holomorphic" a hundred time fancier than "differentiable". For example:
- If you're holomorphic, you're also analytic, i.e. have a power series representation. (Proof left as a very hard exercise, pirate a textbook for elaboration)
- If you're analytic, you're also holomorphic... (Left as an exercise)
- Shocking Conclusion: Any complex function that can be differentiated (i.e. is holomorphic) can be differentiated infinitely many times.

Note that the shocking conclusion above does not hold in real analysis, e.g. the function $f(x) = \begin{cases}x^2, & x \geq 0 \\ -x^2, x < 0\end{cases}$ can only be differentiated once. This shows that complex analysis is super lit.

These facts also show that being holomorphic and being analytic is the same thing in the complex world, so these words are used interchangeably when we're talking about complex analysis.

Theorem (Rigidity): Let $\Omega$ be an open region, $f:\Omega \to \mathbb{C}$ be holomorphic. Suppose the zeroes of $f$ (i.e. the set $\{z \in \Omega : f(z) = 0\}$) has an accumulation point. Then $f$ is the zero function.

Proof. Pirate a textbook. $\blacksquare$

For example, suppose $f:\mathbb{C} \to \mathbb{C}$ is holomorphic.
- If I know that $f$ is zero everywhere in the unit disk $\mathbb{D} := \{z \in \mathbb{C} : |z| < 1\}$, then $f$ has to be zero everywhere else because $\mathbb{D}$ has an accumulation point (in fact any point in the closure $\overline{\mathbb{D}} = \{z \in \mathbb{C} : |z| \leq 1\}$ is an accumulation point).
- If I know that $f(i/n) = 0$ for all $n \in \mathbb{N}$, then $f$ is zero everywhere else too.

But why do we care?

Corollary: Let $\Omega$ be an open region, $f,g:\Omega \to \mathbb{C}$ be holomorphic. Suppose the set for which $f = g$ has an accumulation point. Then $f \equiv g$ everywhere.

Proof. Apply the rigidity theorem on the holomorphic function $f-g$. $\blacksquare$

This lets us show that analytic continuations are unique.

Definition (Analytic Continuation): Let $\Omega_1,\Omega_2$ be open regions with $\Omega_1 \subset \Omega_2$. Let $f_1:\Omega_1 \to \mathbb{C}$ and $f_2,\Omega_2 \to \mathbb{C}$ be holomorphic. We say that $f_2$ is the analytic continuation of $f_1$ if $f_1$ and $f_2$ agree on $\Omega_1$, i.e. $f_1(z) = f_2(z) \quad \forall z \in \Omega_1$.

Theorem: Analytic continuations are unique.

Proof. Unless you're doing something incredibly dumb, $\Omega_1$ is non-empty, and by virtue of being open (look this up if you're unfamiliar) it definitely has an accumulation point. So if $f_2,f_3$ are both continuations of $f_1$ to $\Omega_2$, then surely $f_2,f_3$ must agree on $\Omega_1$. By the corollary it follows that $f_2$ and $f_3$ agree everywhere, i.e. they're the same analytic continuation. $\blacksquare$

With regards to the Riemann Zeta function, we have the "original" definition:
$$\zeta(s) := \sum_{n=1}^\infty \frac{1}{n^s}$$Where does this sum actually converge? Write $s = \sigma + it$. Then if $\sigma > 1$ then:
$$\left|\sum_{n=1}^N \frac{1}{n^s}\right| \leq \sum_{n=1}^N \frac{1}{|n^s|} = \sum_{n=1}^N \frac{1}{|e^{\log(n)\sigma + i\log(n)t}|} = \sum_{n=1}^N \frac{1}{e^{\log(n)\sigma}} = \sum_{n=1}^N \frac{1}{n^\sigma}$$Any I think by sending $N \to \infty$ the RHS converges via an integral test or something. Then absolute convergence implies convergence, so it follows that $\zeta(s)$ converges (and is thus well-defined) for all $s$ with real part greater than $1$.

But these silly mathematicians somehow defined $\zeta(-1)$ and stuff, how could they possibly do that? The short version is that using black magic, you can prove that
$$\zeta(s) = \frac{\pi^{s/2}\xi(s)}{\Gamma(s/2)} \qquad \forall \text{Re}(s) > 1$$and that using additional black magic you can show that $1/\Gamma$ extends everywhere (this proof is actually pretty cool) and $\xi$ extends everywhere except at $s=0,1$ or something, idk I don't remember the proof and this is above my paygrade of zero dollars an hour. Anyways what's important is that an analytic continuation of $\zeta(s)$ exists over $\mathbb{C} \setminus \{1\}$. And by the previous theorem, it's the only possible analytic continuation!

tl;dr pls study complex analysis its fun i promise might want to learn some real analysis first though
This post has been edited 1 time. Last edited by greenturtle3141, May 28, 2021, 4:45 AM
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