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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Wednesday at 3:18 PM
Spring is in full swing and summer is right around the corner, what are your plans? At AoPS Online our schedule has new classes starting now through July, so be sure to keep your skills sharp and be prepared for the Fall school year! Check out the schedule of upcoming classes below.

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0 replies
jlacosta
Wednesday at 3:18 PM
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
An inequality on triangles sides
nAalniaOMliO   7
N 26 minutes ago by navier3072
Source: Belarusian National Olympiad 2025
Numbers $a,b,c$ are lengths of sides of some triangle. Prove the inequality$$\frac{a}{b+c-a}+\frac{b}{c+a-b}+\frac{c}{a+b-c} \geq \frac{a+b}{2c}+\frac{b+c}{2a}+\frac{c+a}{2b}$$
7 replies
nAalniaOMliO
Mar 28, 2025
navier3072
26 minutes ago
D is incenter
Layaliya   3
N 26 minutes ago by rong2020
Source: From my friend in Indonesia
Given an acute triangle \( ABC \) where \( AB > AC \). Point \( O \) is the circumcenter of triangle \( ABC \), and \( P \) is the projection of point \( A \) onto line \( BC \). The midpoints of \( BC \), \( CA \), and \( AB \) are \( D \), \( E \), and \( F \), respectively. The line \( AO \) intersects \( DE \) and \( DF \) at points \( Q \) and \( R \), respectively. Prove that \( D \) is the incenter of triangle \( PQR \).
3 replies
Layaliya
Yesterday at 11:03 AM
rong2020
26 minutes ago
Constructing sequences
SMOJ   6
N an hour ago by lightsynth123
Source: 2018 Singapore Mathematical Olympiad Senior Q5
Starting with any $n$-tuple $R_0$, $n\ge 1$, of symbols from $A,B,C$, we define a sequence $R_0, R_1, R_2,\ldots,$ according to the following rule: If $R_j= (x_1,x_2,\ldots,x_n)$, then $R_{j+1}= (y_1,y_2,\ldots,y_n)$, where $y_i=x_i$ if $x_i=x_{i+1}$ (taking $x_{n+1}=x_1$) and $y_i$ is the symbol other than $x_i, x_{i+1}$ if $x_i\neq x_{i+1}$. Find all positive integers $n>1$ for which there exists some integer $m>0$ such that $R_m=R_0$.
6 replies
SMOJ
Mar 31, 2020
lightsynth123
an hour ago
Orthocenter is the midpoint of the altitude
plagueis   6
N an hour ago by FrancoGiosefAG
Source: Mexican Quarantine Mathematical Olympiad P4
Let $ABC$ be an acute triangle with orthocenter $H$. Let $A_1$, $B_1$ and $C_1$ be the feet of the altitudes of triangle $ABC$ opposite to vertices $A$, $B$, and $C$ respectively. Let $B_2$ and $C_2$ be the midpoints of $BB_1$ and $CC_1$, respectively. Let $O$ be the intersection of lines $BC_2$ and $CB_2$. Prove that $O$ is the circumcenter of triangle $ABC$ if and only if $H$ is the midpoint of $AA_1$.

Proposed by Dorlir Ahmeti
6 replies
plagueis
Apr 26, 2020
FrancoGiosefAG
an hour ago
Inspired by JK1603JK
sqing   3
N an hour ago by SunnyEvan
Source: Own
Let $ a,b,c\geq 0 $ and $ab+bc+ca=1.$ Prove that$$\frac{abc-2}{abc-1}\ge \frac{4(a^2b+b^2c+c^2a)}{a^3b+b^3c+c^3a+1} $$$$\frac{abc-1}{abc-2}\ge \frac{(\sqrt 2-1)(a^2b+b^2c+c^2a+1)}{a^3b+b^3c+c^3a+1} $$
3 replies
sqing
4 hours ago
SunnyEvan
an hour ago
polynomial
tiendat004   0
an hour ago
Let $p$ and $q$ be two prime numbers, with $p$ being a divisor of $q-1$. Prove that there exist integers $a,b,c,d$ such that the polynomial $x^p+cx+d$ is divisible by the polynomial $x^2+ax+b$ with $c$ is a multiple of $q$ and $b\neq 0$.
0 replies
tiendat004
an hour ago
0 replies
IMO 2018 Problem 1
juckter   168
N 2 hours ago by Trasher_Cheeser12321
Let $\Gamma$ be the circumcircle of acute triangle $ABC$. Points $D$ and $E$ are on segments $AB$ and $AC$ respectively such that $AD = AE$. The perpendicular bisectors of $BD$ and $CE$ intersect minor arcs $AB$ and $AC$ of $\Gamma$ at points $F$ and $G$ respectively. Prove that lines $DE$ and $FG$ are either parallel or they are the same line.

Proposed by Silouanos Brazitikos, Evangelos Psychas and Michael Sarantis, Greece
168 replies
juckter
Jul 9, 2018
Trasher_Cheeser12321
2 hours ago
An epitome of config geo
AndreiVila   9
N 2 hours ago by ihategeo_1969
Source: The Golden Digits Contest, December 2024, P3
Let $ABC$ be a scalene acute triangle with incenter $I$ and circumcircle $\Omega$. $M$ is the midpoint of small arc $BC$ on$\Omega$ and $N$ is the projection of $I$ onto the line passing through the midpoints of $AB$ and $AC$. A circle $\omega$ with center $Q$ is internally tangent to $\Omega$ at $A$, and touches segment $BC$. If the circle with diameter $IM$ meets $\Omega$ again at $J$, prove that $JI$ bisects $\angle QJN$.

Proposed by David Anghel
9 replies
AndreiVila
Dec 22, 2024
ihategeo_1969
2 hours ago
Beautiful problem
luutrongphuc   0
2 hours ago
Let triangle $ABC$ be circumscribed about circle $(I)$, and let $H$ be the orthocenter of $\triangle ABC$. The circle $(I)$ touches line $BC$ at $D$. The tangent to the circle $(BHC)$ at $H$ meets $BC$ at $S$. Let $J$ be the midpoint of $HI$, and let the line $DJ$ meet $(I)$ again at $X$. The tangent to $(I)$ parallel to $BC$ meets the line $AX$ at $T$. Prove that $ST$ is tangent to $(I)$.
0 replies
luutrongphuc
2 hours ago
0 replies
functional equation
tiendat004   0
2 hours ago
Let $a,b$ be two nonzero real numbers such that $a^2\neq b^2-2b-1.$ Consider the functions $f:\mathbb{R}\to\mathbb{R}$ and $g:\mathbb{R}\to\mathbb{R}$ satisfying the condition $$g(f(x+by))=a[f(x)+2yg(x)]+2xg(y)+bg(y)[y+g(y)],\quad\forall x,y\in\mathbb{R}.$$(a) Prove that there exists a real number $c$ such that $$g(bx)=\dfrac{b}{a}g(x)+cx,\quad\forall x\in\mathbb{R}.$$(b) Find all functions $f$ and $g$ that satisfy the given condition.
0 replies
tiendat004
2 hours ago
0 replies
inequality ( 4 var
SunnyEvan   0
2 hours ago
Let $ a,b,c,d \in R $ , such that $ a+b+c+d=4 . $ Prove that :
$$ a^4+b^4+c^4+d^4+3 \geq \frac{7}{4}(a^3+b^3+c^3+d^3) $$$$ a^4+b^4+c^4+d^4+ \frac{252}{25} \geq \frac{88}{25}(a^3+b^3+c^3+d^3) $$equality cases : ?
0 replies
SunnyEvan
2 hours ago
0 replies
Square and equilateral triangle
m4thbl3nd3r   2
N 2 hours ago by m4thbl3nd3r
Let $ABCD$ be a square and a point $X$ lies on the interior of $ABCD$ such that triangle $BDX$ is equilateral. Evaluate $\angle AXD$
2 replies
m4thbl3nd3r
3 hours ago
m4thbl3nd3r
2 hours ago
Problem in probability theory
Tip_pay   2
N 2 hours ago by elizhang101412
Find the probability that if four numbers from $1$ to $100$ (inclusive) are selected randomly without repetitions, then either all of them will be odd, or all will be divisible by $3$, or all will be divisible by $5$
2 replies
Tip_pay
Yesterday at 11:00 AM
elizhang101412
2 hours ago
NT Problem
tiendat004   0
3 hours ago
Let $a\in\mathbb{N}_+$ with $a$ is coprime to $21$. It is known that for every $s\in\mathbb{N}_+$, there always exist $r,t\in\mathbb{N}_+$ satisfying $$a+7s^3=r^3+7t^3.$$Prove that $a$ is a cube.
0 replies
tiendat004
3 hours ago
0 replies
Cyclotomic Polynomials in Olympiad Number Theory
dinoboy   14
N Jan 19, 2014 by tc1729
Source: dinoboy
Hello everyone,

Over the past few months I have written an article developing much of the theory on Cyclotomic Polynomials and many applications to Olympiad math. Its been almost finished for the past few weeks, but schoolwork delayed me finishing it for a very long time but now it is finished! There are extremely interesting and applicable to a pretty reasonable set of Olympiad Number Theory problems.

Below is a link to a copy in dropbox, however for some reason the hyperlinks don't render correctly on it which is pretty annoying. Because of this, a copy of the article is also attached to this post which hopefully the hyperlinks work correctly on.
I have also attached the tex file for my article in case they do not render correctly on the pdf, because then you can compile the document yourself. It also is attached to serve as a LaTeX sort of guide, because I myself struggled for quite a while to learn sufficient LaTeX to make a pdf which looked nice.

If you find any errors/typos, please e-mail me or private message the errors to me so I can correct them. Posting them onto this thread is fine as well, but I would prefer you not do that because I want this thread to be used for discussion of the exercises.

I hope you enjoy this article! :)

P.S.

Dropbox Version

EDIT : I have attached newer versions of the tex file/pdf now.
EDIT2: I have attached even newer versions.
14 replies
dinoboy
Jan 23, 2013
tc1729
Jan 19, 2014
Cyclotomic Polynomials in Olympiad Number Theory
G H J
Source: dinoboy
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dinoboy
2903 posts
#1 • 96 Y
Y by baijiangchen, pi37, v_Enhance, Kouichi Nakagawa, PolyaPal, AndrewKwon97, r31415, AkshajK, giratina150, math154, hyperbolictangent, El_Ectric, dantx5, dan23, Binomial-theorem, iarnab_kundu, turkeybob777, negativebplusorminus, GlassBead, exmath89, EricMathPath09, NewAlbionAcademy, kprepaf, siddigss, Amir Hossein, antimonyarsenide, Liebig, Zeref, RiteshR, aZpElr68Cb51U51qy9OM, AwesomeToad, tc1729, proglote, Aref, yugrey, shekast-istadegi, airplanes1, tahanguyen98, Maxima, Pinphong, cire_il, qua96, Potla, pgmath, ahaanomegas, forthegreatergood, bcp123, sicilianfan, Tuxianeer, acupofmath, BOGTRO, Arangeh, Torus121, subham1729, joybangla, minimario, fractals, henrikjb, john111111, fclvbfm934, TheCrafter, mikechen, huricane, hamup1, Wave-Particle, Eugenis, ptxpotterhead, tenplusten, enhanced, XbenX, fatant, mathleticguyyy, AlastorMoody, mathlogician, OlympusHero, myh2910, vsamc, NicoRicci, Flying-Man, Adventure10, Mango247, and 15 other users
Hello everyone,

Over the past few months I have written an article developing much of the theory on Cyclotomic Polynomials and many applications to Olympiad math. Its been almost finished for the past few weeks, but schoolwork delayed me finishing it for a very long time but now it is finished! There are extremely interesting and applicable to a pretty reasonable set of Olympiad Number Theory problems.

Below is a link to a copy in dropbox, however for some reason the hyperlinks don't render correctly on it which is pretty annoying. Because of this, a copy of the article is also attached to this post which hopefully the hyperlinks work correctly on.
I have also attached the tex file for my article in case they do not render correctly on the pdf, because then you can compile the document yourself. It also is attached to serve as a LaTeX sort of guide, because I myself struggled for quite a while to learn sufficient LaTeX to make a pdf which looked nice.

If you find any errors/typos, please e-mail me or private message the errors to me so I can correct them. Posting them onto this thread is fine as well, but I would prefer you not do that because I want this thread to be used for discussion of the exercises.

I hope you enjoy this article! :)

P.S.

Dropbox Version

EDIT : I have attached newer versions of the tex file/pdf now.
EDIT2: I have attached even newer versions.
Attachments:
Cyclotomic Polynomials.pdf (320kb)
Cyclotomic Polynomials.tex (53kb)
This post has been edited 4 times. Last edited by dinoboy, May 16, 2014, 8:49 PM
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v_Enhance
6870 posts
#2 • 15 Y
Y by baijiangchen, kprepaf, siddigss, ArefS, Binomial-theorem, Adventure10, Mango247, and 8 other users
This is great, thanks. I won't comment yet on the math because I've only had a few minutes to skim the article, but I'll point out the typos I've seen in a brief scan:

"Its ok if you ... " in the remark about UFD's.
"Note a key aspect we have above : we had no restictions"
Problem 1 from British MO: looks like you tried to copy a "fi" ligature from a PDF. The word "infinite" is broken.

Also, being a LaTeX geek, I can't help but point out a few things. (In all honesty the LaTeX is making my eyes bleed.)
Rant

If you share the Dropbox file with me, I would be happy to clean up the LaTeX for you. Shoot me a PM.
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dinoboy
2903 posts
#3 • 4 Y
Y by kundu, Adventure10, Mango247, and 1 other user
I have added 3 exercises to the article and corrected a few typos :)

P.S.
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puuhikki
979 posts
#4 • 2 Y
Y by Adventure10, Mango247
Do you mean the following in exercise 5?

Prove that the minimal polynomial of $\cos \left (\frac{2\pi}{n}\right )$ has degree $\frac{\varphi (n)}{2}.$ I haven't heard about the irreducible polynomial of $\cos \left (\frac{2\pi}{n}\right ).$
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dinoboy
2903 posts
#5 • 1 Y
Y by Adventure10
Yes! I have corrected the error. Currently the error is only corrected on the dropbox file, I will update the attached pdf and tex file when more changes have been made.
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mentalgenius
1020 posts
#6 • 2 Y
Y by Adventure10, Mango247
Is the dropbox file always the latest and most up to date?
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v_Enhance
6870 posts
#7 • 4 Y
Y by mentalgenius, Adventure10, Mango247, and 1 other user
Yes, because it's Dropbox. Dropbox has the convenient property that the file is automatically updated everywhere whenever you save. So if dinoboy saves a change, then all references everywhere get updated automatically.
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Jiminhio 10
17 posts
#8 • 1 Y
Y by Adventure10
Can somebody post solutions to the proposed exercises?
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a_math_geek
2 posts
#9 • 2 Y
Y by Adventure10, Mango247
Thanks for the article. Section 2.3, where you ask if the coefficients of all Cyclotomic polynomials are either -1,0 or 1, the answer is negative. The case n=105 gives the first counter-example to that statement, but your explanations give the reader the idea that he/she should try to prove that all coefficients are indeed from the set {-1,0,1} which is false and therefore can't be proven. So, I think you need to mention it to save people's time before they start to attempt to prove something which is false.

Also, when you talk about Cyclotomic polynomials over the field Zp in section 5, maybe it's me, but there is still ambiguity about how they are defined. Do you define them as the way they've already been calculated over Z[x] and then you work with them over Zp[x] by using the canonical epi-morphism from Z[x] to Zp[x]? It'd be easier to understand if you mention how they're defined explicitly.

(I'm new here. How can I type LaTeX codes on this forum?)
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dinoboy
2903 posts
#10 • 3 Y
Y by a_math_geek, Adventure10, and 1 other user
The reason why I don't tell the reader whether its true or false is because in trying to prove it is true you can motivate a lot of the identities in Section 3. These in turn motivate you to look at $\Phi_{pqr}(x)$ for $p,q,r$ odd primes after one proves it is true for $\Phi_{pq}(x)$, and it turns out the first example gives you a counterexample. Perhaps you might toil around wasting your time chasing something which is wrong, but finding the intermediate results will help solidify your understanding a lot.

You are correct that I never explicitly mentioned how I define $\Phi_n(x)$ over $\mathbb{Z}_p$. They are defined as taking the integer polynomial in mod $p$, and then reducing its coefficients mod $p$ (i.e. exactly what you are saying). When I have more time I'll edit this into my article.

To use LaTeX on the forum look here.
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math2468
148 posts
#11 • 2 Y
Y by Adventure10, Mango247
Is this a counterexample to 3.5?

Consider $\Phi_3(2^3) = 8^2 + 8 + 1 = 73$.

This is equal to \[\prod_{d|3}\Phi_{3d}(2) = \Phi_3(2)\Phi_9(2) = (2^2 + 2 + 1)(2^6 + 2^3 + 1) = 7\cdot 73 \ne 73\]
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GlassBead
1583 posts
#12 • 2 Y
Y by Adventure10, Mango247
$\gcd(3, 3) \neq 1$, so your counterexample doesn't apply.
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dinoboy
2903 posts
#13 • 2 Y
Y by Adventure10, Mango247
No, you do not. The article was written to be understandable only knowing concepts in Olympiad mathematics.
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JSGandora
4216 posts
#14 • 1 Y
Y by Adventure10
Can you post a proof of Theorem 3.1? The linked page referenced in the proof seems to be broken.
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tc1729
1221 posts
#15 • 2 Y
Y by Adventure10, Mango247
Hello, see here. But, you don't really need to take logs -- the Mobius inversion formula works multiplicatively for any Abelian group.
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