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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Wednesday at 3:18 PM
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0 replies
jlacosta
Wednesday at 3:18 PM
0 replies
Product of first m primes
joybangla   5
N 7 minutes ago by Erratum
Source: European Mathematical Cup 2013, Junior Division, P1
For $m\in \mathbb{N}$ define $m?$ be the product of first $m$ primes. Determine if there exists positive integers $m,n$ with the following property :
\[ m?=n(n+1)(n+2)(n+3) \]

Proposed by Matko Ljulj
5 replies
joybangla
Jul 3, 2014
Erratum
7 minutes ago
One of a,b,c,d is not greater than -1
WakeUp   5
N 43 minutes ago by Nari_Tom
Source: Baltic Way 2002
Let $a,b,c,d$ be real numbers such that
\[a+b+c+d=-2\]
\[ab+ac+ad+bc+bd+cd=0\]
Prove that at least one of the numbers $a,b,c,d$ is not greater than $-1$.
5 replies
WakeUp
Nov 13, 2010
Nari_Tom
43 minutes ago
a really nice polynomial problem
Etemadi   8
N an hour ago by amirhsz
Source: Iranian TST 2018, third exam day 1, problem 3
$n>1$ and distinct positive integers $a_1,a_2,\ldots,a_{n+1}$ are  given. Does there exist a polynomial $p(x)\in\Bbb{Z}[x]$ of degree  $\le n$ that satisfies the following conditions?
a. $\forall_{1\le i < j\le n+1}: \gcd(p(a_i),p(a_j))>1 $
b. $\forall_{1\le i < j < k\le n+1}: \gcd(p(a_i),p(a_j),p(a_k))=1 $

Proposed by Mojtaba Zare
8 replies
Etemadi
Apr 18, 2018
amirhsz
an hour ago
Japanese NT
pomodor_ap   1
N an hour ago by Tkn
Source: Japan TST 2024 p6
Find all quadruples $(a, b, c, d)$ of positive integers such that
$$2^a3^b + 4^c5^d = 2^b3^a + 4^d5^c.$$
1 reply
pomodor_ap
Oct 5, 2024
Tkn
an hour ago
Geo Mock #5
Bluesoul   3
N 5 hours ago by rhydon516
Consider triangle $ABC$ with $AB=13, BC=14, AC=15$. Denote the orthocenter of $\triangle{ABC}$ as $H$, the intersection of $(BHC)$ and $AC$ as $P\neq C$. Compute the length of $AP$.
3 replies
Bluesoul
Apr 1, 2025
rhydon516
5 hours ago
Inequalities
sqing   0
Today at 3:52 AM
Let $ a,b,c> 0 $ and $  \frac{a}{a^2+ab+c}+\frac{b}{b^2+bc+a}+\frac{c}{c^2+ca+b} \geq 1$. Prove that
$$  a+b+c\leq 3    $$
0 replies
sqing
Today at 3:52 AM
0 replies
Excalibur Identity
jjsunpu   8
N Today at 2:08 AM by MineCuber
proof is below
8 replies
jjsunpu
Yesterday at 3:27 PM
MineCuber
Today at 2:08 AM
New geometry problem
titaniumfalcon   1
N Today at 1:51 AM by mathprodigy2011
Post any solutions you have, with explanation or proof if possible, good luck!
1 reply
titaniumfalcon
Yesterday at 10:40 PM
mathprodigy2011
Today at 1:51 AM
Geo Mock #7
Bluesoul   1
N Yesterday at 8:20 PM by vanstraelen
Consider $\triangle{ABC}$ with $\angle{A}=90^{\circ}$ and $AB=10$. Let $D$ be a point on $AB$ such that $BD=6$. Suppose that the angle bisector of $\angle{C}$ is tangent to the circle with diameter $BD$ and say it intersects $AB$ at point $E$. Find the length of $BE$.
1 reply
Bluesoul
Apr 1, 2025
vanstraelen
Yesterday at 8:20 PM
Number of solutions
Ecrin_eren   3
N Yesterday at 8:00 PM by rchokler
The given equation is:

x³ + 4y³ + 2y = (2024 + 2y)(xy + 1)

The question asks for the number of integer solutions.

3 replies
Ecrin_eren
Yesterday at 11:27 AM
rchokler
Yesterday at 8:00 PM
Geo Mock #8
Bluesoul   1
N Yesterday at 7:06 PM by vanstraelen
Consider acute triangle $ABC$. Denote $M$ as the midpoint of $AB$, and let $O$ be a point on segment $CM$ such that $\angle{AOB}=120^{\circ}$. Find the length of $CM$ given $AO=5, BO=8, \angle{BAC}=60^{\circ}$.
1 reply
Bluesoul
Apr 1, 2025
vanstraelen
Yesterday at 7:06 PM
An inequality
jokehim   6
N Yesterday at 4:47 PM by imnotgoodatmathsorry
Let $a,b,c \in \mathbb{R}: a+b+c=3$ then prove $$\color{black}{\frac{a^2}{a^{2}-2a+3}+\frac{b^2}{b^{2}-2b+3}+\frac{c^2}{c^{2}-2c+3}\ge \frac{3}{2}.}$$
6 replies
jokehim
Mar 21, 2025
imnotgoodatmathsorry
Yesterday at 4:47 PM
Geometry problem
Raul_S_Baz   2
N Yesterday at 4:13 PM by Raul_S_Baz
IMAGE
2 replies
Raul_S_Baz
Wednesday at 8:49 PM
Raul_S_Baz
Yesterday at 4:13 PM
Cyclic inequality
JK1603JK   0
Yesterday at 3:40 PM
Given non-negative real numbers $a,b,c$ satisfying $ab+bc+ca=1.$ Prove that $$\frac{abc-1}{abc-2}\ge \frac{a^2b+b^2c+c^2a}{a^3b+b^3c+c^3a+1}.$$Equality holds at $a=b=1,c=0$ and its cyclic permutations.
0 replies
JK1603JK
Yesterday at 3:40 PM
0 replies
Hard number theory
Hip1zzzil   13
N Mar 30, 2025 by Hip1zzzil
Source: FKMO 2025 P6
Positive integers $a, b$ satisfy both of the following conditions.
For a positive integer $m$, if $m^2 \mid ab$, then $m = 1$.
There exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 = z^2 + w^2$ and $z^2 + w^2 > 0$.
Prove that there exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 + n = z^2 + w^2$, for each integer $n$.
13 replies
Hip1zzzil
Mar 30, 2025
Hip1zzzil
Mar 30, 2025
Hard number theory
G H J
G H BBookmark kLocked kLocked NReply
Source: FKMO 2025 P6
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Hip1zzzil
14 posts
#1
Y by
Positive integers $a, b$ satisfy both of the following conditions.
For a positive integer $m$, if $m^2 \mid ab$, then $m = 1$.
There exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 = z^2 + w^2$ and $z^2 + w^2 > 0$.
Prove that there exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 + n = z^2 + w^2$, for each integer $n$.
This post has been edited 4 times. Last edited by Hip1zzzil, Mar 30, 2025, 1:07 PM
Reason: Better
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Acorn-SJ
59 posts
#2 • 1 Y
Y by seoneo
Please copy the original wording, it’s mildly annoying to watch some phrases missing.
In particular, $m$ is supposed to be a positive integer.

EDIT: $n$ is an integer not a positive integer
This post has been edited 1 time. Last edited by Acorn-SJ, Mar 30, 2025, 11:25 AM
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pokmui9909
185 posts
#3 • 1 Y
Y by seoneo
This would be better:
Quote:
Positive integers $a, b$ satisfy both of the following conditions.
  • For a positive integer $m$, if $m^2 \mid ab$, then $m = 1$.
  • There exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 = z^2 + w^2$ and $z^2 + w^2 > 0$.
Prove that there exist integers $x, y, z, w$ that satisfies the equation $ax^2 + by^2 + n = z^2 + w^2$, for each integer $n$.
This post has been edited 2 times. Last edited by pokmui9909, Mar 30, 2025, 7:10 AM
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whwlqkd
68 posts
#4
Y by
Anyone solved it?
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Acorn-SJ
59 posts
#5
Y by
a failed attempt
This post has been edited 1 time. Last edited by Acorn-SJ, Mar 30, 2025, 6:20 AM
Reason: Typo
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Lufin
9 posts
#6 • 2 Y
Y by Acorn-SJ, jjkim0336
In the original paper, n is an integer, and does not have to be a positive integer.
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seoneo
339 posts
#7
Y by
(Sketch)

From the condition, $a$ and $b$ are square free and coprime.
By the infinite descent method, we have relatively prime $a \alpha, b \beta, \zeta, \omega$ so that $a \alpha^2 +b \beta^2 = \zeta^2 + \omega^2$.

Now consider the expression
\[ (\zeta t + p)^2 + (\omega t +q)^2 -a(\alpha t +r)^2 -b(\beta t +s)^2 = 2(\zeta p + \omega q -a\alpha r -b \beta s)t + p^2 +q^2 -ar^2 -bs^2 \]From Bezout, we have $p,q,r,s$ such that
\[ \zeta p + \omega q -a\alpha r -b \beta s =1 \]so we have all ever, or all odd $n$.

By changing parity of $p^2 +q^2 -ar^2 -bs^2 $, we have all integers.

PS. I thought switching the parity would be easy, but it's actually more subtle than I first thought.
This post has been edited 1 time. Last edited by seoneo, Mar 30, 2025, 8:17 AM
Reason: To add PS
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seoneo
339 posts
#8
Y by
This FKMO problem was not accurately translated at first. If the original wording is not preserved in the current post, it may be better to create a properly translated version and redirect others to that post.

P.S. It looks like it's been fixed now.
This post has been edited 1 time. Last edited by seoneo, Mar 31, 2025, 6:31 AM
Reason: To fix grammer.
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segment
24 posts
#10 • 1 Y
Y by MihaiT
seoneo wrote:
(Sketch)

From the condition, $a$ and $b$ are square free and coprime.
By the infinite descent method, we have relatively prime $a \alpha, b \beta, \zeta, \omega$ so that $a \alpha^2 +b \beta^2 = \zeta^2 + \omega^2$.

Now consider the expression
\[ (\zeta t + p)^2 + (\omega t +q)^2 -a(\alpha t +r)^2 -b(\beta t +s)^2 = 2(\zeta p + \omega q -a\alpha r -b \beta s)t + p^2 +q^2 -ar^2 -bs^2 \]From Bezout, we have $p,q,r,s$ such that
\[ \zeta p + \omega q -a\alpha r -b \beta s =1 \]so we have all ever, or all odd $n$.

By changing parity of $p^2 +q^2 -ar^2 -bs^2 $, we have all integers.

Solved same, the motivation was experimenting $a=b=1$, realizing that we can make a linear function of one variable.
I thought \[ \zeta p + \omega q -a\alpha r -b \beta s =1 \]is not enough for the case when $a,b,\alpha,\beta,\gamma,\delta$ are all odd, so for that case I used \[ \zeta p + \omega q -a\alpha r -b \beta s =2 \]and eventually I got all odds, $4k$s, $4k+2$s.
This post has been edited 2 times. Last edited by segment, Mar 30, 2025, 8:06 AM
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MihaiT
748 posts
#11
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very nice!
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GreekIdiot
157 posts
#12
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Acorn-SJ wrote:
Please copy the original wording, it’s mildly annoying to watch some phrases missing.
In particular, $m$ is supposed to be a positive integer.

Doesnt change a lot if $m$ is negative now, does it?
Also in case $ab$ is not squarefree then $m=1$ is a positive integer...
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Acorn-SJ
59 posts
#13
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GreekIdiot wrote:
Acorn-SJ wrote:
Please copy the original wording, it’s mildly annoying to watch some phrases missing.
In particular, $m$ is supposed to be a positive integer.

Doesnt change a lot if $m$ is negative now, does it?
Also in case $ab$ is not squarefree then $m=1$ is a positive integer...


Yes, you could say that, but better preserve the wording for the sake of archiving. Also, a much more important phrase was omitted, so I added that in.
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GreekIdiot
157 posts
#14
Y by
yeah, just saw the edit
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Hip1zzzil
14 posts
#15
Y by
Thank you for all the replies, Sorry for my bad English. I'm working on it though :)
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