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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
May 1, 2025
May is an exciting month! National MATHCOUNTS is the second week of May in Washington D.C. and our Founder, Richard Rusczyk will be presenting a seminar, Preparing Strong Math Students for College and Careers, on May 11th.

Are you interested in working towards MATHCOUNTS and don’t know where to start? We have you covered! If you have taken Prealgebra, then you are ready for MATHCOUNTS/AMC 8 Basics. Already aiming for State or National MATHCOUNTS and harder AMC 8 problems? Then our MATHCOUNTS/AMC 8 Advanced course is for you.

Summer camps are starting next month at the Virtual Campus in math and language arts that are 2 - to 4 - weeks in duration. Spaces are still available - don’t miss your chance to have an enriching summer experience. There are middle and high school competition math camps as well as Math Beasts camps that review key topics coupled with fun explorations covering areas such as graph theory (Math Beasts Camp 6), cryptography (Math Beasts Camp 7-8), and topology (Math Beasts Camp 8-9)!

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[*]May 21st, 4:00pm PT/7:00pm ET, Mathcamp 2025 Qualifying Quiz Part 2 Math Jam, Problems 5 and 6, Canada/USA Mathcamp staff will discuss solutions to Problems 5 and 6 of the 2025 Mathcamp Qualifying Quiz![/list]
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0 replies
jlacosta
May 1, 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
JBMO TST- Bosnia and Herzegovina 2022 P1
Motion   5
N 34 minutes ago by justaguy_69
Source: JBMO TST 2022 Bosnia and Herzegovina P1
Let $a,b,c$ be real numbers such that $$a^2-bc=b^2-ca=c^2-ab=2$$. Find the value of $$ab+bc+ca$$and find at least one triplet $(a,b,c)$ that satisfy those conditions.
5 replies
Motion
May 21, 2022
justaguy_69
34 minutes ago
Serbian selection contest for the IMO 2025 - P6
OgnjenTesic   0
36 minutes ago
Source: Serbian selection contest for the IMO 2025
For an $n \times n$ table filled with natural numbers, we say it is a divisor table if:
- the numbers in the $i$-th row are exactly all the divisors of some natural number $r_i$,
- the numbers in the $j$-th column are exactly all the divisors of some natural number $c_j$,
- $r_i \ne r_j$ for every $i \ne j$.

A prime number $p$ is given. Determine the smallest natural number $n$, divisible by $p$, such that there exists an $n \times n$ divisor table, or prove that such $n$ does not exist.

Proposed by Pavle Martinović
0 replies
OgnjenTesic
36 minutes ago
0 replies
Serbian selection contest for the IMO 2025 - P5
OgnjenTesic   0
37 minutes ago
Source: Serbian selection contest for the IMO 2025
Determine the smallest positive real number $\alpha$ such that there exists a sequence of positive real numbers $(a_n)$, $n \in \mathbb{N}$, with the property that for every $n \in \mathbb{N}$ it holds that:
\[
        a_1 + \cdots + a_{n+1} < \alpha \cdot a_n.
    \]Proposed by Pavle Martinović
0 replies
OgnjenTesic
37 minutes ago
0 replies
Serbian selection contest for the IMO 2025 - P4
OgnjenTesic   0
37 minutes ago
Source: Serbian selection contest for the IMO 2025
For a permutation $\pi$ of the set $A = \{1, 2, \ldots, 2025\}$, define its colorfulness as the greatest natural number $k$ such that:
- For all $1 \le i, j \le 2025$, $i \ne j$, if $|i - j| < k$, then $|\pi(i) - \pi(j)| \ge k$.
What is the maximum possible colorfulness of a permutation of the set $A$? Determine how many such permutations have maximal colorfulness.

Proposed by Pavle Martinović
0 replies
OgnjenTesic
37 minutes ago
0 replies
Serbian selection contest for the IMO 2025 - P3
OgnjenTesic   0
38 minutes ago
Source: Serbian selection contest for the IMO 2025
Find all functions $f : \mathbb{Z} \to \mathbb{Z}$ such that:
- $f$ is strictly increasing,
- there exists $M \in \mathbb{N}$ such that $f(x+1) - f(x) < M$ for all $x \in \mathbb{N}$,
- for every $x \in \mathbb{Z}$, there exists $y \in \mathbb{Z}$ such that
\[
            f(y) = \frac{f(x) + f(x + 2024)}{2}.
        \]Proposed by Pavle Martinović
0 replies
OgnjenTesic
38 minutes ago
0 replies
Serbian selection contest for the IMO 2025 - P1
OgnjenTesic   0
43 minutes ago
Source: Serbian selection contest for the IMO 2025
Let \( p \geq 7 \) be a prime number and \( m \in \mathbb{N} \). Prove that
\[\left| p^m - (p - 2)! \right| > p^2.\]Proposed by Miloš Milićev
0 replies
OgnjenTesic
43 minutes ago
0 replies
Upper bound on products in sequence
tapir1729   10
N 43 minutes ago by Mathandski
Source: TSTST 2024, problem 7
An infinite sequence $a_1$, $a_2$, $a_3$, $\ldots$ of real numbers satisfies
\[
a_{2n-1} + a_{2n} > a_{2n+1} + a_{2n+2} \qquad \mbox{and} \qquad a_{2n} + a_{2n+1} < a_{2n+2} + a_{2n+3}
\]for every positive integer $n$. Prove that there exists a real number $C$ such that $a_{n} a_{n+1} < C$ for every positive integer $n$.

Merlijn Staps
10 replies
tapir1729
Jun 24, 2024
Mathandski
43 minutes ago
Prove $x+y$ is a composite number.
mt0204   0
44 minutes ago
Let $x, y \in \mathbb{N}^*$ such that $1000 x^{2023}+2024 y^{2023}$ is divisible by $x+y$ and $x+y>2$. Prove that $x+y$ is a composite number.
0 replies
mt0204
44 minutes ago
0 replies
A sharp one with 3 var
mihaig   7
N an hour ago by no_room_for_error
Source: Own
Let $a,b,c\geq0$ satisfying
$$\left(a+b+c-2\right)^2+8\leq3\left(ab+bc+ca\right).$$Prove
$$ab+bc+ca+abc\geq4.$$
7 replies
1 viewing
mihaig
May 13, 2025
no_room_for_error
an hour ago
Find all p(x) such that p(p) is a power of 2
truongphatt2668   6
N an hour ago by truongphatt2668
Source: ???
Find all polynomial $P(x) \in \mathbb{R}[x]$ such that:
$$P(p_i) = 2^{a_i}$$with $p_i$ is an $i$ th prime and $a_i$ is an arbitrary positive integer.
6 replies
truongphatt2668
May 15, 2025
truongphatt2668
an hour ago
External Direct Sum
We2592   1
N 2 hours ago by Acridian9
Q) 1. Let $V$ be external direct sum of vector spaces $U$ and $W$ over a field $\mathbb{F}$.let $\hat{U}={\{(u,0):u\in U\}}$ and $\hat{W}={\{(0,w):w\in W\}}$
show that
i) $\hat{U}$ and $\hat{W}$ is subspaces.
ii)$V=\hat{U}\oplus\hat{W}$

Q)2. Suppose $V=U+W$. Let $\hat{V}$ be the external direct sum of $U$ and $W$. show that $V$ is isomorphic to $\hat{V}$ under the correspondence $v=u+w\leftrightarrow(u,w)$

I face some trouble to solve this problems help me for understanding.
thank you.

1 reply
We2592
Yesterday at 2:45 AM
Acridian9
2 hours ago
How can I know the sequences's convergence value?
Madunglecha   5
N 6 hours ago by teomihai
What is the convergence value of the sequence??
(n^2)*ln(n+1/n)-n
5 replies
Madunglecha
Yesterday at 6:56 AM
teomihai
6 hours ago
Confusion in derivative
Vulch   1
N Today at 10:42 AM by Vulch
Solve the following problem:
1 reply
Vulch
Today at 10:39 AM
Vulch
Today at 10:42 AM
Prove the statement
Butterfly   12
N Today at 9:44 AM by oty
Given an infinite sequence $\{x_n\} \subseteq  [0,1]$, there exists some constant $C$, for any $r>0$, among the sequence $x_n$ and $x_m$ could be chosen to satisfy $|n-m|\ge r $ and $|x_n-x_m|<\frac{C}{|n-m|}$.
12 replies
Butterfly
May 7, 2025
oty
Today at 9:44 AM
Two times derivable real function
Valentin Vornicu   13
N Apr 24, 2025 by solyaris
Source: RMO 2008, 11th Grade, Problem 3
Let $ f: \mathbb R \to \mathbb R$ be a function, two times derivable on $ \mathbb R$ for which there exist $ c\in\mathbb R$ such that
\[ \frac { f(b)-f(a) }{b-a} \neq f'(c) ,\] for all $ a\neq b \in \mathbb R$.

Prove that $ f''(c)=0$.
13 replies
Valentin Vornicu
Apr 30, 2008
solyaris
Apr 24, 2025
Two times derivable real function
G H J
Source: RMO 2008, 11th Grade, Problem 3
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Valentin Vornicu
7301 posts
#1 • 2 Y
Y by Adventure10, Mango247
Let $ f: \mathbb R \to \mathbb R$ be a function, two times derivable on $ \mathbb R$ for which there exist $ c\in\mathbb R$ such that
\[ \frac { f(b)-f(a) }{b-a} \neq f'(c) ,\] for all $ a\neq b \in \mathbb R$.

Prove that $ f''(c)=0$.
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harazi
5526 posts
#2 • 2 Y
Y by Adventure10, Mango247
The image of the function $ g(a,b)=\frac{f(a)-f(b)}{a-b}$ defined for $ a\ne b$ being an interval (connected subset of the real line) and $ f'(c)$ not being in this image, it follows that we may assume that $ f'(c)<g(a,b)$ for all $ a\ne b$. But then $ f'(c)\leq f'(x)$ for all $ x$ and so $ c$ is a minimum point of $ f'$. Of course, this can be written in 11-th grade vocabulary. :D
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Svejk
663 posts
#3 • 1 Y
Y by Adventure10
I tried to solve it harazi's way but I didn't get maximum since I didn't manage to prove that $ g(a,b)-f'(c)$ has the same sign for all $ a,b$.Can you be give me more detailes please :lol: ?The official solution is based on the fact that the function $ g(x)=f(x)-f'(c)\cdot x$ is injective ,hence monotonic.
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harazi
5526 posts
#4 • 1 Y
Y by Adventure10
The set of pairs $ (a,b)$ such that $ a\ne b$ is a connected subset of the plane and the function $ g$ is continuous on this domain, thus its image is a connected subset of the line, thus an interval. I agree however that this is not a solution of an 11-th grade student, but that's how life is. :D I will not be amazed if in a few years I see complex analysis, Lebesgue integration and other such stuff at RMO. It's quite à pity, it gives huges advantages to some people. :(
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enescu
741 posts
#5 • 2 Y
Y by Adventure10, Mango247
harazi wrote:
But then $ f'(c)\leq f'(x)$ for all $ x$ and so $ c$ is a minimum point of $ f'$.
Why for all $ x$?
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harazi
5526 posts
#6 • 2 Y
Y by Adventure10, Mango247
Well, $ f'(c)\leq g(a,x)$ for all $ a\ne x$ and now make $ a$ close to $ x$ and use the definition of derivative.
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harazi
5526 posts
#7 • 1 Y
Y by Adventure10
Well, I said however a very stupid thing: the function $ g$ should be defined on the set of pairs $ (a,b)$ such that $ a<b$ to ensure that its domain is connected. Of course, all the rest works with this modification, I don't know how I could write such a stupid thing. :D
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enescu
741 posts
#8 • 4 Y
Y by adityaguharoy, Adventure10, Mango247, RobertRogo
Well, when I created this problem, I was thinking to the obvious geometric meaning: if $ f''(c) \ne 0$, then $ f$ is strictly concave up or down on some neighbourhood of the point $ c$, thus one can draw a close enough parallel to the line tangent at $ c$ to the function's graph that intersects the graph in two points $ (a,f(a))$ and $ (b,f(b))$. The slope of that tangent would be $ \frac{f(b)-f(a)}{b-a}$, equal to the slope of the tangent at $ c$, that is,$ f'(c)$.
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subham1729
1479 posts
#9 • 1 Y
Y by Adventure10
One of above solutions uses that $C=\mathbb{R}^2-\{(a,a) \mid a \in \mathbb{R}\}$ is connected, but why $C$ is connected ? $C$ has clearly two connected components. However with this spirit we can also solve the problem, extend $g(a,b)=\frac{f(a)-f(b)}{a-b}$ to whole plane defining $g(a,a)=f'(a)$ and now $g$ is continuous on whole plane and do similar thing.
This post has been edited 1 time. Last edited by subham1729, Jun 14, 2016, 4:53 AM
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Raunii
28 posts
#10
Y by
Svejk wrote:
I tried to solve it harazi's way but I didn't get maximum since I didn't manage to prove that $ g(a,b)-f'(c)$ has the same sign for all $ a,b$.Can you be give me more detailes please :lol: ?The official solution is based on the fact that the function $ g(x)=f(x)-f'(c)\cdot x$ is injective ,hence monotonic.

where did you find the official solution?
This post has been edited 1 time. Last edited by Raunii, Mar 15, 2020, 6:05 PM
Reason: .
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Rohit-2006
245 posts
#11
Y by
Too easy for grade 11....
Attachments:
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solyaris
649 posts
#12
Y by
@above: This is not a valid argument. From the MVT you only get for every $(a,b)$ there exists an $m$ with the desired property. So you get $f'(m) \neq f'(c)$ only for values $m$ in some set $M$, which has to be dense in the reals, but $M$ need not be an interval, so the IVP you use later on in your proof does not give a contradiction. (See the solutions above for proofs that avoid this problem.)
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Rohit-2006
245 posts
#13
Y by
Solyaris....can you please elaborate what you are trying to say....I can't get it what you are trying to say....I am just interested that $f'$ is continuous on $\mathbb{R}$ and that is true because $f$ is twice differentiable.
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solyaris
649 posts
#15
Y by
@above. To elaborate: Let $M = \{x \in R : f'(x) \neq f'(c)\}$. In the first paragraph you show that for all $a < b$ $M$ has to contain some $m \in (a,b)$ (which means that $M$ is a dense subset of the real numbers). This part of your proof is fine. But in order to make the proof of the green claim work you need to show that $M$ contains all real numbers. This is missing in your proof (if I interpret you proof correctly).
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