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k a April Highlights and 2025 AoPS Online Class Information
jlacosta   0
Apr 2, 2025
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.

WOOT early bird pricing is in effect, don’t miss out! If you took MathWOOT Level 2 last year, no worries, it is all new problems this year! Our Worldwide Online Olympiad Training program is for high school level competitors. AoPS designed these courses to help our top students get the deep focus they need to succeed in their specific competition goals. Check out the details at this link for all our WOOT programs in math, computer science, chemistry, and physics.

Looking for summer camps in math and language arts? Be sure to check out the video-based summer camps offered at the Virtual Campus that are 2- to 4-weeks in duration. 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)!

Be sure to mark your calendars for the following events:
[list][*]April 3rd (Webinar), 4pm PT/7:00pm ET, Learning with AoPS: Perspectives from a Parent, Math Camp Instructor, and University Professor
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April 9th (Webinar), 4:00pm PT/7:00pm ET, Learn about Video-based Summer Camps at the Virtual Campus
[*]April 10th (Math Jam), 4:30pm PT/7:30pm ET, 2025 MathILy and MathILy-Er Math Jam: Multibackwards Numbers
[*]April 22nd (Webinar), 4:00pm PT/7:00pm ET, Competitive Programming at AoPS (USACO).[/list]
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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
xf(x) + f ^2(y) +2 xf(y) perfect square for all positive integers x,y
parmenides51   8
N 2 minutes ago by E50
Source: Balkan BMO Shortlist 2017 N2
Find all functions $f :Z_{>0} \to Z_{>0}$ such that the number $xf(x) + f ^2(y) + 2xf(y)$ is a perfect square for all positive integers $x,y$.
8 replies
parmenides51
Aug 1, 2019
E50
2 minutes ago
Ratio of lengths in right-angled triangle
DylanN   2
N 5 minutes ago by sunken rock
Source: South African Mathematics Olympiad 2021, Problem 2
Let $PAB$ and $PBC$ be two similar right-angled triangles (in the same plane) with $\angle PAB = \angle PBC = 90^\circ$ such that $A$ and $C$ lie on opposite sides of the line $PB$. If $PC = AC$, calculate the ratio $\frac{PA}{AB}$.
2 replies
DylanN
Aug 11, 2021
sunken rock
5 minutes ago
2 renevant inequalities ?
giangtruong13   6
N 11 minutes ago by centslordm
im confused about 2 inequalities below
1/ Let $a,b,c>0$. Prove that:$$\sum_{cyc} \frac{1+a^2}{1+ab} \geq 3$$2/ Let $a,b,c>0$. Prove that: $$\sum_{cyc} \frac{1+a^4}{1+ab^3} \geq 3$$
6 replies
giangtruong13
Apr 1, 2025
centslordm
11 minutes ago
Function equations
kris_001   1
N 41 minutes ago by pco
Find all solution to $2f(2x)=f(x)+f(1-x),$ $f:[0,1]\rightarrow [0,1].$ I'm interested in what solutions there are other than constant functions.
1 reply
kris_001
Today at 12:35 AM
pco
41 minutes ago
IMO ShortList 1998, number theory problem 5
orl   61
N 42 minutes ago by cursed_tangent1434
Source: IMO ShortList 1998, number theory problem 5
Determine all positive integers $n$ for which there exists an integer $m$ such that ${2^{n}-1}$ is a divisor of ${m^{2}+9}$.
61 replies
orl
Oct 22, 2004
cursed_tangent1434
42 minutes ago
Geometric progression
hzbrl   2
N an hour ago by hzbrl
find all quadruples $(a, b, c, d)$ where they are integers and $a+b+c+d=65$, where $a, b, c, d$ are in a gp.
2 replies
hzbrl
5 hours ago
hzbrl
an hour ago
Number Theory Chain!
JetFire008   18
N an hour ago by whwlqkd
I will post a question and someone has to answer it. Then they have to post a question and someone else will answer it and so on. We can only post questions related to Number Theory and each problem should be more difficult than the previous. Let's start!

Question 1
18 replies
JetFire008
Yesterday at 7:14 AM
whwlqkd
an hour ago
inequality
pennypc123456789   2
N an hour ago by Primeniyazidayi
Let $a_{1} , a_{2} , a_{3} , a_{4} \ge 0  $ . Prove that
$$\dfrac{a_{1} + a_{2} + a_{3} + a_{4} }{4} \ge \sqrt{\dfrac{a_{1}a_{2}+a_{1}a_{3} +a_{1}a_{4}+ a_{2}a_{3} +a_{2} a_{4}+a_{3} , a_{4} }{6}}$$
2 replies
pennypc123456789
3 hours ago
Primeniyazidayi
an hour ago
cricket jumping in dominoes
YLG_123   1
N an hour ago by Bonime
Source: Brazil EGMO TST2 2023 #4
A cricket wants to move across a $2n \times 2n$ board that is entirely covered by dominoes, with no overlap. He jumps along the vertical lines of the board, always going from the midpoint of the vertical segment of a $1 \times 1$ square to another midpoint of the vertical segment, according to the rules:

$(i)$ When the domino is horizontal, the cricket jumps to the opposite vertical segment (such as from $P_2$ to $P_3$);

$(ii)$ When the domino is vertical downwards in relation to its position, the cricket jumps diagonally downwards (such as from $P_1$ to $P_2$);

$(iii)$ When the domino is vertically upwards relative to its position, the cricket jumps diagonally upwards (such as from $P_3$ to $P_4$).

The image illustrates a possible covering and path on the $4 \times 4$ board.
Considering that the starting point is on the first vertical line and the finishing point is on the last vertical line, prove that, regardless of the covering of the board and the height at which the cricket starts its path, the path ends at the same initial height.
1 reply
YLG_123
Jan 29, 2024
Bonime
an hour ago
nice fe with non-linear function being the answer
jjkim0336   1
N an hour ago by aidenkim119
Source: own
f:R+ -> R+

f(xf(y)+y) = y f(y^2 +x)
1 reply
jjkim0336
2 hours ago
aidenkim119
an hour ago
Poly with sequence give infinitely many prime divisors
Assassino9931   1
N an hour ago by bin_sherlo
Source: Bulgaria National Olympiad 2025, Day 1, Problem 3
Let $P(x)$ be a non-constant monic polynomial with integer coefficients and let $a_1, a_2, \ldots$ be an infinite sequence. Prove that there are infinitely many primes, each of which divides at least one term of the sequence $b_n = P(n)^{a_n} + 1$.
1 reply
Assassino9931
3 hours ago
bin_sherlo
an hour ago
ABC is a triangle where D is a point on the side AC such that AB=CD and $\angle
Evo_Zarpic   1
N 2 hours ago by sadat465
Source: Bangladesh National Mathematical Olympiad Junior 2024 P6
$ABC$ is a triangle where $D$ is a point on the side $AC$ such that $AB=CD$ and $\angle B - \frac{\angle A}{2} = 90^\circ$, $\angle ADB - \frac{\angle A}{2} = \angle ABD$. $BD$ is extended to $F$ such that $AB=AF$
Prove That $F$ is the circumcenter of Triangle $ABC$
1 reply
Evo_Zarpic
Feb 3, 2025
sadat465
2 hours ago
Orthocenter config once again
Assassino9931   2
N 2 hours ago by aidenkim119
Source: Bulgaria National Olympiad 2025, Day 2, Problem 4
Let \( ABC \) be an acute triangle with \( AB < AC \), midpoint $M$ of side $BC$, altitude \( AD \) (\( D \in BC \)), and orthocenter \( H \). A circle passes through points \( B \) and \( D \), is tangent to line \( AB \), and intersects the circumcircle of triangle \( ABC \) at a second point \( Q \). The circumcircle of triangle \( QDH \) intersects line \( BC \) at a second point \( P \). Prove that the lines \( MH \) and \( AP \) are perpendicular.
2 replies
Assassino9931
3 hours ago
aidenkim119
2 hours ago
Don't bite me for this straightforward sequence
Assassino9931   1
N 2 hours ago by alexheinis
Source: Bulgaria National Olympiad 2025, Day 1, Problem 1
Determine all infinite sequences $a_1, a_2, \ldots$ of real numbers such that
\[ a_{m^2 + m + n} = a_{m^2} + a_m + a_n\]for all positive integers $m$ and $n$.
1 reply
Assassino9931
3 hours ago
alexheinis
2 hours ago
The Sums of Elements in Subsets
bobaboby1   3
N Apr 2, 2025 by bobaboby1
Given a finite set \( X = \{x_1, x_2, \ldots, x_n\} \), and the pairwise comparison of the sums of elements of all its subsets (with the empty set defined as having a sum of 0), which amounts to \( \binom{2}{2^n} \) inequalities, these given comparisons satisfy the following three constraints:

1. The sum of elements of any non-empty subset is greater than 0.
2. For any two subsets, removing or adding the same elements does not change their comparison of the sums of elements.
3. For any two disjoint subsets \( A \) and \( B \), if the sums of elements of \( A \) and \( B \) are greater than those of subsets \( C \) and \( D \) respectively, then the sum of elements of the union \( A \cup B \) is greater than that of \( C \cup D \).

The question is: Does there necessarily exist a positive solution \( (x_1, x_2, \ldots, x_n) \) that satisfies all these conditions?
3 replies
bobaboby1
Mar 12, 2025
bobaboby1
Apr 2, 2025
The Sums of Elements in Subsets
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G H BBookmark kLocked kLocked NReply
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bobaboby1
35 posts
#1
Y by
Given a finite set \( X = \{x_1, x_2, \ldots, x_n\} \), and the pairwise comparison of the sums of elements of all its subsets (with the empty set defined as having a sum of 0), which amounts to \( \binom{2}{2^n} \) inequalities, these given comparisons satisfy the following three constraints:

1. The sum of elements of any non-empty subset is greater than 0.
2. For any two subsets, removing or adding the same elements does not change their comparison of the sums of elements.
3. For any two disjoint subsets \( A \) and \( B \), if the sums of elements of \( A \) and \( B \) are greater than those of subsets \( C \) and \( D \) respectively, then the sum of elements of the union \( A \cup B \) is greater than that of \( C \cup D \).

The question is: Does there necessarily exist a positive solution \( (x_1, x_2, \ldots, x_n) \) that satisfies all these conditions?
This post has been edited 2 times. Last edited by bobaboby1, Mar 12, 2025, 3:28 PM
Reason: Wrong typing.
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bobaboby1
35 posts
#3
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Can anybody solve it?
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bobaboby1
35 posts
#4
Y by
Maybe the question can be related with $Gordan$ $Theorem$?
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bobaboby1
35 posts
#5
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Too hard?
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N Quick Reply
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