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k a August Highlights and 2025 AoPS Online Class Information
jwelsh   0
Yesterday at 2:14 PM
CONGRATULATIONS to all the competitors at this year’s International Mathematical Olympiad (IMO)! The US Team took second place with 5 gold medals and 1 silver - we are proud to say that each member of the 2025 IMO team has participated in an AoPS WOOT (Worldwide Online Olympiad Training) class!

"As a parent, I'm deeply grateful to AoPS. Tiger has taken very few math courses outside of AoPS, except for a local Math Circle that doesn't focus on Olympiad math. AoPS has been one of the most important resources in his journey. Without AoPS, Tiger wouldn't be where he is today — especially considering he's grown up in a family with no STEM background at all."
— Doreen Dai, parent of IMO US Team Member Tiger Zhang

Interested to learn more about our WOOT programs? Check out the course page here or join a Free Scheduled Info Session. Early bird pricing ends August 19th!:
CodeWOOT Code Jam - Monday, August 11th
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MathWOOT Math Jam - Friday, August 15th

There is still time to enroll in our last wave of summer camps that start in August at the Virtual Campus, our video-based platform, for math and language arts! From Math Beasts Camp 6 (Prealgebra Prep) to AMC 10/12 Prep, you can find an informative 2-week camp before school starts. Plus, our math camps don’t have homework and cover cool enrichment topics like graph theory. Our language arts courses will build the foundation for next year’s challenges, such as Language Arts Triathlon for levels 5-6 and Academic Essay Writing for high school students.

Lastly, Fall is right around the corner! You can plan your Fall schedule now with classes at either AoPS Online, AoPS Academy Virtual Campus, or one of our AoPS Academies around the US. We’ve opened new Academy locations in San Mateo, CA, Pasadena, CA, Saratoga, CA, Johns Creek, GA, Northbrook, IL, and Upper West Side (NYC), New York.

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0 replies
jwelsh
Yesterday at 2:14 PM
0 replies
Some of my less-seen proposals
navid   8
N 3 minutes ago by shaboon
Dear friends,

Since 2003, I have had several nice days in AOPS-- i.e., Mathlinks; as some of you may remember. I decided to share you some of my less-seen proposals. Some of them may be considered as some early ethudes; several of them already appeared on some competitions or journals. I hope you like them and this be a good starting point for working on them. Please take a look at the following link.

https://drive.google.com/file/d/1bntcjZAHZ-WN1lfGbNbz0uyFvhBTMEhz/view?usp=sharing

Best regards,
Navid.
8 replies
1 viewing
navid
Jul 30, 2025
shaboon
3 minutes ago
O_1O_2 passes through the nine-point center
v_Enhance   16
N 40 minutes ago by v4913
Source: ELMO Shortlist 2013: Problem G7, by Michael Kural
Let $ABC$ be a triangle inscribed in circle $\omega$, and let the medians from $B$ and $C$ intersect $\omega$ at $D$ and $E$ respectively. Let $O_1$ be the center of the circle through $D$ tangent to $AC$ at $C$, and let $O_2$ be the center of the circle through $E$ tangent to $AB$ at $B$. Prove that $O_1$, $O_2$, and the nine-point center of $ABC$ are collinear.

Proposed by Michael Kural
16 replies
+1 w
v_Enhance
Jul 23, 2013
v4913
40 minutes ago
Connected graph with k edges
orl   27
N an hour ago by doulai1
Source: IMO 1991, Day 2, Problem 4, IMO ShortList 1991, Problem 10 (USA 5)
Suppose $ \,G\,$ is a connected graph with $ \,k\,$ edges. Prove that it is possible to label the edges $ 1,2,\ldots ,k\,$ in such a way that at each vertex which belongs to two or more edges, the greatest common divisor of the integers labeling those edges is equal to 1.

Note: Graph-Definition. A graph consists of a set of points, called vertices, together with a set of edges joining certain pairs of distinct vertices. Each pair of vertices $ \,u,v\,$ belongs to at most one edge. The graph $ G$ is connected if for each pair of distinct vertices $ \,x,y\,$ there is some sequence of vertices $ \,x = v_{0},v_{1},v_{2},\cdots ,v_{m} = y\,$ such that each pair $ \,v_{i},v_{i + 1}\;(0\leq i < m)\,$ is joined by an edge of $ \,G$.
27 replies
orl
Nov 11, 2005
doulai1
an hour ago
2024 ISL A3
sqing-inequality-BUST   14
N an hour ago by john0512
Source: 2024 ISL A3
Decide whether for every sequence $(a_n)$ of positive real numbers,

$\frac{3^{a_1}+3^{a_2}+\cdots+3^{a_n}}{(2^{a_1}+2^{a_2}+\cdots+2^{a_n})^2} < \frac{1}{2024}$

is true for at least one positive integer $n$.
14 replies
sqing-inequality-BUST
Jul 16, 2025
john0512
an hour ago
No more topics!
A final attempt to make a combinatorics problem
JARP091   4
N May 24, 2025 by JARP091
Source: At the time of posting the problem I do not know the source if any
Let \( N \) be a positive integer and consider the set \( S = \{1, 2, \ldots, N\} \).

Two players alternate moves. On each turn, the current player must select a nonempty subset \( T \subseteq S \) of numbers not previously chosen such that for every distinct \( x, y \in T \), neither \( x \) divides \( y \) nor \( y \) divides \( x \).

After selecting \( T \), all multiples of every element in \( T \), including those in \( T \) itself, are removed from \( S \).

The game continues with the reduced set \( S \) until no moves are possible.
Determine, for each \( N \), which player has a winning strategy if any

Note: It might be wrong or maybe too easy.
4 replies
JARP091
May 24, 2025
JARP091
May 24, 2025
A final attempt to make a combinatorics problem
G H J
Source: At the time of posting the problem I do not know the source if any
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
JARP091
143 posts
#1
Y by
Let \( N \) be a positive integer and consider the set \( S = \{1, 2, \ldots, N\} \).

Two players alternate moves. On each turn, the current player must select a nonempty subset \( T \subseteq S \) of numbers not previously chosen such that for every distinct \( x, y \in T \), neither \( x \) divides \( y \) nor \( y \) divides \( x \).

After selecting \( T \), all multiples of every element in \( T \), including those in \( T \) itself, are removed from \( S \).

The game continues with the reduced set \( S \) until no moves are possible.
Determine, for each \( N \), which player has a winning strategy if any

Note: It might be wrong or maybe too easy.
This post has been edited 1 time. Last edited by JARP091, May 24, 2025, 5:30 AM
Z K Y
The post below has been deleted. Click to close.
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wimpykid
40 posts
#2
Y by
How does a player win?
Z K Y
The post below has been deleted. Click to close.
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JARP091
143 posts
#3
Y by
wimpykid wrote:
How does a player win?

Thanks for replying, a player wins if the other player can not make any move i.e. cannot select a set \( T \) which satisfies the above condition
Z K Y
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ItzsleepyXD
169 posts
#4
Y by
why not first player select $T=\{1\}$ ?
then all the subset gone -> first player win
JARP091 wrote:
Let \( N \) be a positive integer and consider the set \( S = \{1, 2, \ldots, N\} \).

Two players alternate moves. On each turn, the current player must select a nonempty subset \( T \subseteq S \) of numbers not previously chosen such that for every distinct \( x, y \in T \), neither \( x \) divides \( y \) nor \( y \) divides \( x \).

After selecting \( T \), all multiples of every element in \( T \), including those in \( T \) itself, are removed from \( S \).

The game continues with the reduced set \( S \) until no moves are possible.
Determine, for each \( N \), which player has a winning strategy if any

Note: It might be wrong or maybe too easy.
JARP091 wrote:
wimpykid wrote:
How does a player win?

Thanks for replying, a player wins if the other player can not make any move i.e. cannot select a set \( T \) which satisfies the above condition
Z K Y
The post below has been deleted. Click to close.
This post has been deleted. Click here to see post.
JARP091
143 posts
#5
Y by
ItzsleepyXD wrote:
why not first player select $T=\{1\}$ ?
then all the subset gone -> first player win
uff I might never be a problem proposer
Z K Y
N Quick Reply
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