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Contests & Programs AMC and other contests, summer programs, etc.
AMC and other contests, summer programs, etc.
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Contests & Programs AMC and other contests, summer programs, etc.
AMC and other contests, summer programs, etc.
3 M G
BBookmark  VNew Topic kLocked
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Topic
First Poster
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a My Retirement & New Leadership at AoPS
rrusczyk   1322
N 2 minutes ago by REP17
I write today to announce my retirement as CEO from Art of Problem Solving. When I founded AoPS 22 years ago, I never imagined that we would reach so many students and families, or that we would find so many channels through which we discover, inspire, and train the great problem solvers of the next generation. I am very proud of all we have accomplished and I’m thankful for the many supporters who provided inspiration and encouragement along the way. I'm particularly grateful to all of the wonderful members of the AoPS Community!

I’m delighted to introduce our new leaders - Ben Kornell and Andrew Sutherland. Ben has extensive experience in education and edtech prior to joining AoPS as my successor as CEO, including starting like I did as a classroom teacher. He has a deep understanding of the value of our work because he’s an AoPS parent! Meanwhile, Andrew and I have common roots as founders of education companies; he launched Quizlet at age 15! His journey from founder to MIT to technology and product leader as our Chief Product Officer traces a pathway many of our students will follow in the years to come.

Thank you again for your support for Art of Problem Solving and we look forward to working with millions more wonderful problem solvers in the years to come.

And special thanks to all of the amazing AoPS team members who have helped build AoPS. We’ve come a long way from here:IMAGE
1322 replies
+2 w
rrusczyk
Monday at 6:37 PM
REP17
2 minutes ago
k a March Highlights and 2025 AoPS Online Class Information
jlacosta   0
Mar 2, 2025
March is the month for State MATHCOUNTS competitions! Kudos to everyone who participated in their local chapter competitions and best of luck to all going to State! Join us on March 11th for a Math Jam devoted to our favorite Chapter competition problems! Are you interested in training for MATHCOUNTS? Be sure to check out our AMC 8/MATHCOUNTS Basics and Advanced courses.

Are you ready to level up with Olympiad training? Registration is open with early bird pricing available for our WOOT programs: MathWOOT (Levels 1 and 2), CodeWOOT, PhysicsWOOT, and ChemWOOT. What is WOOT? WOOT stands for Worldwide Online Olympiad Training and is a 7-month high school math Olympiad preparation and testing program that brings together many of the best students from around the world to learn Olympiad problem solving skills. Classes begin in September!

Do you have plans this summer? There are so many options to fit your schedule and goals whether attending a summer camp or taking online classes, it can be a great break from the routine of the school year. Check out our summer courses at AoPS Online, or if you want a math or language arts class that doesn’t have homework, but is an enriching summer experience, our AoPS Virtual Campus summer camps may be just the ticket! We are expanding our locations for our AoPS Academies across the country with 15 locations so far and new campuses opening in Saratoga CA, Johns Creek GA, and the Upper West Side NY. Check out this page for summer camp information.

Be sure to mark your calendars for the following events:
[list][*]March 5th (Wednesday), 4:30pm PT/7:30pm ET, HCSSiM Math Jam 2025. Amber Verser, Assistant Director of the Hampshire College Summer Studies in Mathematics, will host an information session about HCSSiM, a summer program for high school students.
[*]March 6th (Thursday), 4:00pm PT/7:00pm ET, Free Webinar on Math Competitions from elementary through high school. Join us for an enlightening session that demystifies the world of math competitions and helps you make informed decisions about your contest journey.
[*]March 11th (Tuesday), 4:30pm PT/7:30pm ET, 2025 MATHCOUNTS Chapter Discussion MATH JAM. AoPS instructors will discuss some of their favorite problems from the MATHCOUNTS Chapter Competition. All are welcome!
[*]March 13th (Thursday), 4:00pm PT/7:00pm ET, Free Webinar about Summer Camps at the Virtual Campus. Transform your summer into an unforgettable learning adventure! From elementary through high school, we offer dynamic summer camps featuring topics in mathematics, language arts, and competition preparation - all designed to fit your schedule and ignite your passion for learning.[/list]
Our full course list for upcoming classes is below:
All classes run 7:30pm-8:45pm ET/4:30pm - 5:45pm PT unless otherwise noted.

Introductory: Grades 5-10

Prealgebra 1 Self-Paced

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Group Theory
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MATHCOUNTS/AMC 8 Basics
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F=ma Problem Series
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WOOT Programs
Visit the pages linked for full schedule details for each of these programs!


MathWOOT Level 1
MathWOOT Level 2
ChemWOOT
CodeWOOT
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Programming

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Relativity
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Mon, Tue, Wed & Thurs, Jun 23 - Jun 26 (meets every day of the week!)
0 replies
jlacosta
Mar 2, 2025
0 replies
k i Suggestion Form
jwelsh   0
May 6, 2021
Hello!

Given the number of suggestions we’ve been receiving, we’re transitioning to a suggestion form. If you have a suggestion for the AoPS website, please submit the Google Form:
Suggestion Form

To keep all new suggestions together, any new suggestion threads posted will be deleted.

Please remember that if you find a bug outside of FTW! (after refreshing to make sure it’s not a glitch), make sure you’re following the How to write a bug report instructions and using the proper format to report the bug.

Please check the FTW! thread for bugs and post any new ones in the For the Win! and Other Games Support Forum.
0 replies
jwelsh
May 6, 2021
0 replies
k i Read me first / How to write a bug report
slester   3
N May 4, 2019 by LauraZed
Greetings, AoPS users!

If you're reading this post, that means you've come across some kind of bug, error, or misbehavior, which nobody likes! To help us developers solve the problem as quickly as possible, we need enough information to understand what happened. Following these guidelines will help us squash those bugs more effectively.

Before submitting a bug report, please confirm the issue exists in other browsers or other computers if you have access to them.

For a list of many common questions and issues, please see our user created FAQ, Community FAQ, or For the Win! FAQ.

What is a bug?
A bug is a misbehavior that is reproducible. If a refresh makes it go away 100% of the time, then it isn't a bug, but rather a glitch. That's when your browser has some strange file cached, or for some reason doesn't render the page like it should. Please don't report glitches, since we generally cannot fix them. A glitch that happens more than a few times, though, could be an intermittent bug.

If something is wrong in the wiki, you can change it! The AoPS Wiki is user-editable, and it may be defaced from time to time. You can revert these changes yourself, but if you notice a particular user defacing the wiki, please let an admin know.

The subject
The subject line should explain as clearly as possible what went wrong.

Bad: Forum doesn't work
Good: Switching between threads quickly shows blank page.

The report
Use this format to report bugs. Be as specific as possible. If you don't know the answer exactly, give us as much information as you know. Attaching a screenshot is helpful if you can take one.

Summary of the problem:
Page URL:
Steps to reproduce:
1.
2.
3.
...
Expected behavior:
Frequency:
Operating system(s):
Browser(s), including version:
Additional information:


If your computer or tablet is school issued, please indicate this under Additional information.

Example
Summary of the problem: When I click back and forth between two threads in the site support section, the content of the threads no longer show up. (See attached screenshot.)
Page URL: http://artofproblemsolving.com/community/c10_site_support
Steps to reproduce:
1. Go to the Site Support forum.
2. Click on any thread.
3. Click quickly on a different thread.
Expected behavior: To see the second thread.
Frequency: Every time
Operating system: Mac OS X
Browser: Chrome and Firefox
Additional information: Only happens in the Site Support forum. My tablet is school issued, but I have the problem at both school and home.

How to take a screenshot
Mac OS X: If you type ⌘+Shift+4, you'll get a "crosshairs" that lets you take a custom screenshot size. Just click and drag to select the area you want to take a picture of. If you type ⌘+Shift+4+space, you can take a screenshot of a specific window. All screenshots will show up on your desktop.

Windows: Hit the Windows logo key+PrtScn, and a screenshot of your entire screen. Alternatively, you can hit Alt+PrtScn to take a screenshot of the currently selected window. All screenshots are saved to the Pictures → Screenshots folder.

Advanced
If you're a bit more comfortable with how browsers work, you can also show us what happens in the JavaScript console.

In Chrome, type CTRL+Shift+J (Windows, Linux) or ⌘+Option+J (Mac).
In Firefox, type CTRL+Shift+K (Windows, Linux) or ⌘+Option+K (Mac).
In Internet Explorer, it's the F12 key.
In Safari, first enable the Develop menu: Preferences → Advanced, click "Show Develop menu in menu bar." Then either go to Develop → Show Error console or type Option+⌘+C.

It'll look something like this:
IMAGE
3 replies
slester
Apr 9, 2015
LauraZed
May 4, 2019
k i Community Safety
dcouchman   0
Jan 18, 2018
If you find content on the AoPS Community that makes you concerned for a user's health or safety, please alert AoPS Administrators using the report button (Z) or by emailing sheriff@aops.com . You should provide a description of the content and a link in your message. If it's an emergency, call 911 or whatever the local emergency services are in your country.

Please also use those steps to alert us if bullying behavior is being directed at you or another user. Content that is "unlawful, harmful, threatening, abusive, harassing, tortuous, defamatory, vulgar, obscene, libelous, invasive of another's privacy, hateful, or racially, ethnically or otherwise objectionable" (AoPS Terms of Service 5.d) or that otherwise bullies people is not tolerated on AoPS, and accounts that post such content may be terminated or suspended.
0 replies
dcouchman
Jan 18, 2018
0 replies
usamOOK geometry
KevinYang2.71   85
N 14 minutes ago by Mathandski
Source: USAMO 2025/4, USAJMO 2025/5
Let $H$ be the orthocenter of acute triangle $ABC$, let $F$ be the foot of the altitude from $C$ to $AB$, and let $P$ be the reflection of $H$ across $BC$. Suppose that the circumcircle of triangle $AFP$ intersects line $BC$ at two distinct points $X$ and $Y$. Prove that $C$ is the midpoint of $XY$.
85 replies
+1 w
KevinYang2.71
Mar 21, 2025
Mathandski
14 minutes ago
Good AIME/Olympiad Level Number Theory Books
MathRook7817   1
N 29 minutes ago by rbcubed13
Hey guys, do you guys have any good AIME/USAJMO Level Number Theory book suggestions?
I'm trying to get 10+ on next year's AIME and hopefully qual for USAJMO.
1 reply
MathRook7817
33 minutes ago
rbcubed13
29 minutes ago
[TEST RELEASED] Mock Geometry Test for College Competitions
Bluesoul   22
N 32 minutes ago by QuestionSourcer
Hi AOPSers,

I have finished writing a mock geometry test for fun and practice for the real college competitions like HMMT/PUMaC/CMIMC... There would be 10 questions and you should finish the test in 60 minutes, the test would be close to the actual test (hopefully). You could sign up under this thread, PM me your answers!. The submission would close on March 31st at 11:59PM PST.

I would create a private discussion forum so everyone could discuss after finishing the test. This is the first mock I've written, please sign up and enjoy geometry!!

~Bluesoul

Discussion forum: Discussion forum

Leaderboard
22 replies
Bluesoul
Feb 24, 2025
QuestionSourcer
32 minutes ago
what the yap
KevinYang2.71   25
N 40 minutes ago by Mathandski
Source: USAMO 2025/3
Alice the architect and Bob the builder play a game. First, Alice chooses two points $P$ and $Q$ in the plane and a subset $\mathcal{S}$ of the plane, which are announced to Bob. Next, Bob marks infinitely many points in the plane, designating each a city. He may not place two cities within distance at most one unit of each other, and no three cities he places may be collinear. Finally, roads are constructed between the cities as follows: for each pair $A,\,B$ of cities, they are connected with a road along the line segment $AB$ if and only if the following condition holds:
[center]For every city $C$ distinct from $A$ and $B$, there exists $R\in\mathcal{S}$ such[/center]
[center]that $\triangle PQR$ is directly similar to either $\triangle ABC$ or $\triangle BAC$.[/center]
Alice wins the game if (i) the resulting roads allow for travel between any pair of cities via a finite sequence of roads and (ii) no two roads cross. Otherwise, Bob wins. Determine, with proof, which player has a winning strategy.

Note: $\triangle UVW$ is directly similar to $\triangle XYZ$ if there exists a sequence of rotations, translations, and dilations sending $U$ to $X$, $V$ to $Y$, and $W$ to $Z$.
25 replies
KevinYang2.71
Mar 20, 2025
Mathandski
40 minutes ago
Pressing 'go down button' always creates a gray box on the last post
Craftybutterfly   20
N Yesterday at 12:03 AM by Craftybutterfly
Summary of the problem: Pressing go down to last post button always creates a gray box overlapping last post
Page URL: any forum
Steps to reproduce:
1. Go to any topic in a forum
2. The gray box at the bottom overlaps part of the first post
Expected behavior: Should not show a gray box
Frequency: 100% of the time
Operating system(s): Linux HP EliteBook 835 G8 Notebook PC
Browser(s), including version: Chrome 133.0.6943.142 (Official Build) (64-bit) (cohort: Stable)
Additional information: It works on any other device, on my iPhone XR, a MacOS, and my iPad. Took the screenshot a month ago. The gray box still appears
20 replies
Craftybutterfly
Mar 12, 2025
Craftybutterfly
Yesterday at 12:03 AM
k The avatars are not consistent
Craftybutterfly   45
N Monday at 11:03 PM by Demetri
Summary of the problem: The avatars are not consistent
Page URL: idk
Steps to reproduce:
1. change your avatar
2.reload a topic you posted in
3. do #2 to a different topic with your post in it
Expected behavior: Avatars are the same
Frequency: 100%
Operating system(s): MacOS
Browser(s), including version: Chrome latest version
Additional information: refreshing does not help, neither does logging out and in
45 replies
Craftybutterfly
Mar 20, 2025
Demetri
Monday at 11:03 PM
k Mr. Rusczyk is retiring!
SmartGroot   58
N Monday at 8:33 PM by AmethystC
Has anyone else got the email? Mr. Rusczyks retiring :o
58 replies
SmartGroot
Monday at 4:12 PM
AmethystC
Monday at 8:33 PM
Python exit() module decriptions appear as "undefined"
SoaringHigh   8
N Mar 22, 2025 by Major_Monogram
Summary of the problem: When using exit() (or quit()) in the Python windows on AoPS the "Description" and "To fix" options show up as "undefined"
sample program
Page URL: N/A
Steps to reproduce:
1. Use the AoPS Python module to execute the exit() or quit() functions in a program. (try running the sample program)
Expected behavior: The "Description" and "To fix" sections give a description of SystemExit
Frequency: Always
Operating system(s): Windows 11 Home
Browser(s), including version: Microsoft Edge 130.0.2849.46
Additional information: N/A
8 replies
SoaringHigh
Oct 22, 2024
Major_Monogram
Mar 22, 2025
Help about topics on alcumus
GreenBanana666   10
N Mar 22, 2025 by Major_Monogram
My ratio basics score used to be 99 a 5 days ago. But it is 84 now. What and why did that happen?


Now after one hour and zero questions done on Ratio basics, it has risen by 5 rating. :ewpu:
10 replies
GreenBanana666
Oct 29, 2024
Major_Monogram
Mar 22, 2025
k Happy 1795^2 day
maxamc   2
N Mar 22, 2025 by cheltstudent
$30222025=1795^2$.

$3^2$ states are taking mathcounts state and $1^2$ territories are taking mathcounts state ($6^2$, $2^2$ national qualifiers each).

$5^2$ perfect square dates this century, $5$ this year.
2 replies
maxamc
Mar 22, 2025
cheltstudent
Mar 22, 2025
k AMC 12a 2024 Problems and Solutions Pages Misaligned?
ViolinGod   5
N Mar 21, 2025 by jlacosta
I was looking at the past AMC problems and it seems like the problems listed on this page:

https://artofproblemsolving.com/wiki/index.php/2024_AMC_12A_Problems

have solution links that do not all correspond to the correct problem. For example, for problem 2, the solution page is linked as:

https://artofproblemsolving.com/wiki/index.php/2024_AMC_12A_Problems/Problem_2, but in the first link the problems are different.
5 replies
ViolinGod
Mar 21, 2025
jlacosta
Mar 21, 2025
k Number of posts is not consistent
LawofCosine   3
N Mar 21, 2025 by LawofCosine
The number of posts is not consistent. see attachments. Thank you.
3 replies
LawofCosine
Mar 21, 2025
LawofCosine
Mar 21, 2025
k How to add members to forums?
Bnn81351   2
N Mar 20, 2025 by Craftybutterfly
How do I add members to forums?
2 replies
Bnn81351
Mar 20, 2025
Craftybutterfly
Mar 20, 2025
k AoPS Academy: Exporting rich format results in broken BBCode.
Minium   1
N Mar 19, 2025 by jlacosta
When exporting a rich format document in the writing tab into the message board, bold formatting specifically is broken and results in broken BBCode.
Page URL: virtual.aopsacademy.org/class/<any writing class works here>/writing

TO REPRODUCE
1. enter "Lorem ipsum".
2. apply bold to "Lorem"
3. apply italic to "ip".
4. click the Post Draft on Message Board
5. read the contents of the message board post.

FOR EXAMPLE
When I format "Lorem ipsum" (in the writing tab of course), but when I export to post it, I get

[code]Lorem[/b] ipsum[/code].

Notice that the first bolding does not start, only ends.
1 reply
Minium
Mar 18, 2025
jlacosta
Mar 19, 2025
Distributing cupcakes
KevinYang2.71   19
N Mar 24, 2025 by sixoneeight
Source: USAMO 2025/6
Let $m$ and $n$ be positive integers with $m\geq n$. There are $m$ cupcakes of different flavors arranged around a circle and $n$ people who like cupcakes. Each person assigns a nonnegative real number score to each cupcake, depending on how much they like the cupcake. Suppose that for each person $P$, it is possible to partition the circle of $m$ cupcakes into $n$ groups of consecutive cupcakes so that the sum of $P$'s scores of the cupcakes in each group is at least $1$. Prove that it is possible to distribute the $m$ cupcakes to the $n$ people so that each person $P$ receives cupcakes of total score at least $1$ with respect to $P$.
19 replies
KevinYang2.71
Mar 21, 2025
sixoneeight
Mar 24, 2025
Distributing cupcakes
G H J
G H BBookmark kLocked kLocked NReply
Source: USAMO 2025/6
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KevinYang2.71
409 posts
#1 • 5 Y
Y by ihatemath123, lpieleanu, RoyalPrince, LostDreams, Countmath1
Let $m$ and $n$ be positive integers with $m\geq n$. There are $m$ cupcakes of different flavors arranged around a circle and $n$ people who like cupcakes. Each person assigns a nonnegative real number score to each cupcake, depending on how much they like the cupcake. Suppose that for each person $P$, it is possible to partition the circle of $m$ cupcakes into $n$ groups of consecutive cupcakes so that the sum of $P$'s scores of the cupcakes in each group is at least $1$. Prove that it is possible to distribute the $m$ cupcakes to the $n$ people so that each person $P$ receives cupcakes of total score at least $1$ with respect to $P$.
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Countmath1
176 posts
#2 • 41 Y
Y by arfekete, Equinox8, balllightning37, Pengu14, EpicBird08, Ilikeminecraft, Awesomeness_in_a_bun, OronSH, KevinYang2.71, ihatemath123, lpieleanu, Amkan2022, BS2012, GrantStar, aidan0626, ninjaforce, tediousbear, the_math_prodigy, megarnie, mathfan2020, Sedro, LostDreams, GeronimoStilton, github, Marcus_Zhang, Toinfinity, elasticwealth, centslordm, DreamineYT, MathRook7817, Exponent11, williamxiao, eg4334, CyclicISLscelesTrapezoid, sdfgfjh, Geometry285, xHypotenuse, watery, eduD_looC, AlexWin0806, Jack_w
how many partials for throwing up 2 mins into the test then leaving
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YaoAOPS
1500 posts
#3 • 3 Y
Y by peace09, KevinYang2.71, centslordm
Communicated to me by the orz orz orz jatloe.

Define the $P$ person's partitions as bubbles. Then we get a matching between the $n$ people $P$ and the first person's $n$ bubbles $B$. Connect a person to a bubble if they value that bubble as more than $1$, so one person is connected to all the bubbles on this bipartite graph $G \subset P \times B$. If Hall's condition is satisfied on these people, take that matching to win. Else, there exists some set $X$ of people such that they match less than $|X|$ bubbles. Remove that set, and repeatedly remove any set with $|N_G(X)| < |X|$. We must end up with some non-empty set of people $M$ such that $M$ and $N_G(M)$ have a matching, and no person in $P \setminus M$ matches any bubble in $N_G(M)$. In this case, we may take a matching on $M$ and $N_G(M)$, and remove this matching and the corresponding bubbles; since the remaining people aren't connected to these bubbles, this inductively preserves the score condition for them.
This post has been edited 1 time. Last edited by YaoAOPS, Mar 21, 2025, 1:13 PM
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HamstPan38825
8857 posts
#4 • 3 Y
Y by krithikrokcs, bjump, centslordm
This problem is ultra cool. Here is a solution communicated to me by (the goat himself) Lerchen Zhong. We perform a strong induction on $n$.

We call a continuous arc of cupcakes $\mathcal A$ delicious for person $P_i$ if the sum of $P_i$'s scores across the cupcakes in that arc is at least $1$. Consider a partition of the circle $\mathcal C$ into disjoint counterclockwise-oriented arcs $\mathcal A_1, \mathcal A_2, \dots, \mathcal A_n$ such that
  • $\mathcal A_1$ is the shortest arc starting at some cupcake $C$ that is delicious for someone;
  • for each $i$, $\mathcal A_i$ is the shortest arc starting at the first cupcake counterclockwise of $\mathcal A_{i-1}$ that is delicious for someone.
By the given condition, we can always perform such a partition. Fix these arcs $\mathcal A_1, \dots, \mathcal A_n$ now. For each arc $\mathcal A_i$, define the set $S_i$ to consist of all the people for which $\mathcal A_i$ is delicious. Then $S_i$ is nonempty for every $i$. In fact, this is all the information we need to solve the problem.

We call a complete family $\mathcal F$ of the sets $S_i$ a family of subsets such that there exists a positive integer $k$ with
  • the union $\left|\bigcup_{S_i \in \mathcal F} S_i\right| = k$;
  • a perfect matching from $P_1, \dots, P_k$ (indexed without loss of generality) to the sets in $\mathcal F$ (under the obvious condition).
If there exists a complete family $\mathcal F$, we can simply distribute the cupcakes in $S_i$ according to the matching and induct down. Otherwise, assume for the sake of contradiction that the result is not true; then there is no perfect matching between the $S_i$ and $P_i$, so by the converse of Hall's marriage lemma, Hall's condition should fail.

Thus, take a minimal $k \geq 2$ such that $|S_1 \cup \cdots \cup S_k| < k$. But then Hall's condition holds for $k-1$, i.e. we can take that perfect matching (which notably contains only $k-1$ elements in the union) and induct down. This completes the proof.

Remark: [Rambling] I think Hall's marriage lemma is a natural idea to apply here: you have some sort of condition that says that there are ``enough" cupcakes for each person individually, so there should be a perfect matching. I found the difficulty of the problem to be the splitting into arcs $\mathcal A_i$ then realizing that this is sufficient to solve the problem. While the later Hall arguments are nontrivial, they are also quite unexpected (you want some kind of perfect matching and also a tight union set to induct, and those just sort of come together!), and while I did write down the $\mathcal A_i$ partition, I didn't think much of it. So in a sense this problem has a similar pitfall as USAMO2: you have to be confident that once you do have the $S_i$ reformulation that this information is sufficient to solve the problem.
This post has been edited 1 time. Last edited by HamstPan38825, Mar 21, 2025, 1:24 PM
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plang2008
329 posts
#5
Y by
YaoAOPS wrote:
Communicated to me by the orz orz orz jatloe.

Define the $P$ person's partitions as bubbles. Then we get a matching between the $n$ people $P$ and the first person's $n$ bubbles $B$. Connect a person to a bubble if they value that bubble as more than $1$, so one person is connected to all the bubbles on this bipartite graph $G \subset P \times B$. If Hall's condition is satisfied on these people, take that matching to win. Else, there exists some set $X$ of people such that they match less than $|X|$ bubbles. Remove that set, and repeatedly remove any set with $|N_G(X)| < |X|$. We must end up with some non-empty set of people $M$ such that $M$ and $N_G(M)$ have a matching, and no person in $P \setminus M$ matches any bubble in $N_G(M)$. In this case, we may take a matching on $M$ and $N_G(M)$, and remove this matching and the corresponding bubbles; since the remaining people aren't connected to these bubbles, this inductively preserves the score condition for them.

How many points for getting up to the Hall’s graph, but with flawed, kinda flipped sign of hall’s: I said if there exists $|N_G(X)| > |X|$ we can remove them and induct down (which is false) otherwise (this case doesn’t even exist??) there exists a bubble not matched with anyone which we can induct down on preserving score. I missed the immediate match and win case and I think my second case is a subset of my first which is skullers. Like I’m pretty sure I have a solve if in my first case instead of matching and winning I match and consider the complement which eventually reduces to something like my second case.

This is what happens when you figure out how p6 works with 20 minutes left, not having any time to recall Hall’s correctly or write down detailed proof, since you misread p5 and made it harder than it asked for
This post has been edited 1 time. Last edited by plang2008, Mar 21, 2025, 3:01 PM
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USJL
530 posts
#6 • 6 Y
Y by ihatemath123, NaturalSelection, aidan0626, Exponent11, i3435, MathRook7817
This is proposed by me and Cheng-Ying Chang. Probably the whackiest hall's I've ever done.
This post has been edited 1 time. Last edited by USJL, Yesterday at 12:56 AM
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Mathandski
725 posts
#7 • 10 Y
Y by peace09, YaoAOPS, aidan0626, megarnie, Sedro, KevinYang2.71, EpicBird08, balllightning37, LostDreams, sixoneeight
In all seriousness though I was planning on reviewing hall but forgot so it is 100% my own fault I missed this. Posting this just because it aged humorously
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This post has been edited 1 time. Last edited by Mathandski, Mar 21, 2025, 3:47 PM
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plang2008
329 posts
#8
Y by
Mathandski wrote:
In all seriousness though I was planning on reviewing hall but forgot so it is 100% my own fault I missed this. Posting this just because it aged humorously

I learned Hall’s for the first time at Mathcamp but never did the provided hw so when the idea of Hall’s came to me on P6 I couldn’t formulate it fast enough because I just 1) have never applied it and 2) don’t have the condition ingrained in my head.

Maybe this is a sign to do your homework guys
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KevinYang2.71
409 posts
#9
Y by
then theres me who read Diestel so i know Hall and Konig and Tutte and literally every matching theorem there exists
but i still couldnt solve this problem with 50 minutes left (i didnt try)
This post has been edited 1 time. Last edited by KevinYang2.71, Mar 21, 2025, 5:31 PM
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ihatemath123
3440 posts
#10 • 3 Y
Y by peace09, Mintylemon66, centslordm
FWIW, the posts in #3 and #4 gloss over the exact deletion process, and even though the Hall's lemma is the actual hard part of the proof, it's not a complete solution unless you explain how the deletion works too. Unless I'm going crazy, it's not just a trivial thing you can say in one sentence?

(Btw, I didn't solve this problem either. The solution was communicated to me by Andrew Carratu, who heard it from Lerchen I think. Or maybe Apra?)
Call the $s$ people who you are about to delete settled. For the people who aren't settled, you need to make sure they lose exactly $s$ groups in the deletion, and that each of their remaining groups still has value at least 1. For each of the $s$ settled people, delete the group they match with one at a time. Call the group we are about to delete $G$. By the hypothesis (this is what the Hall's was for), the $n-s$ unsettled people value $G$ at most $1$. In particular, none of these unsettled people will possess a group that is contained strictly within $G$. They either own two adjacent groups whose border lies inside $G$, or they own one group that contains $G$ (or is equal to $G$).
  • Suppose an unsettled person falls under the former case. Before, the two adjacent groups had a total value of at least $2$, and after deleting $G$, at most $1$ total value of cupcake was deleted from the union of those groups. Gluing the remains of those two groups together thus creates a new group with total value at least $1$, maintaining the condition. In particular, gluing these groups together decreases this unsettled person's total number of groups by one, which is what we want.
  • Suppose an unsettled person falls under the latter case. So, by deleting $G$, we put a hole in this unsettled person's group. Then, they simply glue the two remains of this group together with another group to the left. The value of this new group is at least $1$ (in fact, equality holds only when there the remains of the group totaled $0$ in value).
We obviously don't need to worry about regrouping the settled people. So, repeating this process $s$ times on each of the settled people will leave us with only the $n-s$ unsettled people remaining, each of whom have $n-s$ groups.

Notably, the deletion process is the only part of the problem where you use (a) the fact that the value of a group is determined by the sum of its individual cupcake values, and not some other function of the values, and (b) the fact that the groups are contiguous. It's important to find a maximal set of settled people to delete at once, since we take advantage of the fact that we don't need to regroup the settled people.
This post has been edited 5 times. Last edited by ihatemath123, Mar 21, 2025, 7:41 PM
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plang2008
329 posts
#13 • 1 Y
Y by eg4334
Ok here’s an attempt. I’m still crashing out over how many wrong statements I wrote about hall’s yet have the right idea all along


Call each person's consecutive group of cupcakes their bubbles. WLOG each bubble has a total score of $1$ by scaling down; clearly we can scale back up later.

Consider a specific person $P$. Let $X$ be the set of the other $n - 1$ people and $Y$ be the set of bubbles of $P$. Match a person with a bubble if the bubble's score for that person is greater than $1$. We have a bipartite graph.

Now we consider a reduction step. Suppose we give $P$ a bubble $B$, then $P$ is happy. If no other person $Q$ matches with $B$, then $Q$ is happy too, and since $B$ does not fully contain any bubbles of $Q$, so we can join the up to two bubbles of $Q$ that was split by $B$, preserving the score condition. If someone matches with $B$, then we may run into issues.

Now apply Hall's. If Hall's condition is satisfied, then we can easily match all $n-1$ people with a bubble and give $P$ their last bubble. This reduces the problem to a smaller $n$. If Hall's condition is not satisfied, this means there is some subset $S$ such that if the set of its matches is $N(S)$, then $|S| > |N(S)|$.

Thus consider $X \setminus S$ and $Y \setminus N(S)$. Since $|X| < |Y|$, clearly $|X \setminus S| < |Y \setminus N(S)|$ still. Additionally, note that no person in $S$ matches with any bubble in $Y \setminus N(S)$. Thus, we return to our original hypothesis.

Thus we can continue reducing until Hall's condition is satisfied (in which case we match and reduce), or until $X \setminus S = \emptyset$. In this case, clearly $Y \setminus N(S) \neq \emptyset$, so there is indeed an empty bubble. Apply the reduction step mentioned earlier. This concludes the reduction.

And of course, $n = 1$ is trivial.
This post has been edited 2 times. Last edited by plang2008, Mar 21, 2025, 11:08 PM
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S.Das93
707 posts
#14 • 1 Y
Y by elasticwealth
plang2008 wrote:
Ok here’s an attempt. I’m still crashing out over how many wrong statements I wrote about hall’s yet have the right idea all along


Call each person's consuective group of cupcakes their bubbles. WLOG each bubble has a total score of $1$ by scaling down; clearly we can scale back up later.

Consider a specific person $P$. Let $X$ be the set of the other $n - 1$ people and $Y$ be the set of bubbles of $P$. Match a person with a bubble if the bubble's score for that person is greater than $1$. We have a bipartite graph.

Now we consider a reduction step. Suppose we give $P$ a bubble $B$, then $P$ is happy. If no other person $Q$ matches with $B$, then $Q$ is happy too, and since $B$ does not fully contain any bubbles of $Q$, so we can join the up to two bubbles of $Q$ that was split by $B$, preserving the score condition. If someone matches with $B$, then we may run into issues.

Now apply Hall's. If Hall's condition is satisfied, then we can easily match all $n-1$ people with a bubble and give $P$ their last bubble. If Hall's condition is not satifisfied, this means there is some subset $S$ such that if the set of its matches is $N(S)$, then $|S| > |N(S)|$.

Thus consider $X \setminus S$ and $Y \setminus N(S)$. Since $|X| < |Y|$, clearly $|X \setminus S| < |Y \setminus N(S)|$ still. Additionally, note that no person in $S$ matches with any bubble in $Y \setminus N(S)$. Thus, we return to our original hypothesis.

Thus we can continue reducing until Hall's condition is satisfied (in which case we match and win), or until $X \setminus S = \emptyset$. In this case, clearly $Y \setminus N(S) \neq \emptyset$, so there is indeed an empty bubble. Apply the reduction step mentioned earlier. This concludes the reduction.

And of course, $n = 1$ is trivial.


0/7, you spelled consecutive wrong
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plang2008
329 posts
#15
Y by
S.Das93 wrote:

0/7, you spelled consecutive wrong

well i’m already 0/7 for writing hall’s condition wrong
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S.Das93
707 posts
#16
Y by
hall's divorce
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v_Enhance
6870 posts
#17 • 6 Y
Y by H_Taken, Exponent11, aidan0626, ihatemath123, EpicBird08, ostriches88
Amazing problem. Here's my write-up of it. When working on it myself, I had the idea to match one person's arcs with Hall but didn't see the deletion argument.

Arbitrarily pick any one person --- call her Pip --- and her $n$ arcs. The initial idea is to try to apply Hall's marriage lemma to match the $n$ people with Pip's arcs (such that each such person is happy with their matched arc). To that end, construct the obvious bipartite graph $\mathfrak{G}$ between the people and the arcs for Pip.
We now consider the following algorithm, which takes several steps.
  • If a perfect matching of $\mathfrak{G}$ exists, we're done!
  • We're probably not that lucky. Per Hall's condition, this means there is a bad set $\mathcal{B}_1$ of people, who are compatible with fewer than $|\mathcal{B}_1|$ of the arcs. Let's imagine deleting $\mathcal{B}_1$ and those neighbors of $\mathcal{B}_1$, then try to find a matching on the remaining graph.
  • If a matching exists, terminate the algorithm. Otherwise, that means there's another bad set $\mathcal{B}_2$ for the remaining graph. We again delete $\mathcal{B}_2$ and the fewer than $\mathcal{B}_2$ neighbors.
  • Repeat until some perfect matching $\mathfrak{M}$ is possible in the remaining graph, i.e.\ there are no more bad sets (and then terminate once that occurs).
    Since Pip is a universal vertex, it's impossible to delete Pip, so the algorithm does indeed terminate with nonempty $\mathcal{M}$.
A cartoon of this picture is shown below.
[asy]usepackage("amssymb");
size(8cm); dotfactor *= 1.3; real w = 3; real eps = 0.4; label("People", (-w,10)); label("Arcs of Pip", (w,10)); filldraw(box((-w-eps, 9+eps), (-w+eps,7-eps)), invisible, red+1.2); filldraw(box((-w-eps, 6+eps), (-w+eps,5-eps)), invisible, orange+1.2); filldraw(box((-w-eps, 4+eps), (-w+eps,2-eps)), invisible, brown+1.2);
filldraw(box((w-eps, 9+eps), (w+eps,8-eps)), invisible, red+1.2); filldraw(box((w-eps, 7+eps), (w+eps,7-eps)), invisible, orange+1.2); filldraw(box((w-eps, 6+eps), (w+eps,5-eps)), invisible, brown+1.2);
draw((-w+eps, 9+eps)--(w-eps, 9+eps), red+dashed); draw((-w+eps, 7-eps)--(w-eps, 8-eps), red+dashed); draw((-w+eps, 6+eps)--(w-eps, 9+eps), orange+dashed); draw((-w+eps, 5-eps)--(w-eps, 7-eps), orange+dashed); draw((-w+eps, 4+eps)--(w-eps, 9+eps), brown+dashed); draw((-w+eps, 2-eps)--(w-eps, 5-eps), brown+dashed);
label((-w-eps, 8), "Bad set $\mathcal{B}_1$", dir(180), black); label((-w-eps, 5.5), "Bad set $\mathcal{B}_2$", dir(180), black); label((-w-eps, 3), "Bad set $\mathcal{B}_3$", dir(180), black);
draw((-w,1)--(w,1), deepgreen+1.3); draw((-w,0)--(w,0), deepgreen+1.3); label((0, 0.5), "Final perfect matching $\mathfrak{M}$", deepgreen);
for (int i=0; i<10; ++i) { dot((w,i), blue); dot((-w,i), blue); } label("Pip", (-w,0), dir(180), blue);
[/asy]
We commit to assigning each of person in $\mathcal{M}$ their matched arc (in particular if there are no bad sets at all, the problem is already solved). Now we finish the problem by induction on $n$ (for the remaining people) by simply deleting the arcs used up by $\mathcal{M}$.
To see why this deletion-induction works, consider any particular person $Q$ not in $\mathcal{M}$. By definition, $Q$ is not happy with any of the arcs in $\mathcal{M}$ So when an arc $\mathcal{A}$ of $\mathfrak{M}$ is deleted, it had value less than $1$ for $Q$, so in particular it couldn't contain entirely any of $Q$'s arcs. Hence at most one endpoint among $Q$'s arcs was in the deleted arc $\mathcal{A}$. If so, this causes two arcs of $Q$ to merge, and the merged value is \[ (\ge 1) + (\ge 1) - (\le 1) \qquad \ge \qquad 1 \]meaning the induction is OK. See below for a cartoon of the deletion, where Pip's arcs are drawn in blue while $Q$'s arcs and scores are drawn in red (in this example $n=3$).
[asy]
size(13cm); usepackage("amsmath"); pair O = (0,0); picture before; picture after;
real r = 1; real s = 0.9; real t = 0.65;
draw(before, arc(O, s, -20, 80), red+1.3); draw(before, arc(O, s, 100, 200), red+1.3); draw(before, arc(O, s, 220, 320), red+1.3);
draw(before, "Pip arc to delete", arc(O, r,  40, 140), blue+1.3); draw(before, rotate(-60)*"Pip arc", arc(O, r, 160, 260), blue+1.3); draw(before, rotate( 60)*"Pip arc", arc(O, r, 280, 380), blue+1.3);
label(before, "$\boxed{0.2}$", t*dir(120), red); label(before, "$\boxed{0.3}$", t*dir( 60), red); label(before, "$\boxed{0.8}$", t*dir(180), red); label(before, "$\boxed{0.43}$", t*dir(240), red); label(before, "$\boxed{0.57}$", t*dir(300), red); label(before, "$\boxed{0.7}$", t*dir(  0), red); label(before, "$Q$'s values", O, red);
draw(after, arc(O, s, -20, 200), red+1.3); draw(after, arc(O, s, 220, 320), red+1.3);
draw(after, rotate(-60)*"Pip arc", arc(O, r, 160, 260), blue+1.3); draw(after, rotate( 60)*"Pip arc", arc(O, r, 280, 380), blue+1.3);
label(after, "$\boxed{0.8}$", t*dir(180), red); label(after, "$\boxed{0.43}$", t*dir(240), red); label(after, "$\boxed{0.57}$", t*dir(300), red); label(after, "$\boxed{0.7}$", t*dir(  0), red); label(after, "$Q$'s values", O, red);
add(before); add(shift(3.2,0)*after);
[/asy]

Remark: This deletion argument can be thought of in some special cases even before the realization of Hall, in the case where $\mathcal{M}$ has only one person (Pip). This amounts to saying that if one of Pip's arcs isn't liked by anybody, then that arc can be deleted and the induction carries through.
This post has been edited 1 time. Last edited by v_Enhance, Mar 23, 2025, 12:27 PM
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ihatemath123
3440 posts
#19 • 1 Y
Y by v_Enhance
@above in the deletion process, isn't it also possible for $Q$ to contain the deleted arc entirely? (so zero of the endpoints are in the deleted arc)
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plang2008
329 posts
#20
Y by
ihatemath123 wrote:
@above in the deletion process, isn't it also possible for $Q$ to contain the deleted arc entirely? (so zero of the endpoints are in the deleted arc)

then you can just arbitrarily join that arc of $Q$ with a neighboring one (which is valid since $1 + (\geq0) \geq 1$)
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deduck
177 posts
#21
Y by
is this possible to do without halls because i wasted 30 mins and have no idea what is going on
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MathLuis
1466 posts
#22 • 1 Y
Y by KevinYang2.71
Pick the $n$ partitions of an arbitrary person and we will try to match them with the $n$ people so that all of them are happy on their arc. Clearly this isn't always possible, so we make our abritrary person pick so that when making the search for the maximal matching we get the biggest out of all possible outcomes (by fixing the $n$ arcs of a different person).
Now if a complete matching was not possible at all by Hall's this means that there does exist a set of people $X$ that can be happy in less than $|X|$ arcs as a group, so what we wil do is just to delete $X$ and its neighbourhood as well. So now we just repeat this until we find a perfect matching, clearly the process terminates at some non-degenerate point because the intial person cannot be deleted as it is always happily matched.
Consider this final matching graph to be $\mathcal H$ then we will now perform induction on $n$ for the remaining people lying outside $\mathcal H$, it is clear that if we pick a random person outside $\mathcal H$ it is because it was not happy with any of the arcs on $\mathcal H$ and thus with respect of that person, each of thise arcs has value less than $1$ so clearly no arc that made this person happy was in $\mathcal H$ however this means by instead now removing the whole matching $\mathcal H$ we can induct down on the next set of people as we have seen these are completely disjoint and inductivity will preserve the happyness condition by merging (basically observe that $(>1)+(>1)-(<1) \ge (>1)$ can be considered), thus we are done :cool:.
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sixoneeight
1134 posts
#23
Y by
Mathandski wrote:
In all seriousness though I was planning on reviewing hall but forgot so it is 100% my own fault I missed this. Posting this just because it aged humorously

That's my bad guys (the guy who said no is me)
This post has been edited 1 time. Last edited by sixoneeight, Mar 24, 2025, 12:49 AM
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