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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
Thursday at 11:16 PM
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)!

Be sure to mark your calendars for the following upcoming events:
[list][*]May 9th, 4:30pm PT/7:30pm ET, Casework 2: Overwhelming Evidence — A Text Adventure, a game where participants will work together to navigate the map, solve puzzles, and win! All are welcome.
[*]May 19th, 4:30pm PT/7:30pm ET, What's Next After Beast Academy?, designed for students finishing Beast Academy and ready for Prealgebra 1.
[*]May 20th, 4:00pm PT/7:00pm ET, Mathcamp 2025 Qualifying Quiz Part 1 Math Jam, Problems 1 to 4, join the Canada/USA Mathcamp staff for this exciting Math Jam, where they discuss solutions to Problems 1 to 4 of the 2025 Mathcamp Qualifying Quiz!
[*]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
Thursday at 11:16 PM
0 replies
s(I)=2019
math90   8
N 2 minutes ago by MathSaiyan
Source: IMC 2019 Day 2 P8
Let $x_1,\ldots,x_n$ be real numbers. For any set $I\subset\{1,2,…,n\}$ let $s(I)=\sum_{i\in I}x_i$. Assume that the function $I\to s(I)$ takes on at least $1.8^n$ values where $I$ runs over all $2^n$ subsets of $\{1,2,…,n\}$. Prove that the number of sets $I\subset \{1,2,…,n\}$ for which $s(I)=2019$ does not exceed $1.7^n$.

Proposed by Fedor Part and Fedor Petrov, St. Petersburg State University
8 replies
math90
Jul 31, 2019
MathSaiyan
2 minutes ago
Does the sequence log(1+sink)/k converge?
tom-nowy   5
N an hour ago by P_Fazioli
Source: Question arising while viewing https://artofproblemsolving.com/community/c7h3556569
Does the sequence $$ \frac{\ln(1+\sin k)}{k} \;\;\;(k=1,2,3,\ldots) $$converge?
5 replies
tom-nowy
Apr 30, 2025
P_Fazioli
an hour ago
Cauchy's functional equation with f({max{x,y})=max{f(x),f(y)}
tom-nowy   1
N 2 hours ago by Filipjack
Source: https://x.com/D_atWork/status/1788496152855560470, Problem 4
Determine all functions $f: \mathbb{R} \to \mathbb{R}$ satisfying the following two conditions for all $x,y \in \mathbb{R}$:
\[ f(x+y)=f(x)+f(y), \;\;\; f \left( \max \{x, y \} \right) = \max \left\{ f(x),f(y) \right\}. \]
1 reply
tom-nowy
Today at 2:23 PM
Filipjack
2 hours ago
A problem in point set topology
tobylong   1
N 4 hours ago by alexheinis
Source: Basic Topology, Armstrong
Let $f:X\to Y$ be a closed map with the property that the inverse image of each point in $Y$ is a compact subset of $X$. Prove that $f^{-1}(K)$ is compact whenever $K$ is compact in $Y$.
1 reply
tobylong
Today at 3:14 AM
alexheinis
4 hours ago
No more topics!
Differential equations , Matrix theory
c00lb0y   1
N Apr 19, 2025 by loup blanc
Source: RUDN MATH OLYMP 2024 problem 4
Any idea?? Diff equational system combined with Matrix theory.
Consider the equation dX/dt=X^2, where X(t) is an n×n matrix satisfying the condition detX=0. It is known that there are no solutions of this equation defined on a bounded interval, but there exist non-continuable solutions defined on unbounded intervals of the form (t ,+∞) and (−∞,t). Find n.
1 reply
c00lb0y
Apr 17, 2025
loup blanc
Apr 19, 2025
Differential equations , Matrix theory
G H J
Source: RUDN MATH OLYMP 2024 problem 4
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c00lb0y
6 posts
#1 • 1 Y
Y by FFA21
Any idea?? Diff equational system combined with Matrix theory.
Consider the equation dX/dt=X^2, where X(t) is an n×n matrix satisfying the condition detX=0. It is known that there are no solutions of this equation defined on a bounded interval, but there exist non-continuable solutions defined on unbounded intervals of the form (t ,+∞) and (−∞,t). Find n.
This post has been edited 1 time. Last edited by c00lb0y, Apr 18, 2025, 7:13 AM
Reason: unsolved yet!
Z K Y
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loup blanc
3591 posts
#2
Y by
Your post is badly written.
"The condition" is -I think so- : for every $t$, $\det(X(t))=0$.
"There are no solutions..." ; you mean : "there are no maximal solutions...".
The considered property is false for $n=1$ and true for $n\geq 2$.
For $n\geq 2$, choose $X(t)=diag(\dfrac{-1}{t+C},0_{n-1})$.
EDIT.
For $n=2$, there are non-diagonal solutions with $2$ constants: $X(t)=\begin{pmatrix}0&\dfrac{\lambda}{t+\mu}\\0&\dfrac{-1}{t+\mu}\end{pmatrix}$.
This post has been edited 2 times. Last edited by loup blanc, Apr 20, 2025, 2:48 PM
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