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Divisibility on 101 integers
BR1F1SZ   3
N an hour ago by ClassyPeach
Source: Argentina Cono Sur TST 2024 P2
There are $101$ positive integers $a_1, a_2, \ldots, a_{101}$ such that for every index $i$, with $1 \leqslant i \leqslant 101$, $a_i+1$ is a multiple of $a_{i+1}$. Determine the greatest possible value of the largest of the $101$ numbers.
3 replies
BR1F1SZ
Aug 9, 2024
ClassyPeach
an hour ago
BMO 2021 problem 3
VicKmath7   19
N an hour ago by NuMBeRaToRiC
Source: Balkan MO 2021 P3
Let $a, b$ and $c$ be positive integers satisfying the equation $(a, b) + [a, b]=2021^c$. If $|a-b|$ is a prime number, prove that the number $(a+b)^2+4$ is composite.

Proposed by Serbia
19 replies
VicKmath7
Sep 8, 2021
NuMBeRaToRiC
an hour ago
USAMO 2002 Problem 4
MithsApprentice   89
N 2 hours ago by blueprimes
Let $\mathbb{R}$ be the set of real numbers. Determine all functions $f: \mathbb{R} \to \mathbb{R}$ such that \[ f(x^2 - y^2) = x f(x) - y f(y)  \] for all pairs of real numbers $x$ and $y$.
89 replies
MithsApprentice
Sep 30, 2005
blueprimes
2 hours ago
pqr/uvw convert
Nguyenhuyen_AG   8
N 3 hours ago by Victoria_Discalceata1
Source: https://github.com/nguyenhuyenag/pqr_convert
Hi everyone,
As we know, the pqr/uvw method is a powerful and useful tool for proving inequalities. However, transforming an expression $f(a,b,c)$ into $f(p,q,r)$ or $f(u,v,w)$ can sometimes be quite complex. That's why I’ve written a program to assist with this process.
I hope you’ll find it helpful!

Download: pqr_convert

Screenshot:
IMAGE
IMAGE
8 replies
Nguyenhuyen_AG
Apr 19, 2025
Victoria_Discalceata1
3 hours ago
Inspired by hlminh
sqing   2
N 3 hours ago by SPQ
Source: Own
Let $ a,b,c $ be real numbers such that $ a^2+b^2+c^2=1. $ Prove that $$ |a-kb|+|b-kc|+|c-ka|\leq \sqrt{3k^2+2k+3}$$Where $ k\geq 0 . $
2 replies
sqing
Yesterday at 4:43 AM
SPQ
3 hours ago
A cyclic inequality
KhuongTrang   3
N 3 hours ago by KhuongTrang
Source: own-CRUX
IMAGE
https://cms.math.ca/.../uploads/2025/04/Wholeissue_51_4.pdf
3 replies
KhuongTrang
Monday at 4:18 PM
KhuongTrang
3 hours ago
Tiling rectangle with smaller rectangles.
MarkBcc168   60
N 3 hours ago by cursed_tangent1434
Source: IMO Shortlist 2017 C1
A rectangle $\mathcal{R}$ with odd integer side lengths is divided into small rectangles with integer side lengths. Prove that there is at least one among the small rectangles whose distances from the four sides of $\mathcal{R}$ are either all odd or all even.

Proposed by Jeck Lim, Singapore
60 replies
MarkBcc168
Jul 10, 2018
cursed_tangent1434
3 hours ago
ALGEBRA INEQUALITY
Tony_stark0094   2
N 3 hours ago by Sedro
$a,b,c > 0$ Prove that $$\frac{a^2+bc}{b+c} + \frac{b^2+ac}{a+c} + \frac {c^2 + ab}{a+b} \geq a+b+c$$
2 replies
Tony_stark0094
4 hours ago
Sedro
3 hours ago
Checking a summand property for integers sufficiently large.
DinDean   2
N 3 hours ago by DinDean
For any fixed integer $m\geqslant 2$, prove that there exists a positive integer $f(m)$, such that for any integer $n\geqslant f(m)$, $n$ can be expressed by a sum of positive integers $a_i$'s as
\[n=a_1+a_2+\dots+a_m,\]where $a_1\mid a_2$, $a_2\mid a_3$, $\dots$, $a_{m-1}\mid a_m$ and $1\leqslant a_1<a_2<\dots<a_m$.
2 replies
DinDean
Yesterday at 5:21 PM
DinDean
3 hours ago
Bunnies hopping around in circles
popcorn1   22
N 4 hours ago by awesomeming327.
Source: USA December TST for IMO 2023, Problem 1 and USA TST for EGMO 2023, Problem 1
There are $2022$ equally spaced points on a circular track $\gamma$ of circumference $2022$. The points are labeled $A_1, A_2, \ldots, A_{2022}$ in some order, each label used once. Initially, Bunbun the Bunny begins at $A_1$. She hops along $\gamma$ from $A_1$ to $A_2$, then from $A_2$ to $A_3$, until she reaches $A_{2022}$, after which she hops back to $A_1$. When hopping from $P$ to $Q$, she always hops along the shorter of the two arcs $\widehat{PQ}$ of $\gamma$; if $\overline{PQ}$ is a diameter of $\gamma$, she moves along either semicircle.

Determine the maximal possible sum of the lengths of the $2022$ arcs which Bunbun traveled, over all possible labellings of the $2022$ points.

Kevin Cong
22 replies
1 viewing
popcorn1
Dec 12, 2022
awesomeming327.
4 hours ago
A, I, P, Q concyclic wanted, AD+AE=BC, circle passing through B,C
parmenides51   0
Nov 28, 2022
Source: 2010 JJMO p4 - Junior Japan Math Olympiad Finals
Given a triangle $ABC$. A circle $\omega$ passing through points $B$ and $C$, intersects line segments $AB$ and $AC$ (not including endpoints) at points $D$ and $E$, respectively, and $AD+AE=BC$ holds. Let $I$ be the incenter of the triangle $ABC$. Let $P$ and $Q$ be the points, other than $B,C$ , where the straight lines $BI$ and $CI$ intersect the circle $\omega$ . Show that $A, I, P$ and $Q$ lie on the same circle.
0 replies
parmenides51
Nov 28, 2022
0 replies
A, I, P, Q concyclic wanted, AD+AE=BC, circle passing through B,C
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Source: 2010 JJMO p4 - Junior Japan Math Olympiad Finals
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parmenides51
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Given a triangle $ABC$. A circle $\omega$ passing through points $B$ and $C$, intersects line segments $AB$ and $AC$ (not including endpoints) at points $D$ and $E$, respectively, and $AD+AE=BC$ holds. Let $I$ be the incenter of the triangle $ABC$. Let $P$ and $Q$ be the points, other than $B,C$ , where the straight lines $BI$ and $CI$ intersect the circle $\omega$ . Show that $A, I, P$ and $Q$ lie on the same circle.
This post has been edited 2 times. Last edited by parmenides51, Nov 28, 2022, 1:38 PM
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