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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)!

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[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
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[*]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
number theory
Levieee   1
N 24 minutes ago by Lil_flip38
Idk where it went wrong, marks was deducted for this solution
$\textbf{Question}$
Show that for a fixed pair of distinct positive integers \( a \) and \( b \), there cannot exist infinitely many \( n \in \mathbb{Z} \) such that
\[
\sqrt{n + a} + \sqrt{n + b} \in \mathbb{Z}.
\]
$\textbf{Solution}$

Let
\[
x = \sqrt{n + a} + \sqrt{n + b} \in \mathbb{N}.
\]
Then,
\[
x^2 = (\sqrt{n + a} + \sqrt{n + b})^2 = (n + a) + (n + b) + 2\sqrt{(n + a)(n + b)}.
\]So:
\[
x^2 = 2n + a + b + 2\sqrt{(n + a)(n + b)}.
\]
Therefore,
\[
\sqrt{(n + a)(n + b)} \in \mathbb{N}.
\]
Let
\[
(n + a)(n + b) = k^2.
\]Assume \( n + a \neq n + b \). Then we have:
\[
n + a \mid k \quad \text{and} \quad k \mid n + b,
\]or it could also be that \( k \mid n + a \quad \text{and} \quad n + b \mid k \).

Without loss of generality, we take the first case:
\[
(n + a)k_1 = k \quad \text{and} \quad kk_2 = n + b.
\]
Thus,
\[
k_1 k_2 = \frac{n + b}{n + a}.
\]
Since \( k_1 k_2 \in \mathbb{N} \), we have:
\[
k_1 k_2 = 1 + \frac{b - a}{n + a}.
\]
For infinitely many \( n \), \( \frac{b - a}{n + a} \) must be an integer, which is not possible.

Therefore, there cannot be infinitely many such \( n \).
1 reply
+1 w
Levieee
an hour ago
Lil_flip38
24 minutes ago
f(x+y+f(y)) = f(x) + f(ay)
the_universe6626   5
N 34 minutes ago by deduck
Source: Janson MO 4 P5
For a given integer $a$, find all functions $f:\mathbb{Z}\rightarrow\mathbb{Z}$ such that
\[f(x+y+f(y))=f(x)+f(ay)\]holds for all $x,y\in\mathbb{Z}$.

(Proposed by navi_09220114)
5 replies
the_universe6626
Feb 21, 2025
deduck
34 minutes ago
a, b subset
MithsApprentice   19
N an hour ago by Maximilian113
Source: USAMO 1996
Determine (with proof) whether there is a subset $X$ of the integers with the following property: for any integer $n$ there is exactly one solution of $a + 2b = n$ with $a,b \in X$.
19 replies
MithsApprentice
Oct 22, 2005
Maximilian113
an hour ago
Parallelograms and concyclicity
Lukaluce   28
N an hour ago by Jupiterballs
Source: EGMO 2025 P4
Let $ABC$ be an acute triangle with incentre $I$ and $AB \neq AC$. Let lines $BI$ and $CI$ intersect the circumcircle of $ABC$ at $P \neq B$ and $Q \neq C$, respectively. Consider points $R$ and $S$ such that $AQRB$ and $ACSP$ are parallelograms (with $AQ \parallel RB, AB \parallel QR, AC \parallel SP$, and $AP \parallel CS$). Let $T$ be the point of intersection of lines $RB$ and $SC$. Prove that points $R, S, T$, and $I$ are concyclic.
28 replies
Lukaluce
Apr 14, 2025
Jupiterballs
an hour ago
No more topics!
Transform the sequence
steven_zhang123   1
N Apr 13, 2025 by vgtcross
Given a sequence of \( n \) real numbers \( a_1, a_2, \ldots, a_n \), we can select a real number \( \alpha \) and transform the sequence into \( |a_1 - \alpha|, |a_2 - \alpha|, \ldots, |a_n - \alpha| \). This transformation can be performed multiple times, with each chosen real number \( \alpha \) potentially being different
(i) Prove that it is possible to transform the sequence into all zeros after a finite number of such transformations.
(ii) To ensure that the above result can be achieved for any given initial sequence, what is the minimum number of transformations required?
1 reply
steven_zhang123
Apr 13, 2025
vgtcross
Apr 13, 2025
Transform the sequence
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steven_zhang123
411 posts
#1
Y by
Given a sequence of \( n \) real numbers \( a_1, a_2, \ldots, a_n \), we can select a real number \( \alpha \) and transform the sequence into \( |a_1 - \alpha|, |a_2 - \alpha|, \ldots, |a_n - \alpha| \). This transformation can be performed multiple times, with each chosen real number \( \alpha \) potentially being different
(i) Prove that it is possible to transform the sequence into all zeros after a finite number of such transformations.
(ii) To ensure that the above result can be achieved for any given initial sequence, what is the minimum number of transformations required?
Z K Y
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vgtcross
2 posts
#2
Y by
(i) If there are two distinct values in the sequence, we can choose alpha to be the arithmetic mean of two such values - the transformation will map both original values to the same new value, decreasing the number of distinct values by at least one. Thus, after at most n-1 moves we will have only one distinct value. As all values in the sequence are equal, we can choose alpha to be equal to them and get a sequence of all zeros.

(ii) The above construction proves an upper bound of n on the answer. I think a sequence can be constructed which requires n transformations, but I don't have a full solution yet. I might come back to the problem later.
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