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k a May Highlights and 2025 AoPS Online Class Information
jlacosta   0
May 1, 2025
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.

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[*]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
May 1, 2025
0 replies
F.E....can you solve it?
Jackson0423   0
2 minutes ago
Find all functions \( f : \mathbb{R} \to \mathbb{R} \) such that
\[
f\left(\frac{x^2 - f(x)}{f(x) - 1}\right) = x
\]for all real numbers \( x \) satisfying \( f(x) \neq 1 \).
0 replies
Jackson0423
2 minutes ago
0 replies
Functional equation
Math-wiz   25
N 28 minutes ago by Adywastaken
Source: IMOC SL A1
Find all functions $f:\mathbb{R}\to\mathbb{R}$ such that for all $x,y\in\mathbb{R}$,
$$f(xy+f(x))=f(xf(y))+x$$
25 replies
Math-wiz
Dec 15, 2019
Adywastaken
28 minutes ago
Nice numer theory
GeoArt   5
N 43 minutes ago by Primeniyazidayi
$p$ is a prime number, $m, x, y$ are natural numbers ($m, x, y > 1$). It is known that $\frac{x^p + y^p}{2}$ $=$ $(\frac{x+y}{2} )^m$. Prove that $p = m$.
5 replies
+1 w
GeoArt
Jan 7, 2021
Primeniyazidayi
43 minutes ago
Prove XBY equal to angle C
nataliaonline75   2
N an hour ago by starchan
Let $M$ be the midpoint of $BC$ on triangle $ABC$. Point $X$ lies on segment $AC$ such that $AX=BX$ and $Y$ on line $AM$ such that $XY//AB$. Prove that $\angle XBY = \angle ACB$.
2 replies
nataliaonline75
Yesterday at 2:47 PM
starchan
an hour ago
Rolles theorem
sasu1ke   4
N Yesterday at 6:13 PM by sasu1ke

Let \( f: \mathbb{R} \to \mathbb{R} \) be a twice differentiable function such that
\[
f(0) = 2, \quad f'(0) = -2, \quad \text{and} \quad f(1) = 1.
\]Prove that there exists a point \( \xi \in (0, 1) \) such that
\[
f(\xi) \cdot f'(\xi) + f''(\xi) = 0.
\]

4 replies
sasu1ke
Saturday at 9:00 PM
sasu1ke
Yesterday at 6:13 PM
Putnam 1947 B1
sqrtX   3
N Yesterday at 5:38 PM by Levieee
Source: Putnam 1947
Let $f(x)$ be a function such that $f(1)=1$ and for $x \geq 1$
$$f'(x)= \frac{1}{x^2 +f(x)^{2}}.$$Prove that
$$\lim_{x\to \infty} f(x)$$exists and is less than $1+ \frac{\pi}{4}.$
3 replies
sqrtX
Apr 3, 2022
Levieee
Yesterday at 5:38 PM
Double Sum
P162008   2
N Yesterday at 3:01 PM by Etkan
Evaluate $\sum_{a=1}^{\infty} \sum_{b=1}^{a + 2} \frac{1}{ab(a + 2)}.$
2 replies
P162008
Yesterday at 12:19 PM
Etkan
Yesterday at 3:01 PM
Summation
Saucepan_man02   4
N Yesterday at 2:47 PM by Etkan
If $P = \sum_{r=1}^{50} \sum_{k=1}^{r} (-1)^{r-1} \frac{\binom{50}{r}}{k}$, then find the value of $P$.

Ans
4 replies
Saucepan_man02
May 3, 2025
Etkan
Yesterday at 2:47 PM
Trigo + Series
P162008   0
Yesterday at 11:59 AM
$F(r,x) = \tan\left(\frac{\pi}{3} + 3^r x\right)$ and $G(r,x) = \cot\left(\frac{\pi}{6} + 3^r x\right)$

Let $A = \sum_{r=0}^{24} \frac{d(F(r,x))}{dx} \left[\frac{1}{F(r,x) + \frac{1}{F(r,x)}}\right]$

$B = \sum_{r=0}^{24} \frac{-d(G(r,x))}{dx} \left[\frac{1}{G(r,x) + \frac{1}{G(r,x)}}\right]$

$P = \prod_{r=0}^{24} F(r,x), Q = \prod_{r=0}^{24} G(r,x)$

and, $R = \frac{A - B}{\tan x\left[PQ - \frac{1}{3^{25}}\right]}$ where $x \in R$

Find the remainder when $R^{2025}$ is divided by $11.$
0 replies
P162008
Yesterday at 11:59 AM
0 replies
Integration Bee in Czechia
Assassino9931   2
N Yesterday at 9:35 AM by pi_quadrat_sechstel
Source: Vojtech Jarnik IMC 2025, Category II, P3
Evaluate the integral $\int_0^{\infty} \frac{\log(x+2)}{x^2+3x+2}\mathrm{d}x$.
2 replies
Assassino9931
May 2, 2025
pi_quadrat_sechstel
Yesterday at 9:35 AM
Alternating series and integral
jestrada   4
N Yesterday at 4:06 AM by bakkune
Source: own
Prove that for all $\alpha\in\mathbb{R}, \alpha>-1$, we have
$$ \frac{1}{\alpha+1}-\frac{1}{\alpha+2}+\frac{1}{\alpha+3}-\frac{1}{\alpha+4}+\cdots=\int_0^1 \frac{x^{\alpha}}{x+1}  \,dx. $$
4 replies
jestrada
Saturday at 10:56 PM
bakkune
Yesterday at 4:06 AM
Find all continuous functions
bakkune   3
N Yesterday at 3:58 AM by bakkune
Source: Own
Find all continuous function $f, g\colon\mathbb{R}\to\mathbb{R}$ satisfied
$$
(x - k)f(x) = \int_k^x g(y)\mathrm{d}y 
$$for all $x\in\mathbb{R}$ and all $k\in\mathbb{Z}$.
3 replies
bakkune
May 3, 2025
bakkune
Yesterday at 3:58 AM
Equivalent condition of the uniformly continuous fo a function
Alphaamss   2
N Yesterday at 2:05 AM by Alphaamss
Source: Personal
Let $f_{a,b}(x)=x^a\cos(x^b),x\in(0,\infty)$. Get all the $(a,b)\in\mathbb R^2$ such that $f_{a,b}$ is uniformly continuous on $(0,\infty)$.
2 replies
Alphaamss
May 3, 2025
Alphaamss
Yesterday at 2:05 AM
A problem in point set topology
tobylong   2
N Yesterday at 12:00 AM by cosmicgenius
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 $Y$.
2 replies
tobylong
May 3, 2025
cosmicgenius
Yesterday at 12:00 AM
Interesting Function
Kei0923   4
N Apr 30, 2025 by CrazyInMath
Source: 2024 JMO preliminary p8
Function $f:\mathbb{Z}_{\geq 0}\rightarrow\mathbb{Z}$ satisfies
$$f(m+n)^2=f(m|f(n)|)+f(n^2)$$for any non-negative integers $m$ and $n$. Determine the number of possible sets of integers $\{f(0), f(1), \dots, f(2024)\}$.
4 replies
Kei0923
Jan 9, 2024
CrazyInMath
Apr 30, 2025
Interesting Function
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G H BBookmark kLocked kLocked NReply
Source: 2024 JMO preliminary p8
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Kei0923
95 posts
#1
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Function $f:\mathbb{Z}_{\geq 0}\rightarrow\mathbb{Z}$ satisfies
$$f(m+n)^2=f(m|f(n)|)+f(n^2)$$for any non-negative integers $m$ and $n$. Determine the number of possible sets of integers $\{f(0), f(1), \dots, f(2024)\}$.
This post has been edited 1 time. Last edited by Kei0923, Jan 9, 2024, 8:41 AM
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nguyenloc1712
62 posts
#3
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answer :maybe:
sol
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sami1618
902 posts
#5
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The answer is $\boxed{992}$. Let $P(m,n)$ be the given assertion. Notice $P(0,0)$ gives $f(0)^2=2f(0)$ so either $f(0)=0$ or $f(0)=2$.

Case 1: $f(0)=0$
The assertion $P(m,0)$ gives $f(m)^2=0\Rightarrow f(m)=0$. Which can be checked to work.

Case 2: $f(0)=2$
The assertion $P(0,n)$ gives $f(n)^2=2+f(n^2)$. The assertion $P(m,0)$ gives $f(m)^2=f(2m)+2$. Combined we get $f(n^2)=f(2n)$. The assertion $P(0,1)$ gives $f(1)^2=2+f(1)$ so either $f(1)=2$ or $f(1)=-1$. If $f(1)=-1$ then $f(2)=-1$ and $P(1,1)$ gives a contradiction. Now assume $f(1)=2\Rightarrow f(2)=2$. We get from $P(m,1)$ that $f(m+1)^2=f(2m)+2\Rightarrow f(2m+2)=f(2m)\Rightarrow$ $f(2m)=2\Rightarrow f(m^2)=2$. Thus we get $f(n)^2=4\Rightarrow f(n)=\pm 2$. Thus $f(x)=2$ whenever $x$ is even or a square and $f(x)$ can be either $2$ or $-2$ when $x$ is not even or a square. It is easy to check that all such functions work.
This post has been edited 1 time. Last edited by sami1618, May 2, 2024, 11:47 AM
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JerryXi
2 posts
#8
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I think the correct answer should be 991, because the number of possible sets in Case 2 should be 990. It seems like you may have made a small counting mistake. The number of integers from 3 to 2024 that are neither even nor perfect squares is 990.
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CrazyInMath
457 posts
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$P(0, 0)\Longrightarrow f(0)=0, 2$
if $f(0)=0$, then $P(m, 0)\Longrightarrow f(m)^2=0\Longrightarrow f(m)=0$

Let $f(0)=2$ from now on
$P(0, 1)\Longrightarrow f(1)^2=2+f(1)\Longrightarrow f(1)=-1, 2$
if $f(1)=-1$ then $P(1, 1)\Longrightarrow f(2)^2=-2$, contradiction, so $f(1)=2$
$P(m, 1)\Longrightarrow f(m+1)^2=f(2m)+2$, $P(m, 0)\Longrightarrow f(m)^2=f(2m)+2$, so $f(m+1)=\pm f(m)$, and so $f(m)=\pm2$
and so the original FE is now $4=f(2m)+f(n^2)$, which means even numbers and perfect squares must have $f(a)=2$, the others can have $f(a)=2, -2$

The number of sets would be just $2$ as sets don't count duplicates. If it is multiset then the answer is $991$.
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