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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.

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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
Theory of Equations
P162008   1
N 25 minutes ago by vanstraelen
Let $a,b,c,d$ and $e\in [-2,2]$ such that $\sum_{cyc} a = 0, \sum_{cyc} a^3 = 0, \sum_{cyc} a^5 = 10.$ Find the value of $\sum_{cyc} a^2.$
1 reply
P162008
Today at 11:27 AM
vanstraelen
25 minutes ago
inequalities of elements in set
toanrathay   1
N an hour ago by Lankou
Let \( m \) be a positive integer such that \( m \geq 4 \), and let the set
\[
A = \{a_1, a_2, a_3, \ldots, a_m\}
\]consist of distinct positive integers not exceeding 2025. Suppose that for every \( a, b \in A \), with \( a \ne b \), if \( a + b \leq 2025 \), then \( a + b \in A \) as well. Prove that

\[
\frac{a_1 + a_2 + a_3 + \cdots + a_m}{m} \geq 1013.
\]
1 reply
toanrathay
5 hours ago
Lankou
an hour ago
2024 PUMaC Team Round, Question 14 Inquiry
A22-4   0
2 hours ago
2024 PUMaC Team Round Question 14 reads as follows:

What is the largest value of $m$ for which I can find nonnegative integers $a_1, a_2, \cdots, a_m<2024$ such that for all indices $i>j$, $17$ divides $\binom{a_i}{a_j}$?
(Note: This should say "... nonnegative integers $a_1<a_2<\cdots<a_m<2024$ ...")

I interpreted this correction to mean the following:
What is the largest value of $m$ for which I there exists nonnegative integers $a_1<a_2<\cdots<a_m<2024$ such that for all indices $i>j$, $17$ divides $\binom{a_i}{a_j}$?

The official answer (https://static1.squarespace.com/static/570450471d07c094a39efaed/t/67421bd74806e80a7ab11c7d/1732385751115/PUMaC_2024_Team__Final_.pdf) is 107. However, I believe I have a construction with $108$ integers - take the set of all integers with a digit sum of $19$ in base $17$, then append $2023_{10}=700_{17}$ to the list.

I checked this with Python using the following code:
[code]def digit_sum_base(n, base):
total = 0
while n > 0:
total += n % base
n //= base
return total

target_sum = 19
base = 17
limit = 2024
qualified_numbers = [n for n in range(limit) if digit_sum_base(n, base) == target_sum]

qualified_numbers.append(2023)

from math import comb

all_divisible = True
for i in range(len(qualified_numbers)):
for j in range(i):
a, b = qualified_numbers, qualified_numbers[j]
if comb(a, b) % 17 != 0:
all_divisible = False
break
if not all_divisible:
break

print(len(qualified_numbers), all_divisible)[/code]

Am I wrong or are they wrong? Any insight would be appreciated!
0 replies
A22-4
2 hours ago
0 replies
How many ways can we indistribute n different marbles into 6 identical boxes
Taiharward   10
N 3 hours ago by MathBot101101
How many ways can we distribute n indifferent marbles into 6 identical boxes and one jar?
10 replies
Taiharward
Today at 2:14 AM
MathBot101101
3 hours ago
Putnam 1972 A2
sqrtX   2
N 4 hours ago by KAME06
Source: Putnam 1972
Let $S$ be a set with a binary operation $\ast$ such that
1) $a \ast(a\ast b)=b$ for all $a,b\in S$.
2) $(a\ast b)\ast b=a$ for all $a,b\in S$.
Show that $\ast$ is commutative and give an example where $\ast$ is not associative.
2 replies
sqrtX
Feb 17, 2022
KAME06
4 hours ago
Limit with sin^2x
Quantum_fluctuations   7
N Today at 7:25 AM by P162008

Evaluate:

$\lim_{x \to 0} \left( 1^{1/\sin^2 x} + 2^{1/\sin^2 x} + 3^{1/\sin^2 x} + .  .  . + n^{1/\sin^2 x} \right)^{\sin^2 x}$
7 replies
Quantum_fluctuations
Apr 26, 2020
P162008
Today at 7:25 AM
Decimal number defined recursively by digit sums modulo 10
fermion13pi   2
N Today at 7:20 AM by solyaris
Source: Competição Elon Lages Lima
Consider the real number written in decimal notation:
r = 0.235831...
where, starting from the third digit after the decimal point, each digit is equal to the remainder when the sum of the previous two digits is divided by 10.

Which of the following statements is true?

(a) (10⁶⁰ - 1).r is an integer
(b) (10²⁵ - 1).r is an integer
(c) (10¹⁷ - 1).r is an integer
(d) r is an irrational algebraic number
(e) r is an irrational transcendental number

(Recall that a complex number is called algebraic if it is a root of a non-zero polynomial with integer coefficients.)
2 replies
fermion13pi
Yesterday at 11:14 PM
solyaris
Today at 7:20 AM
Integrable function: + and - on every subinterval.
SPQ   3
N Today at 7:06 AM by solyaris
Provide a function integrable on [a, b] such that f takes on positive and negative values on every subinterval (c, d) of [a, b]. Prove your function satisfies both conditions.
3 replies
SPQ
Today at 2:40 AM
solyaris
Today at 7:06 AM
Putnam 1999 A4
djmathman   7
N Today at 7:05 AM by P162008
Sum the series \[\sum_{m=1}^\infty\sum_{n=1}^\infty\dfrac{m^2n}{3^m(n3^m+m3^n)}.\]
7 replies
djmathman
Dec 22, 2012
P162008
Today at 7:05 AM
Find the greatest possible value of the expression
BEHZOD_UZ   1
N Today at 6:34 AM by alexheinis
Source: Yandex Uzbekistan Coding and Math Contest 2025
Let $a, b, c, d$ be complex numbers with $|a| \le 1, |b| \le 1, |c| \le 1, |d| \le 1$. Find the greatest possible value of the expression $$|ac+ad+bc-bd|.$$
1 reply
BEHZOD_UZ
Yesterday at 11:56 AM
alexheinis
Today at 6:34 AM
Problem with lcm
snowhite   2
N Today at 6:21 AM by snowhite
Prove that $\underset{n\to \infty }{\mathop{\lim }}\,\sqrt[n]{lcm(1,2,3,...,n)}=e$
Please help me! Thank you!
2 replies
snowhite
Today at 5:19 AM
snowhite
Today at 6:21 AM
combinatorics
Hello_Kitty   2
N Yesterday at 10:23 PM by Hello_Kitty
How many $100$ digit numbers are there
- not including the sequence $123$ ?
- not including the sequences $123$ and $231$ ?
2 replies
Hello_Kitty
Apr 17, 2025
Hello_Kitty
Yesterday at 10:23 PM
Sequence of functions
Squeeze   2
N Yesterday at 10:22 PM by Hello_Kitty
Q) let $f_n:[-1,1)\to\mathbb{R}$ and $f_n(x)=x^{n}$ then is this uniformly convergence on $(0,1)$ comment on uniformly convergence on $[0,1]$ where in general it is should be uniformly convergence.

My I am trying with some contradicton method like chose $\epsilon=1$ and trying to solve$|f_n(a)-f(a)|<\epsilon=1$
Next take a in (0,1) and chose a= 2^1/N but not solution
How to solve like this way help.
2 replies
Squeeze
Apr 18, 2025
Hello_Kitty
Yesterday at 10:22 PM
A in M2(prime), A=B^2 and det(B)=p^2
jasperE3   1
N Yesterday at 9:59 PM by KAME06
Source: VJIMC 2012 1.2
Determine all $2\times2$ integer matrices $A$ having the following properties:

$1.$ the entries of $A$ are (positive) prime numbers,
$2.$ there exists a $2\times2$ integer matrix $B$ such that $A=B^2$ and the determinant of $B$ is the square of a prime number.
1 reply
jasperE3
May 31, 2021
KAME06
Yesterday at 9:59 PM
MN // AD if AB = BC =CD 2004 Rusanovsky Lyceum Olympiad p89
parmenides51   0
Jun 28, 2022
Given a convex quadrilateral $ABCD$, in which $AB = BC =CD$. Let $M$ be the intersection point of the bisector of the external angle at the vertex $B$ with the line $CD$, and let $N$ be the intersection point of the bisector of the external angle at the vertex $C$ with the line $AB$. Prove that $MN \parallel AD$.
0 replies
parmenides51
Jun 28, 2022
0 replies
MN // AD if AB = BC =CD 2004 Rusanovsky Lyceum Olympiad p89
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parmenides51
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Given a convex quadrilateral $ABCD$, in which $AB = BC =CD$. Let $M$ be the intersection point of the bisector of the external angle at the vertex $B$ with the line $CD$, and let $N$ be the intersection point of the bisector of the external angle at the vertex $C$ with the line $AB$. Prove that $MN \parallel AD$.
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