Found problems: 15460
2016 Saudi Arabia Pre-TST, 2.3
Let $u$ and $v$ be positive rational numbers with $u \ne v$. Assume that there are infinitely many positive integers $n$ with the property that $u^n - v^n$ are integers. Prove that $u$ and $v$ are integers.
2024 Kyiv City MO Round 1, Problem 1
Find all pairs of positive integers $(a, b)$ such that $4b - 1$ is divisible by $3a + 1$ and $3a - 1$ is divisible by $2b + 1$.
2014 NIMO Problems, 8
For positive integers $a$, $b$, and $c$, define \[ f(a,b,c)=\frac{abc}{\text{gcd}(a,b,c)\cdot\text{lcm}(a,b,c)}. \] We say that a positive integer $n$ is $f@$ if there exist pairwise distinct positive integers $x,y,z\leq60$ that satisfy $f(x,y,z)=n$. How many $f@$ integers are there?
[i]Proposed by Michael Ren[/i]
1954 Moscow Mathematical Olympiad, 262
Are there integers $m$ and $n$ such that $m^2 + 1954 = n^2$?
2021 Latvia Baltic Way TST, P16
A function $f:\mathbb{N} \to \mathbb{N}$ is given. If $a,b$ are coprime, then $f(ab)=f(a)f(b)$. Also, if $m,k$ are primes (not necessarily different), then $$f(m+k-3)=f(m)+f(k)-f(3).$$ Find all possible values of $f(11)$.
2022 Cyprus TST, 2
Determine for how many positive integers $n\in\{1, 2, \ldots, 2022\}$ it holds that $402$ divides at least one of
\[n^2-1, n^3-1, n^4-1\]
2016 India Regional Mathematical Olympiad, 6
Show that the infinite arithmetic progression $\{1,4,7,10 \ldots\}$ has infinitely many 3 -term sub sequences in harmonic progression such that for any two such triples $\{a_1, a_2 , a_3 \}$ and $\{b_1, b_2 ,b_3\}$ in harmonic progression , one has $$\frac{a_1} {b_1} \ne \frac {a_2}{b_2}$$.
2012 Pan African, 2
Find all positive integers $m$ and $n$ such that $n^m - m$ divides $m^2 + 2m$.
2004 IMO Shortlist, 6
Given an integer ${n>1}$, denote by $P_{n}$ the product of all positive integers $x$ less than $n$ and such that $n$ divides ${x^2-1}$. For each ${n>1}$, find the remainder of $P_{n}$ on division by $n$.
[i]Proposed by John Murray, Ireland[/i]
1998 Tournament Of Towns, 4
For every three-digit number, we take the product of its three digits. Then we add all of these products together. What is the result?
(G Galperin)
2007 Estonia Math Open Senior Contests, 1
Let $ a_n \equal{} 1 \plus{} 2 \plus{} ... \plus{} n$ for every $ n \ge 1$; the numbers $ a_n$ are called triangular. Prove that if $ 2a_m \equal{} a_n$ then $ a_{2m \minus{} n}$ is a perfect square.
2002 Romania Team Selection Test, 1
Find all sets $A$ and $B$ that satisfy the following conditions:
a) $A \cup B= \mathbb{Z}$;
b) if $x \in A$ then $x-1 \in B$;
c) if $x,y \in B$ then $x+y \in A$.
[i]Laurentiu Panaitopol[/i]
2022 Baltic Way, 18
Find all pairs $(a, b)$ of positive integers such that $a \le b$ and
$$ \gcd(x, a) \gcd(x, b) = \gcd(x, 20) \gcd(x, 22) $$
holds for every positive integer $x$.
2015 Grand Duchy of Lithuania, 4
We denote by gcd (...) the greatest common divisor of the numbers in (...). (For example, gcd$(4, 6, 8)=2$ and gcd $(12, 15)=3$.) Suppose that positive integers $a, b, c$ satisfy the following four conditions:
$\bullet$ gcd $(a, b, c)=1$,
$\bullet$ gcd $(a, b + c)>1$,
$\bullet$ gcd $(b, c + a)>1$,
$\bullet$ gcd $(c, a + b)>1$.
a) Is it possible that $a + b + c = 2015$?
b) Determine the minimum possible value that the sum $a+ b+ c$ can take.
2011 IMO Shortlist, 1
Given any set $A = \{a_1, a_2, a_3, a_4\}$ of four distinct positive integers, we denote the sum $a_1 +a_2 +a_3 +a_4$ by $s_A$. Let $n_A$ denote the number of pairs $(i, j)$ with $1 \leq i < j \leq 4$ for which $a_i +a_j$ divides $s_A$. Find all sets $A$ of four distinct positive integers which achieve the largest possible value of $n_A$.
[i]Proposed by Fernando Campos, Mexico[/i]
1994 North Macedonia National Olympiad, 1
Let $ a_1, a_2, ..., a_ {1994} $ be integers such that $ a_1 + a_2 + ... + a_{1994} = 1994 ^{1994} $ .
Determine the remainder of the division of $ a ^ 3_1 + a ^ 3_2 + ... + a ^ 3_{1994} $ with $6$.
2011 May Olympiad, 1
Find a positive integer $x$ such that the sum of the digits of $x$ is greater than $2011$ times the sum of the digits of the number $3x$ ($3$ times $x$).
1999 Dutch Mathematical Olympiad, 5
Let $c$ be a nonnegative integer, and define $a_n = n^2 + c$ (for $n \geq 1)$. Define $d_n$ as the greatest common divisor of $a_n$ and $a_{n + 1}$.
(a) Suppose that $c = 0$. Show that $d_n = 1,\ \forall n \geq 1$.
(b) Suppose that $c = 1$. Show that $d_n \in \{1,5\},\ \forall n \geq 1$.
(c) Show that $d_n \leq 4c + 1,\ \forall n \geq 1$.
1995 Baltic Way, 1
Find all triples $(x,y,z)$ of positive integers satisfying the system of equations
\[\begin{cases} x^2=2(y+z)\\ x^6=y^6+z^6+31(y^2+z^2)\end{cases}\]
2015 Danube Mathematical Competition, 4
Given an integer $n \ge 2$ ,determine the numbers that written in the form $a_1$$a_2$$+$$a_2$$a_3$$+$$...$$a_{k-1}$$a_k$ , where $k$ is an integer greater than or equal to 2, and $a_1$ ,... $a_k$ are positive integers with sum $n$.
2025 Poland - Second Round, 2
Determine all integers $n\ge 2$ with the following property: the number $2^k\cdot n-1$ is prime for all $k\in\{2,3,\ldots,n\}$.
1999 IMO Shortlist, 5
Let $n,k$ be positive integers such that n is not divisible by 3 and $k \geq n$. Prove that there exists a positive integer $m$ which is divisible by $n$ and the sum of its digits in decimal representation is $k$.
2014 China Team Selection Test, 2
Let $A$ be a finite set of positive numbers , $B=\{\frac{a+b}{c+d} |a,b,c,d \in A \}$.
Show that: $\left | B \right | \ge 2\left | A \right |^2-1 $,
where $|X| $ be the number of elements of the finite set $X$.
(High School Affiliated to Nanjing Normal University )
2003 Irish Math Olympiad, 1
find all solutions, not necessarily positive integers for $(m^2+ n)(m+ n^2)= (m+ n)^3$
2006 Princeton University Math Competition, 7
Find the largest possible value of the expression $x+y+z$, $x,y, z \in Z$, given that the equation $10x^3 +20y^3+2006xyz = 2007z^3$ holds.