Olympiad problems

2015 BMT Spring, 6

Tags: number theory

An integer-valued function $f$ satisfies $f(2) = 4$ and $f(mn) = f(m)f(n)$ for all integers $m$ and $n$. If $f$ is an increasing function, determine $f(2015)$.

2014 JBMO TST - Macedonia, 3

Tags: number theory , digit , divisibility

Find all positive integers $n$ which are divisible by 11 and satisfy the following condition: all the numbers which are generated by an arbitrary rearrangement of the digits of $n$, are also divisible by 11.

2017 Princeton University Math Competition, A6/B8

Tags: number theory

Find the least positive integer $N$ such that the only values of $n$ for which $1 + N \cdot 2^n$ is prime are multiples of $12$.

2010 Indonesia TST, 3

Tags: function , induction , number theory

Let $ \mathbb{Z}$ be the set of all integers. Define the set $ \mathbb{H}$ as follows: (1). $ \dfrac{1}{2} \in \mathbb{H}$, (2). if $ x \in \mathbb{H}$, then $ \dfrac{1}{1\plus{}x} \in \mathbb{H}$ and also $ \dfrac{x}{1\plus{}x} \in \mathbb{H}$. Prove that there exists a bijective function $ f: \mathbb{Z} \rightarrow \mathbb{H}$.

2001 District Olympiad, 3

Tags: number theory

Conside a positive odd integer $k$ and let $n_1<n_2<\ldots<n_k$ be $k$ positive odd integers. Prove that: \[n_1^2-n_2^2+n_3^2-n_4^2+\ldots+n_k^2\ge 2k^2-1\] [i]Titu Andreescu[/i]

1991 IberoAmerican, 5

Tags: algebra , polynomial , quadratic , number theory

Let $P(x,\, y)=2x^{2}-6xy+5y^{2}$. Let us say an integer number $a$ is a value of $P$ if there exist integer numbers $b$, $c$ such that $P(b,\, c)=a$. a) Find all values of $P$ lying between 1 and 100. b) Show that if $r$ and $s$ are values of $P$, then so is $rs$.

2019 Czech and Slovak Olympiad III A, 5

Tags: number theory

Prove that there are infinitely many integers which cannot be expressed as $2^a+3^b-5^c$ for non-negative integers $a,b,c$.

VI Soros Olympiad 1999 - 2000 (Russia), 9.2

Tags: number theory , diophantine equation , diophantine

Find the smallest natural number n such that for all integers $m > n$ there are positive integers $x$ and $y$ for which the equality 1$7x + 23y = m$ holds

2014 Iran Team Selection Test, 4

Tags: quadratic , number theory

$n$ is a natural number. We shall call a permutation $a_1,\dots,a_n$ of $1,\dots,n$ a quadratic(cubic) permutation if $\forall 1\leq i \leq n-1$ we have $a_ia_{i+1}+1$ is a perfect square(cube). $(a)$ Prove that for infinitely many natural numbers $n$ there exists a quadratic permutation. $(b)$ Prove that for no natural number $n$ exists a cubic permutation.

2017 Taiwan TST Round 3, 2

Tags: ellipse , number theory , conic

Choose a rational point $P_0(x_p,y_p)$ arbitrary on ellipse $C:x^2+2y^2=2098$. Define $P_1,P_2,\cdots$ recursively by the following rules: $(1)$ Choose a lattice point $Q_i=(x_i,y_i)\notin C$ such that $|x_i|<50$ and $|y_i|<50$. $(2)$ Line $P_iQ_i$ intersects $C$ at another point $P_{i+1}$. Prove that for any point $P_0$ we can choose suitable points $Q_0,Q_1,\cdots$ such that $\exists k\in\mathbb{N}\cup\{0\}$, $\overline{OP_k}^2=2017$.

2002 APMO, 2

Tags: number theory

Find all positive integers $a$ and $b$ such that \[ {a^2+b\over b^2-a}\quad\mbox{and}\quad{b^2+a\over a^2-b} \] are both integers.

2022 Junior Balkan Team Selection Tests - Moldova, 2

Tags: number theory , combinatorics

Let n be the natural number ($n\ge 2$). All natural numbers from $1$ up to $n$ ,inclusive, are written on the board in some order: $a_1$, $a_2$ , $...$ , $a_n$. Determine all natural numbers $n$ ($n\ge 2$), for which the product $$P = (1 + a_1) \cdot (2 + a_2) \cdot ... \cdot (n + a_n)$$ is an even number, whatever the arrangement of the numbers written on the board.

2015 Cono Sur Olympiad, 5

Tags: number theory , divisibility , sequence

Determine if there exists an infinite sequence of not necessarily distinct positive integers $a_1, a_2, a_3,\ldots$ such that for any positive integers $m$ and $n$ where $1 \leq m < n$, the number $a_{m+1} + a_{m+2} + \ldots + a_{n}$ is not divisible by $a_1 + a_2 + \ldots + a_m$.

2010 Kyrgyzstan National Olympiad, 6

Tags: modular arithmetic , number theory

Let $p$ - a prime, where $p> 11$. Prove that there exists a number $k$ such that the product $p \cdot k$ can be written in the decimal system with only ones.

2023 Harvard-MIT Mathematics Tournament, 7

Tags: number theory

If $a, b, c$, and $d$ are pairwise distinct positive integers that satisfy $lcm (a, b, c, d) < 1000$ and $a+b = c+d$, compute the largest possible value of $a + b$.

2001 Croatia National Olympiad, Problem 3

Tags: number theory , combinatorics

Let there be given triples of integers $(r_j,s_j,t_j),~j=1,2,\ldots,N$, such that for each $j$, $r_j,t_j,s_j$ are not all even. Show that one can find integers $a,b,c$ such that $ar_j+bs_j+ct_j$ is odd for at least $\frac{4N}7$ of the indices $j$.

2015 India National Olympiad, 2

Tags: induction , number theory

For any natural number $n > 1$ write the finite decimal expansion of $\frac{1}{n}$ (for example we write $\frac{1}{2}=0.4\overline{9}$ as its infinite decimal expansion not $0.5)$. Determine the length of non-periodic part of the (infinite) decimal expansion of $\frac{1}{n}$.

2024 ELMO Shortlist, N3

Tags: number theory , ratio

Given a positive integer $k$, find all polynomials $P$ of degree $k$ with integer coefficients such that for all positive integers $n$ where all of $P(n)$, $P(2024n)$, $P(2024^2n)$ are nonzero, we have $$\frac{\gcd(P(2024n), P(2024^2n))}{\gcd(P(n), P(2024n))}=2024^k.$$ [i]Allen Wang[/i]

2023 Saint Petersburg Mathematical Olympiad, 3

Tags: number theory

The infinite periodic fractions $\frac{a} {b}$ and $\frac{c} {d}$ with $(a, b)=(c, d)=1$ are such that every finite block of digits in the first fraction after the decimal point appears in the second fraction as well (again after the decimal point). Show that $b=d$.

1983 Spain Mathematical Olympiad, 6

Tags: number theory , system of equations

In a cafeteria, a glass of lemonade, three sandwiches and seven biscuits have cost $1$ shilling and $2$ pence, and a glass of lemonade, four sandwiches and $10$ biscuits they are worth $1$ shilling and $5$ pence. Find the price of: a) a glass of lemonade, a sandwich and a cake; b) two glasses of lemonade, three sandwiches and five biscuits. ($1$ shilling = $12$ pence).

2018-2019 Winter SDPC, 6

Tags: function , number theory

Let $S$ be the set of positive perfect squares that are of the form $\overline{AA}$, i.e. the concatenation of two equal integers $A$. (Integers are not allowed to start with zero.) (a) Prove that $S$ is infinite. (b) Does there exist a function $f:S\times S \rightarrow S$ such that if $a,b,c \in S$ and $a,b | c$, then $f(a,b) | c$? (If such a function $f$ exists, we call $f$ an LCM function)

2024 Belarus Team Selection Test, 4.1

Tags: algebra , system of equations , number theory

Six integers $a,b,c,d,e,f$ satisfy: $\begin{cases} ace+3ebd-3bcf+3adf=5 \\ bce+acf-ade+3bdf=2 \end{cases}$ Find all possible values of $abcde$ [i]D. Bazyleu[/i]

2022 IFYM, Sozopol, 6

Tags: number theory

Let $D$ be an infinite in both sides sequence of $0$s and $1$s. For each positive integer $n$ we denote with $a_n$ the number of different subsequences of $0$s and $1$s in $D$ of length $n$. Does there exist a sequence $D$ for which for each $n\geq 22$ the number $a_n$ is equal to the $n$-th prime number?

2003 India IMO Training Camp, 10

Tags: modular arithmetic , number theory

Let $n$ be a positive integer greater than $1$, and let $p$ be a prime such that $n$ divides $p-1$ and $p$ divides $n^3-1$. Prove that $4p-3$ is a square.

2011 AIME Problems, 1

Tags: ratio , number theory , relatively prime

Jar A contains four liters of a solution that is $45\%$ acid. Jar B contains five liters of a solution that is $48\%$ acid. Jar C contains one liter of a solution that is $k\%$ acid. From jar C, $\tfrac{m}{n}$ liters of the solution is added to jar A, and the remainder of the solution in jar C is added to jar B. At the end, both jar A and jar B contain solutions that are $50\%$ acid. Given that $m$ and $n$ are relatively prime positive integers, find $k+m+n$.