Found problems: 526
Find all polynomials $f(x)$ with integer coefficients such that $f(n)$ and $f(2^{n})$ are co-prime for all natural numbers $n$.
How many three-digit numbers leave remainder $2$ when divided by $5$ and leave remainder $7$ when divided by $9$?
$\mathrm{(A) \ } 20 \qquad \mathrm{(B) \ } 21 \qquad \mathrm {(C) \ } 22 \qquad \mathrm{(D) \ } 23 \qquad \mathrm{(E) \ } 24$
Given positive integers $a,c$ and integer $b$, prove that there exists a positive integer $x$ such that
\[ a^x + x \equiv b \pmod c, \]
that is, there exists a positive integer $x$ such that $c$ is a divisor of $a^x + x - b$.
Let $P(x)$ be a non-constant polynomial with integer coefficients and let $n$ be a positive integer. The sequence $a_0,a_1,\ldots$ is defined as follows: $a_0=n$ and $a_k=P(a_{k-1})$ for all positive integers $k.$ Assume that for every positive integer $b$ the sequence contains a $b$th power of an integer greater than $1.$ Show that $P(x)$ is linear.
Find the number of polynomials $f(x)=ax^3+bx$ satisfying both following conditions:
(i) $a,b\in\{1,2,\ldots,2013\}$;
(ii) the difference between any two of $f(1),f(2),\ldots,f(2013)$ is not a multiple of $2013$.
Let a sequence $\{a_n\}$, $n \in \mathbb{N}^{*}$ given, satisfying the condition
\[0 < a_{n+1} - a_n \leq 2001\]
for all $n \in \mathbb{N}^{*}$
Show that there are infinitely many pairs of positive integers $(p, q)$ such that $p < q$ and $a_p$ is divisor of $a_q$.
Let $a>1$ be a positive integer and $f\in \mathbb{Z}[x]$ with positive leading coefficient. Let $S$ be the set of integers $n$ such that
\[n \mid a^{f(n)}-1.\]
Prove that $S$ has density $0$; that is, prove that $\lim_{n\rightarrow \infty} \frac{|S\cap \{1,...,n\}|}{n}=0$.
Let $n>1$ be an integer, and let $k$ be the number of distinct prime divisors of $n$. Prove that there exists an integer $a$, $1<a<\frac{n}{k}+1$, such that $n \mid a^2-a$.
The number obtained from the last two nonzero digits of $ 90!$ is equal to $ n$. What is $ n$?
$ \textbf{(A)}\ 12 \qquad
\textbf{(B)}\ 32 \qquad
\textbf{(C)}\ 48 \qquad
\textbf{(D)}\ 52 \qquad
\textbf{(E)}\ 68$
Prove that there exists a real number $\varepsilon>0$ such that there are infinitely many sequences of integers $0<a_1<a_2<\hdots<a_{2025}$ satisfying
\[\gcd(a_1^2+1, a_2^2+1,\hdots, a_{2025}^2+1) > a_{2025}^{1+\varepsilon}.\]
[i]Pitchayut Saengrungkongka[/i]
Let $k$ be a fixed integer greater than 1, and let ${m=4k^2-5}$. Show that there exist positive integers $a$ and $b$ such that the sequence $(x_n)$ defined by \[x_0=a,\quad x_1=b,\quad x_{n+2}=x_{n+1}+x_n\quad\text{for}\quad n=0,1,2,\dots,\] has all of its terms relatively prime to $m$.
[i]Proposed by Jaroslaw Wroblewski, Poland[/i]
Find the natural numbers $ n\ge 2 $ which have the property that the ring of integers modulo $ n $ has exactly an element that is not a sum of two squares.
Let $m$ be the least positive integer divisible by $17$ whose digits sum to $17$. Find $m$.
Let $a > 1$ be a positive integer and $d > 1$ be a positive integer coprime to $a$. Let $x_1=1$, and for $k\geq 1$, define
$$x_{k+1} = \begin{cases}
x_k + d &\text{if } a \text{ does not divide } x_k \\
x_k/a & \text{if } a \text{ divides } x_k
\end{cases}$$
Find, in terms of $a$ and $d$, the greatest positive integer $n$ for which there exists an index $k$ such that $x_k$ is divisible by $a^n$.
Does there exist a sequence of positive integers $a_1,a_2,...$ such that every positive integer occurs exactly once and that the number $\tau (na_{n+1}^n+(n+1)a_n^{n+1})$ is divisible by $n$ for all positive integer.
Here $\tau (n)$ denotes the number of positive divisor of $n$.
Prove that there exists two strictly increasing sequences $(a_{n})$ and $(b_{n})$ such that $a_{n}(a_{n}+1)$ divides $b^{2}_{n}+1$ for every natural n.
Consider the sequence $(a^n + 1)_{n\geq 1}$, with $a>1$ a fixed integer.
i) Prove there exist infinitely many primes, each dividing some term of the sequence.
ii) Prove there exist infinitely many primes, none dividing any term of the sequence.
[i](Dan Schwarz)[/i]
Find all triples $(a,b,c)$ of positive integers such that if $n$ is not divisible by any prime less than $2014$, then $n+c$ divides $a^n+b^n+n$.
[i]Proposed by Evan Chen[/i]
For distinct positive integers $a, b<2012$, define $f(a, b)$ to be the number of integers $k$ with $1\le k<2012$ such that the remainder when $ak$ divided by $2012$ is greater than that of $bk$ divided by $2012$. Let $S$ be the minimum value of $f(a, b)$, where $a$ and $b$ range over all pairs of distinct positive integers less than $2012$. Determine $S$.
A lattice point in the coordinate plane with origin $O$ is called invisible if the segment $OA$ contains a lattice point other than $O,A$. Let $L$ be a positive integer. Show that there exists a square with side length $L$ and sides parallel to the coordinate axes, such that all points in the square are invisible.
For $ x \in (0, 1)$ let $ y \in (0, 1)$ be the number whose $ n$-th digit after the decimal point is the $ 2^{n}$-th digit after the decimal point of $ x$. Show that if $ x$ is rational then so is $ y$.
[i]Proposed by J.P. Grossman, Canada[/i]
For a positive integer $n$, we will say that a sequence $a_1, a_2, \dots a_n$ where $a_i \in \{1, 2, \dots , n\}$ for all $i$ is $n$[i]-highly divisible[/i] if, for every positive integer $d$ that divides $n$ and every nonnegative integer $k$ less than $\frac{n}{d}$ we have that \[ d\;\Bigg\vert \sum_{i=kd+1}^{(k+1)d} a_i. \] Let $\chi(n)$ be the probability that a sequence $a_1, a_2, \dots, a_n$ where $a_i$ is chosen randomly from $\{1, 2, \dots n\}$ independently for all $i$ is $n$-highly divisible. Suppose that $n$ is a positive integer such that there exists a positive integer $m$ not divisible by 3 such that $3^{40}\chi(n)=\frac{1}{m}$. Compute the sum of all possible values of $n$.
[i]Proposed by Jaedon Whyte[/i]
Let $f$ be a non-constant function from the set of positive integers into the set of positive integer, such that $a-b$ divides $f(a)-f(b)$ for all distinct positive integers $a$, $b$. Prove that there exist infinitely many primes $p$ such that $p$ divides $f(c)$ for some positive integer $c$.
[i]Proposed by Juhan Aru, Estonia[/i]
Prove that for every given positive integer $k$, there exist infinitely many $n$, such that $2^{n}+3^{n}-1, 2^{n}+3^{n}-2,\ldots, 2^{n}+3^{n}-k$ are all composite numbers.
Find all positive integers $n$ such that for any integer $k$ there exists an integer $a$ for which $a^3+a-k$ is divisible by $n$.
[i]Warut Suksompong, Thailand[/i]