Given natural number n such that, for any natural $a,b$ number $2^a3^b+1$ is not divisible by $n$.Prove that $2^c+3^d$ is not divisible by $n$ for any natural $c$ and $d$

We say that an integer $a$ is a quadratic, cubic, or quintic residue modulo $n$ if there exists an integer $x$ such that $x^2\equiv a \pmod n$, $x^3 \equiv a \pmod n$, or $x^5 \equiv a \pmod n$, respectively. Further, an integer $a$ is a primitive residue modulo $n$ if it is exactly one of these three types of residues modulo $n$. How many integers $1 \le a \le 2015$ are primitive residues modulo $2015$? [i] Proposed by Michael Ren [/i]

Let $n$ be a natural number with more than $2021$ digits, none of which are $8$ or $9$. Suppose that $n$ has no common factors with $2021$. Prove that it is possible to increase one of the digits of $n$ by at most $2$ so that the resulting number is a multiple of 2021.

Find all positive integer pairs $(a,n)$ such that $\frac{(a+1)^n-a^n}{n}$ is an integer.

Consider a finite group $ G $ and the sequence of functions $ \left( A_n \right)_{n\ge 1} :G\longrightarrow \mathcal{P} (G) $ defined as $ A_n(g) = \left\{ x\in G|x^n=g \right\} , $ where $ \mathcal{P} (G) $ is the power of $ G. $ [b]a)[/b] Prove that if $ G $ is commutative, then for any natural numbers $ n, $ either $ A_n(g) =\emptyset , $ or $ \left| A_n(g) \right| =\left| A_n(1) \right| . $ [b]b)[/b] Provide an example of what $ G $ could be in the case that there exists an element $ g_0 $ of $ G $ and a natural number $ n_0 $ such that $ \left| A_{n_0}\left( g_0 \right) \right| >\left| A_{n_0}(1) \right| . $ [i]Sorin Rădulescu[/i] and [i]Ion Savu[/i]

In assigning dormitory rooms, a college gives preference to pairs of students in this order: $$AA,\, AB ,\, AC, \, BB , \, BC ,\, AD , \, CC, \, BD, \, CD, \, DD$$ in which $AA$ means two seniors, $AB$ means a senior and a junior, etc. Determine numerical values to assign to $A,B,C$ and $D$ so that the set of numbers $A+A, A+B, A+C, B+B, \ldots $ corresponding to the order above will be in descending order. Find the general solution and the solution in least positive integers.

Find all positive integer pairs $(a,n)$ such that $\frac{(a+1)^n-a^n}{n}$ is an integer.

Find all pairs $(p,q)$ of prime numbers such that $$p^q-4~|~q^p-1.$$ [i](Vlad Robu)[/i]

Let \( \mathbb{N} \) be the set of all positive integers. We say that a function \( f: \mathbb{N} \to \mathbb{N} \) is Georgian if \( f(1) = 1 \) and, for every positive integer \( n \), there exists a positive integer \( k \) such that \[ f^{(k)}(n) = 1, \quad \text{where } f^{(k)} = f \circ f \cdots \circ f \quad \text{(applied } k \text{ times)}. \] If \( f \) is a Georgian function, we define, for each positive integer \( n \), \( \text{ord}(n) \) as the smallest positive integer \( m \) such that \( f^{(m)}(n) = 1 \). Determine all positive real numbers \( c \) for which there exists a Georgian function such that, for every positive integer \( n \geq 2024 \), it holds that \( \text{ord}(n) \geq cn - 1 \).

Let $N(n)$ denote the smallest positive integer $N$ such that $x^N =e$ for every element $x$ of the symmetric group $S_n$, where $e$ denotes the identity permutation. Prove that if $n>1,$ $$\frac{N(n)}{N(n-1)} =\begin{cases} p \;\text{if}\; n\; \text{is a power of a prime } p\\ 1\; \text{otherwise}. \end{cases}$$

Find all pairs $(p,q)$ of prime numbers such that $$p^q-4~|~q^p-1.$$ [i](Vlad Robu)[/i]

Find all ordered triples of primes $(p, q, r)$ such that \[ p \mid q^r + 1, \quad q \mid r^p + 1, \quad r \mid p^q + 1. \] [i]Reid Barton[/i]

Given positive odd integers $m$ and $n$ where the set of all prime factors of $m$ is the same as the set of all prime factors $n$, and $n \vert m$. Let $a$ be an arbitrary integer which is relatively prime to $m$ and $n$. Prove that: \[ o_m(a) = o_n(a) \times \frac{m}{\gcd(m, a^{o_n(a)}-1)} \] where $o_k(a)$ denotes the smallest positive integer such that $a^{o_k(a)} \equiv 1$ (mod $k$) holds for some natural number $k > 1$.

Let $x$ and $y$ be integers and let $p$ be a prime number. Suppose that there exist realatively prime positive integers $m$ and $n$ such that $$x^m \equiv y^n \pmod p$$ Prove that there exists an unique integer $z$ modulo $p$ such that $$x \equiv z^n \pmod p \quad \text{and} \quad y \equiv z^m \pmod p$$