Looks difficult number mock#5

by Physicsknight, Apr 14, 2025, 1:51 PM

Consider $a_1,a_2,\hdots,a_b$ be distinct prime numbers. Let $\alpha_i=\sqrt{a}, \,\mathrm{A}=\mathbb{Q}[\alpha_1,\alpha_2,\hdots,\alpha_b]$
Let $\gamma=\sum\,\alpha_i$
  • Prove that $[\mathrm{A}:\mathbb{Q}]=2^b$
  • Prove that $\mathrm{A}=\mathbb{Q}[\gamma];$ and deduce that the minimum polynomial $f(X)$ of $\gamma$ over $\mathbb{Q}$ has degree $2^b.$
  • Prove that $f(X)$ factors in $\mathbb{Z}_a[X]$ into a product of polynomials of degree $\leq 4 \,(a\ne 2)$ either of degree $\leq 8\,(a=2)$

Turbo's en route to visit each cell of the board

by Lukaluce, Apr 14, 2025, 11:01 AM

Let $n > 1$ be an integer. In a configuration of an $n \times n$ board, each of the $n^2$ cells contains an arrow, either pointing up, down, left, or right. Given a starting configuration, Turbo the snail starts in one of the cells of the board and travels from cell to cell. In each move, Turbo moves one square unit in the direction indicated by the arrow in her cell (possibly leaving the board). After each move, the arrows in all of the cells rotate $90^{\circ}$ counterclockwise. We call a cell good if, starting from that cell, Turbo visits each cell of the board exactly once, without leaving the board, and returns to her initial cell at the end. Determine, in terms of $n$, the maximum number of good cells over all possible starting configurations.

Proposed by Melek Güngör, Turkey
This post has been edited 1 time. Last edited by Lukaluce, Yesterday at 11:54 AM

Nice FE from Canada Winter Camp

by AshAuktober, Apr 12, 2025, 10:47 AM

Find all functions $f:\mathbb{R}\to\mathbb{Z}$ such that $f(x+y)<f(x)+f(y)$ and $f(f(x))=\lfloor x\rfloor+2$ for all reals $x,y$.

equal angles

by jhz, Mar 26, 2025, 12:56 AM

In convex quadrilateral $ABCD, AB \perp AD, AD = DC$. Let $ E$ be a point on side $BC$, and $F$ be a point on the extension of $DE$ such that $\angle ABF = \angle DEC>90^{\circ}$. Let $O$ be the circumcenter of $\triangle CDE$, and $P$ be a point on the side extension of $FO$ satisfying $FB =FP$. Line BP intersects AC at point Q. Prove that $\angle AQB =\angle DPF.$

Simple FE on National Contest

by somebodyyouusedtoknow, Aug 29, 2023, 6:48 AM

Determine all functions $f : \mathbb{R} \to \mathbb{R}$ such that the following equation holds for every real $x,y$:
\[ f(f(x) + y) = \lfloor x + f(f(y)) \rfloor. \]Note: $\lfloor x \rfloor$ denotes the greatest integer not greater than $x$.

Four-variable FE mod n

by TheUltimate123, Jul 11, 2023, 6:27 PM

Let \(n\) be a positive integer, and let \(\mathbb Z/n\mathbb Z\) denote the integers modulo \(n\). Determine the number of functions \(f:(\mathbb Z/n\mathbb Z)^4\to\mathbb Z/n\mathbb Z\) satisfying \begin{align*}     &f(a,b,c,d)+f(a+b,c,d,e)+f(a,b,c+d,e)\\     &=f(b,c,d,e)+f(a,b+c,d,e)+f(a,b,c,d+e). \end{align*}for all \(a,b,c,d,e\in\mathbb Z/n\mathbb Z\).

BMO Shortlist 2021 A6

by Lukaluce, May 8, 2022, 5:01 PM

Find all functions $f: \mathbb{R} \rightarrow \mathbb{R}$ such that
$$f(xy) = f(x)f(y) + f(f(x + y))$$holds for all $x, y \in \mathbb{R}$.

IMO 2014 Problem 5

by codyj, Jul 9, 2014, 11:59 AM

For each positive integer $n$, the Bank of Cape Town issues coins of denomination $\frac1n$. Given a finite collection of such coins (of not necessarily different denominations) with total value at most most $99+\frac12$, prove that it is possible to split this collection into $100$ or fewer groups, such that each group has total value at most $1$.
This post has been edited 3 times. Last edited by codyj, Jul 12, 2014, 12:54 AM

IMO 2011 Problem 5

by orl, Jul 19, 2011, 12:00 PM

Let $f$ be a function from the set of integers to the set of positive integers. Suppose that, for any two integers $m$ and $n$, the difference $f(m) - f(n)$ is divisible by $f(m- n)$. Prove that, for all integers $m$ and $n$ with $f(m) \leq f(n)$, the number $f(n)$ is divisible by $f(m)$.

Proposed by Mahyar Sefidgaran, Iran

Very easy number theory

by darij grinberg, Aug 6, 2004, 11:42 AM

Determine all positive integers $ n\geq 2$ that satisfy the following condition: for all $ a$ and $ b$ relatively prime to $ n$ we have \[a \equiv b \pmod n\qquad\text{if and only if}\qquad ab\equiv 1 \pmod n.\]
This post has been edited 1 time. Last edited by djmathman, Oct 3, 2016, 3:32 AM
Reason: adjusted formatting

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