## IMO 2013 – Day 2

4. Let $ABC$ be an acute triangle with orthocenter $H$, and let $W$ be a point on the side $BC$, between $B$ and $C$. The points $M$ and $N$ are the feet of the altitudes drawn from $B$ and $C$, respectively. $\omega_1$ is the circumcircle of triangle $BWN$, and $X$ is a point such that $WX$ is a diameter of $\omega_1$. Similarly, $\omega_2$ is the circumcircle of triangle $CWM$, and $Y$ is a point such that $WY$ is a diameter of $\omega_2$. Show that the points $X, Y$, and $H$ are collinear.

5. Let $\mathbb Q_{>0}$ be the set of all rational numbers greater than zero. Let $f:\mathbb Q_{>0} \to \mathbb R$ be a function satisfying the following conditions

(i) $f(x)f(y) \geq f(xy)$ for all $x, y \in \mathbb Q_{>0}$,

(ii) $f(x+y) \geq f(x) + f(y)$ for all $x, y \in \mathbb Q_{>0}$,

(iii) There exists a rational number $a> 1$ such that $f (a) = a$.

Show that $f(x) = x$ for all $x\in\mathbb Q_{>0}$.

6. Let $n\geq 3$ be an integer, and consider a circle with $n+1$ equally spaced points marked on it. Consider all labellings of these points with the numbers $0,1,\dots, n$ such that each label is used exactly once; two such labellings are considered to be the same if one can be obtained from the other by a rotation of the circle. A labelling is called beautiful if, for any four labels $a with $a+d=b+c$, the chord joining the points labelled $a$ and $d$ does not intersect the chord joining the points labelled $b$ and $c$.

Let $M$ be the number of beautiful labellings and let $N$ be the number of ordered pairs $(x,y)$ of positive integers such that $x+y\leq n$ and $\gcd(x,y)=1$. Prove that $M=N+1.$

## IMO 2013 – Day 1

1. Prove that for any two positive integers $k, n$ there exist positive integers $m_1, m_2, \ldots, m_k$ such that $1+\dfrac{2^k-1}{n}=\left(1+\dfrac{1}{m_1}\right)\left(1+\dfrac{1}{m_2}\right)\dots\left(1+\dfrac{1}{m_k}\right).$

2. Given $2013$ red and $2014$ blue points in the plane, no three of them on a line. We aim to split the plane by lines (not passing through these points) into regions such that there are no regions containing points of both the colors. What is the least number of lines that always suffice?

3. Let $ABC$ be a triangle and let $A_1$, $B_1$, and $C_1$ be points of contact of the excircles with the sides $BC$, $AC$, and $AB$, respectively. Prove that if the circumcenter of $\triangle A_1B_1C_1$ lies on the circumcircle of $\triangle ABC$, then $\triangle ABC$ is a right triangle.

## SEAMS School “Algebraic curves” VIASM, July 8-19, 2013

This school provides basic materials on Commutative Algebra and on algebraic curves.

Dates: July 8, 2013 – July 19, 2013

Organizing Institutions:

– Vietnam Institute for Advanced Study in Mathematics (VIASM)

– Southeast Asian Mathematical Society (SEAMS)

– VIASM

– Centre International de Mathématiques Pures et Appliquées (CIMPA)

Organizers:

Ngo Bao Chau (Chicago and VIASM, Director), Doan Trung Cuong (Institute of Mathematics Hanoi), Le Tuan Hoa (VIASM) and Nguyen Chu Gia Vuong (Institute of Mathematics Hanoi).

Lecturers:

Prof. Ngo Bao Chau, Prof. Nguyen Tu Cuong, Dr. Doan Trung Cuong, Dr. Dinh Thanh Trung and Dr. Nguyen Chu Gia Vuong.

Registration: Before June 15th 2013.

Undergraduate students and graduate students can apply to participate in this school by sending application (form below) via email at vseams@viasm.edu.vn

– Application form for international students: VSEAMS2013-Application-EN

– Application form for Vietnamese students: VSEAMS2013-Application-VN

Selection announcement of participants and grants will be made on June 20th 2013.

Language: English.
Visit the conference site for more details: http://viasm.edu.vn/activities/school_ac2013/