# International Mathematics Competition for University Students 2019

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IMC 2022
 Information Results Problems & Solutions

## IMC2018: Problems on Day 2

Problem 6. Let $\displaystyle k$ be a positive integer. Find the smallest positive integer $\displaystyle n$ for which there exist $\displaystyle k$ nonzero vectors $\displaystyle v_1,\ldots,v_k$ in $\displaystyle \mathbb R^n$ such that for every pair $\displaystyle i,j$ of indices with $\displaystyle |i-j|> 1$ the vectors $\displaystyle v_i$ and $\displaystyle v_j$ are orthogonal.

(Proposed by Alexey Balitskiy, Moscow Institute of Physics and Technology and M.I.T.)

Problem 7. Let $\displaystyle (a_n)_{n=0}^\infty$ be a sequence of real numbers such that $\displaystyle a_0=0$ and

$\displaystyle a_{n+1}^3=a_n^2-8 \quad \text{for} \quad n=0,1,2,\ldots$

Prove that the following series is convergent:

 $\displaystyle \sum_{n=0}^\infty|a_{n+1}-a_n|.$ $\displaystyle (1)$

(Proposed by Orif Ibrogimov, National University of Uzbekistan)

Problem 8. Let $\displaystyle \Omega={(x,y,z)\in \mathbb{Z}^3: y+1\ge x\ge y\ge z\ge 0}$. A frog moves along the points of $\displaystyle \Omega$ by jumps of length $\displaystyle 1$. For every positive integer $\displaystyle n$, determine the number of paths the frog can take to reach $\displaystyle (n,n,n)$ starting from $\displaystyle (0,0,0)$ in exactly $\displaystyle 3n$ jumps.

(Proposed by Fedor Petrov and Anatoly Vershik, St. Petersburg State University)

Problem 9. Determine all pairs $\displaystyle P(x)$, $\displaystyle Q(x)$ of complex polynomials with leading coefficient $\displaystyle 1$ such that $\displaystyle P(x)$ divides $\displaystyle Q(x)^2+1$ and $\displaystyle Q(x)$ divides $\displaystyle P(x)^2+1$.

(Proposed by Rodrigo Angelo, Princeton University and Matheus Secco, PUC, Rio de Janeiro)

Problem 10. For $\displaystyle R>1$ let $\displaystyle \mathcal{D}_R = {(a,b)\in\mathbb{Z}^2 \colon 0<a^2+b^2<R}$. Compute

$\displaystyle \lim_{R\rightarrow \infty} \sum_{(a,b) \in \mathcal{D}_R} \dfrac{(-1)^{a+b}}{a^2+b^2}.$

(Proposed by Rodrigo Angelo, Princeton University and Matheus Secco, PUC, Rio de Janeiro) IMC1994 IMC1995 IMC1996 IMC1997 IMC1998 IMC1999 IMC2000 IMC2001 IMC2002 IMC2003 IMC2004 IMC2005 IMC2006 IMC2007 IMC2008 IMC2009 IMC2010 IMC2011 IMC2012 IMC2013 IMC2014 IMC2015 IMC2016 IMC2017 IMC2018 IMC 2019 IMC2020 IMC2021 IMC2022 